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Cite TR page numbers, SA38 ch. 2, PCjr HMS (uploaded) and NMAH; add prices, maker, units, dimensions, weights, models; fix memory aliasing (only without expansion), keyboard-replacement and TR-edition claim, PSU fuse/trip values; remove uncited Tandy, 'PCjr cliff', developer; copyedit
 
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| name          = IBM PCjr
| name          = IBM PCjr
| logo          = [[File:IBM PCjr wordmark.svg|150px]]
| logo          = [[File:IBM PCjr wordmark.svg|150px]]
| image        = [[File:IBM PCjr with display.jpg|260px]]
| image        = [[File:IBM PCjr with display.jpg|260px|frameless]]
| caption      = IBM PCjr (4860) with its companion IBM 4863 display
| caption      = IBM PCjr (4860) with the IBM PCjr Color Display (4863) and the 62-key cordless keyboard
| developer    = IBM Entry Systems Division, Boca Raton
| manufacturer  = IBM (built by Teledyne in Lewisburg, Tennessee)
| manufacturer  = IBM
| type          = Home personal computer
| type          = Home / consumer personal computer
| release date  = March 1984 (announced November 1983)
| release date  = March 1984 (announced 1 November 1983)
| discontinued  = March 1985
| discontinued  = March 1985
| cpu          = Intel 8088 @ 4.77 MHz
| price        = US$669 (64 KB); US$1,269 (128 KB with diskette drive)
| memory        = 64 KB on the system board (4860-004) or 128 KB (4860-067); expandable by sidecar
| units_sold    = About 270,000
| storage      = One 5.25" 360 KB half-height diskette drive (4860-067 only); cassette port; two ROM cartridge slots
| cpu          = Intel 8088 at 4.77 MHz
| display      = Built-in colour/graphics subsystem using a Motorola 6845 CRTC and IBM's Video Gate Array; direct-drive RGB, composite video, and RF through the optional TV Connector
| memory        = 64 KB on the system board; 128 KB with the 64KB Memory and Display Expansion; IBM lists expansion to 512 KB
| sound        = Texas Instruments '''SN76496N''' complex sound generator, plus the PC speaker, selected by an 8255
| storage      = Optional 5.25-inch 360 KB diskette drive (one only); cassette interface; two ROM cartridge slots
| os            = IBM PC DOS 2.1; IBM Cassette BASIC in system ROM, with Cartridge BASIC for Advanced BASIC
| display      = Built-in video: Motorola 6845 CRT controller and IBM Video Gate Array; RGB direct drive, composite video and TV connector
| predecessor  = none — new product line
| sound        = Texas Instruments SN76496N (three tone channels and noise), 8253 timer, cassette audio
| successor    = none — line ended after one year
| os            = Cassette BASIC in ROM; Cartridge BASIC; PC DOS 2.1
| model        = 4860
| dimensions    = 97 × 354 × 290 mm (3.8 × 13.9 × 11.4 in)
| weight        = 3.71 kg with diskette drive; 2.61 kg without
| model        = 4860 Model 04, 4860 Model 67
}}
}}


The '''IBM PCjr''' (model '''4860''') was IBM's attempt at a low-cost home personal computer. It was announced on 1 November 1983, shipped from March 1984, and withdrawn a year later. It is broadly compatible with the [[IBM PC (5150)]] at the BIOS level, but enough is different — no DMA controller, a different memory map, a gate-array video subsystem with extra graphics modes, a sound chip instead of just a speaker, ROM cartridges instead of expansion slots, an infrared keyboard and a sidecar bus instead of the I/O channel — that it is a separate platform in practice.
The '''IBM PCjr''' (type 4860) is IBM's home computer of 1984. It uses the same 8088 processor as the [[IBM PC (5150)]] but is a different design: its video is built into the system board and uses main memory as the display buffer, it has a three-voice sound chip, ROM cartridge slots and an infrared keyboard, and it expands through side-mounted "sidecar" modules instead of PC expansion slots. IBM announced the PCjr in November 1983, shipped it from March 1984 and discontinued it in March 1985 after selling about 270,000.<ref name="nmah">National Museum of American History, [https://americanhistory.si.edu/collections/object/nmah_905913 "IBM PCjr Microcomputer"], object 2002.0090.01. Announcement and availability dates, prices, manufacture by Teledyne in Lewisburg, Tennessee, discontinuation and units sold.</ref>
 
== History ==
 
The PCjr was sold at US$669 with 64 KB of memory and US$1,269 with 128 KB and a diskette drive. Teledyne built it for IBM in Lewisburg, Tennessee.<ref name="nmah" /> Criticism of the machine centred on the original keyboard, which had small calculator-style keys.<ref name="nmah" /> IBM replaced it with a version that is electronically identical but has bigger keys with the legends on the key tops.<ref name="sim2">IBM, ''Personal Computer Family Service Information Manual'', SA38-0037-00, July 1989, chapter 2, "4860 PCjr", pp. 2-1 to 2-3, hosted as [[IBM Personal Computer Family Service Information Manual]]. Models 04 and 67, unit specifications, interface locations, the power supply, both keyboard versions, and the keyboard as a non-repairable FRU.</ref><ref name="brutman">Mike Brutman, [https://www.brutman.com/PCjr/pcjr_hardware.html "PCjr Hardware Notes"], 2000, updated 2010. Community reference. Graphics modes, cartridge address regions, the missing DMA controller and its effects, the diskette drive and fan, DOS 2.1, the COM port numbering, the replacement keyboard, and the short (33 W) and long (45 W) Power Board versions.</ref>


== Models ==
== Models ==
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<templatestyles src="Template:StyledTable/styles.css" />
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''IBM PCjr submodels'''
|+'''IBM PCjr models'''
! Submodel !! On-board RAM !! Diskette drive !! Modem
! Model !! Memory !! Diskette drive !! Launch price
|-
|-
| 4860-004 (Entry) || 64 KB || none || none
| 4860-04 || 64 KB || None || US$669
|-
|-
| 4860-067 (Enhanced) || 128 KB || 360 KB 5.25-inch || optional internal
| 4860-67 || 128 KB || 360 KB 5.25-inch || US$1,269
|}
|}
IBM's service manual names the two models 04 and 67.<ref name="sim2" /> The 128 KB machine has the 64KB Memory and Display Expansion card fitted.<ref name="nmah" /><ref name="trjr217">IBM, ''IBM PCjr Technical Reference'', revised edition, November 1983, "64K RAM", p. 2-17, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref>


== Architecture ==
== Architecture ==


An '''Intel 8088 at 4.77 MHz''', the same CPU clock as the 5150 and 5160. The machine has '''no DMA controller''', so all I/O including diskette transfer is done by the CPU in tight programmed-I/O loops.<ref name="brutman">Mike Brutman, [https://www.brutman.com/PCjr/pcjr_hardware.html IBM PCjr Hardware]. Long-running community reference for the machine. Source for the split supply description, the short and long Power Board variants rated at 33 W and 45 W, the roughly 400 mA of +5 V left for sidecars on an internally fully configured machine, the behaviour of overloaded sidecar configurations, the existence of power sidecars using the same external transformer, the absence of a DMA controller and its consequences, and the point that the system has no fan of its own while a diskette-equipped machine has a fan on the drive.</ref>
=== Processor and memory ===


=== Memory ===
The PCjr uses an Intel 8088 at 4.77 MHz.<ref name="nmah" /> Early boards have the 8088 in a socket; later boards have it soldered in, and some use an AMD D8088.<ref name="brutman" /> The PCjr has no DMA controller, so the processor handles diskette transfers itself.<ref name="brutman" /><ref name="trjr413">IBM, ''IBM PCjr Technical Reference'', November 1983, "Hardware Differences", pp. 4-12 to 4-13, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref>


The system board's 64 KB of RAM is built from '''eight 64K-by-1, 150 ns''' devices &mdash; Motorola MCM6665AL15, Texas Instruments TMS4164-15 or equivalent &mdash; and has '''no parity'''.<ref name="trjr">IBM, ''IBM PCjr Technical Reference'' &mdash; hosted on this wiki as [[:File:IBM PCjr Technical Reference.pdf]] and documented as [[IBM PCjr Technical Reference]]. "System Power Supply" and its Operating Characteristics, Over-Voltage/Over-Current Protection and Connector Specifications subsections for the two-stage 33 W supply, the transformer's short-circuit-protected extra-low ac output and its "Pin 1 - 17 Vac, Pin 2 - GND, Pin 3 - 17 Vac" connector specification, the internal Power Board that converts that ac to three dc levels, the +5 V / +12 V / &minus;6 V rails with their load currents and regulation tolerances, the 1.5 A minimum +5 V load required for &minus;6 V to be present, the input requirements, the power available at the I/O connector on a fully configured machine, and the input and output protection tables including the five-second reset time. "Hardware Differences" for cassette BASIC residing in system ROM and cartridge BASIC providing Advanced BASIC capability in ROM.</ref> It is mapped at the bottom of the 1 MB address space, and the same physical storage is aliased into the next 64 KB of address space.
The system board's 64 KB of RAM uses eight 64K × 1, 150 ns dynamic RAMs (Motorola MCM6665AL15, Texas Instruments TMS4164-15 or equivalent) and has no parity.<ref name="trjr217" /> It sits at the bottom of the address space. Without the 64KB Memory and Display Expansion, the same 64 KB also appears in the next 64 KB of address space. With the expansion fitted, the expansion card takes the odd addresses and the system board the even addresses across the bottom 128 KB. The 6845 and the gate array refresh the RAM and arbitrate between display and processor cycles.<ref name="trjr217" />


'''The video buffer is system RAM.''' The same memory serves both as the display buffer and as the 8088's main RAM: the processor can reach it at 00000 upwards, or through a 16 KB window starting at B8000, with a page register deciding which physical page a read or write lands on.<ref name="trjr" /> This is the root of the so-called "PCjr cliff" &mdash; software written to assume the 5150's fixed video addresses does not necessarily work here.
This RAM is both the display buffer and the 8088's main memory. The processor reaches it either directly from address 00000 or through a 16 KB window at B8000, with a page register choosing which part of RAM the window points at.<ref name="trjr247">IBM, ''IBM PCjr Technical Reference'', November 1983, "Video Color/Graphics Subsystem", pp. 2-44 to 2-50 and 2-64 to 2-65, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref> Brutman gives the practical cost: a 128 KB PCjr in a standard CGA mode leaves 112 KB for programs, and 96 KB in the 16-colour modes.<ref name="brutman" />


ROM is built from '''32K-by-8 modules''' (Mostek 38000, TMM23256P or equivalent), also without parity, at 250 ns access and 375 ns cycle time.<ref name="trjr" /> The BIOS, diagnostics and cassette BASIC occupy F0000 to FFFFF; two standard application cartridge areas sit at E0000 and E8000, with D8000 reserved for a further cartridge.<ref name="trjr" />
=== ROM and cartridges ===


=== Video ===
The system ROM is 64 KB at the top of the address space, built from 32K × 8 modules (Mostek MK38000, TMM23256P or equivalent) with no parity, 250 ns access and 375 ns cycle time. It holds the BIOS, the diagnostics and Cassette BASIC from F0000 to FFFFF. The two standard application cartridge areas are E0000 and E8000, and D0000 and D8000 are reserved for further cartridges.<ref name="trjr219">IBM, ''IBM PCjr Technical Reference'', November 1983, "ROM Subsystem", p. 2-19, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref> Each cartridge is wired for a fixed region, and two cartridges that use the same region usually lock the machine up. Inserting or removing a cartridge resets the machine.<ref name="brutman" />


The colour/graphics subsystem is implemented with '''a Motorola 6845 CRT controller and IBM's Video Gate Array (VGA, LSI5220)''', a CMOS gate array that generates the storage timing (RAS, CAS, WE), the direct-drive and composite-colour outputs and the status signals.<ref name="trjr" /> The gate array also holds a '''16-entry palette''' used in all alphanumeric modes, with any input maskable off for modes that do not support the full 16 colours.<ref name="trjr" />
Cassette BASIC is in the system ROM, and Cartridge BASIC supplies the Advanced BASIC features from ROM, so neither takes user memory.<ref name="trjr413" />


That palette and the shared-RAM buffer are what give the PCjr its two well-known extra modes beyond the standard [[IBM Color Graphics Adapter|CGA]] set &mdash; 320 &times; 200 in 16 colours and 640 &times; 200 in 4 colours. Tandy reused the same arrangement in the Tandy 1000, which is why software calls them "PCjr/Tandy" modes.
=== Video ===


The character generator is an MCM68A316E or equivalent, 350 ns static, pin-compatible with 2716 and 2732 EPROMs &mdash; which makes a custom font a realistic modification.<ref name="trjr" />
The colour/graphics subsystem uses a Motorola 6845 CRT controller and IBM's Video Gate Array (LSI5220), a CMOS gate array that generates the memory timing (RAS, CAS, WE), the direct-drive and composite-colour outputs and the status signals.<ref name="trjr247" /> The gate array has a 16-entry palette. The ROM character generator is an MCM68A316E or equivalent, 2 KB, 350 ns, pin-compatible with 2716 and 2732 EPROMs.<ref name="trjr247" /> Outputs are RGB direct drive, composite video and the connector for the optional IBM Connector for Television.<ref name="trjr247" />


Outputs are direct-drive RGB, composite video and the connector for the optional IBM PCjr TV Connector; all are right-angle-mounted connectors through the rear panel.<ref name="trjr" />
The PCjr has the CGA modes plus 320 × 200 in 16 colours and 640 × 200 in 4 colours, and a 160 × 200 16-colour mode.<ref name="brutman" /> IBM classes 80-column text, 640 × 200 4-colour and 320 × 200 16-colour as high-bandwidth modes that need the 64KB Memory and Display Expansion, so a machine without the expansion cannot use them.<ref name="trjr247" />


=== Sound ===
=== Sound ===


A Texas Instruments '''SN76496N''' complex sound generator: three channels of programmable frequency that can be mixed to form chords, plus a noise generator, each of the four independently attenuated. An '''8255''' selects which source reaches the audio output.<ref name="trjr" /> Output is available on the rear audio jack and, through the TV Connector, on the television's speaker.
An analogue multiplexer, controlled by bits PB5 and PB6 of the 8255, selects one of four sources: the Texas Instruments SN76496N complex sound generator, the 8253 timer, cassette audio and audio from the I/O channel. The SN76496N has three tone channels and a noise generator, each with its own attenuation. The output is 1 V peak to peak through a 4.8 kHz low-pass filter, and goes to the external-amplifier connector and the television connector; an external speaker needs its own amplifier.<ref name="trjr287">IBM, ''IBM PCjr Technical Reference'', November 1983, "Sound Subsystem", pp. 2-87 to 2-88, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref>


== Keyboard ==
== Keyboard ==


The PCjr keyboard is a '''low-profile, 62-key, detached keyboard with full-travel keys''', laid out as a standard typewriter keyboard with a function key and cursor-control keys added. The key tops are unmarked for their alternate functions; overlays carry the functional descriptions.<ref name="trjr" /> The first production keyboard used flat rubber keys and was widely criticised; IBM replaced it in 1984 with the full-travel version the technical reference describes.
IBM describes the cordless keyboard as a low-profile, 62-key detached keyboard with full-travel keys in a typewriter layout, plus a function key and cursor keys. On the original keyboard the keys are unmarked for their alternate functions and an overlay carries the descriptions.<ref name="trjr2101">IBM, ''IBM PCjr Technical Reference'', November 1983, "IBM PCjr Cordless Keyboard", pp. 2-101 to 2-105, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref> It runs on four AA batteries. An 80C48 microcontroller stays in a power-down standby mode until a key is pressed, then scans, sends the scan code and goes back to standby, so the keyboard has no on/off switch.<ref name="trjr2101" /><ref name="sim2" />


Inside is an '''80C48''' CMOS microcontroller. It normally sits in a standby power-down mode, wakes on a keypress, scans, transmits, and goes back to sleep &mdash; which is why the keyboard runs on '''four batteries''' and has no on/off switch.<ref name="trjr" /> It buffers up to 16 scan codes and performs key debounce.
The link to the system unit is infrared: 11-bit words with odd parity, biphase-encoded with a 440 µs bit cell, each logical 1 sent as a 40 kHz burst.<ref name="trjr2101" /> The optional keyboard cord carries the same data. Plugging it in disconnects the batteries and the infrared transmitter, and its −CABLE CONNECT signal disables the system unit's infrared receiver, so other infrared devices no longer interfere.<ref name="trjr2101" />


Two behaviours are specific to this keyboard:
Unlike other IBM PC keyboards, the PCjr keyboard has phantom-key detection. When three or more keys are pressed together in an invalid combination, it generates a hex 55 code and ignores all the keys pressed at that moment; BIOS ignores the code.<ref name="trjr2101" /> Scan codes match the PC and XT at BIOS level, and each break code is the make code plus hex 80.<ref name="trjr2101" />


* '''Phantom-key detection.''' Unlike every other keyboard in the IBM PC family, the PCjr's detects invalid combinations of three or more simultaneous keys, sends a hex 55 scan code to its own processor, and discards all keys pressed at that moment. BIOS ignores the resulting code.<ref name="trjr" /> Games that expect three-key combinations will not get them.
== Expansion ==
* '''The keyboard cord changes the electronics.''' Plugging the optional cord into the modular jack '''disconnects the batteries''' and '''disables the infrared transmitter'''; the &minus;CABLE CONNECTED signal disables the infrared receiver in the system unit as well, so other infrared devices in the room stop interfering.<ref name="trjr" />


Data reaches the system unit the same way over either link: serially encoded words with '''odd parity''', biphase-encoded by the 80C48 with a 440 &micro;s bit cell, over a simplex link.<ref name="trjr" /> Scan codes are compatible with the PC and XT at the BIOS level; every key is typematic and produces a make code and a break code, the break code being the make code plus hex 80.<ref name="trjr" />
The system board has internal connectors for the 64KB Memory and Display Expansion, the internal modem and the diskette-drive adapter.<ref name="sim2" /> Only one diskette drive is supported.<ref name="brutman" /> On the right-hand side is the I/O expansion bus for sidecars, which is not the PC's I/O channel, so PC and XT cards do not fit.<ref name="sim2" /><ref name="brutman" /> IBM's launch options include the Parallel Printer Attachment, which is a sidecar, and the internal modem, which fits inside.<ref name="brutman" /><ref name="hms">IBM, ''Personal Computer PCjr Hardware Maintenance and Service'', first edition, September 1983, hosted as [[:File:IBM PCjr Hardware Maintenance and Service (September 1983).pdf]]. Options list, p. 1-4; parts catalogue, pp. 6-7 to 6-13.</ref>


== Cartridge and Sidecar Expansion ==
With no internal modem, the external serial port is COM1 but uses the COM2 address and interrupt (2F8h, IRQ 3); with the modem fitted, the modem becomes COM1 and the serial port COM2.<ref name="brutman" />
 
Two '''ROM cartridge slots''' on the front of the system unit map into the reserved cartridge areas at E0000, E8000 and D8000.<ref name="trjr" /> Cartridge BASIC, which supplies the PCjr's Advanced BASIC capability, is one of these &mdash; and because it is in ROM it takes no user memory, unlike ''BASICA'' on a 5150.<ref name="trjr" />
 
On the right-hand side of the system unit, behind a removable cover, is a proprietary '''sidecar bus'''. Sidecars latch on and extend the machine horizontally, and can be chained. IBM's own sidecars include the Memory Expansion Attachment, the Parallel Printer Attachment, the Power Expansion Attachment, the internal modem and the Cluster Attachment. The sidecar bus is '''not''' the 5150's I/O channel and PC/XT cards do not fit it.
 
The power budget is the practical limit on sidecars rather than the bus: see below.


== Power ==
== Power ==


'''The external brick is a transformer and nothing else.''' IBM describes the supply as a "33 Watt, three voltage-level, two-stage supply": the first stage is a power transformer providing a single, short-circuit-protected, extra-low '''ac''' output, and the second stage is a printed-circuit board '''inside the system unit''' &mdash; the Power Board &mdash; which converts that ac into the machine's three dc levels.<ref name="trjr" /> IBM's connector specification for the transformer output reads "Pin 1 &ndash; 17 Vac, Pin 2 &ndash; GND, Pin 3 &ndash; 17 Vac".<ref name="trjr" />
IBM's supply is a 33 W, three-voltage, two-stage design. The first stage is an external transformer with a single-fuse-protected, extra-low-voltage AC output. The second stage is the Power Board, a card mounted vertically in the system board, which converts that AC into three DC levels. The power switch is on the Power Board at the rear.<ref name="trjr2135">IBM, ''IBM PCjr Technical Reference'', November 1983, "System Power Supply", pp. 2-135 to 2-137, hosted as [[:File:IBM PCjr Technical Reference.pdf]].</ref> Brutman reports a short 33 W Power Board and a later long 45 W version.<ref name="brutman" />
 
There is therefore '''no rectifier, no regulator and no high-voltage bulk capacitor inside the brick'''. Rectification, regulation and all of the over-current and over-voltage protection are on the Power Board.


{| class="wikitable styled-table" style="width:85%; text-align:center;"
{| class="wikitable styled-table" style="width:85%; text-align:center;"
|+'''IBM PCjr &mdash; dc outputs (IBM figures)'''
|+'''IBM PCjr DC outputs (Technical Reference p. 2-136)'''
! Rail !! Minimum load !! Maximum load !! Regulation tolerance
! Rail !! Minimum load !! Maximum load !! Regulation
|-
|-
| +5 Vdc || 1.5 A || 3.6 A || &plusmn;5%
| +5 V || 1.5 A || 3.6 A || ±5 %
|-
|-
| +12 Vdc || 0.04 A || 1.2 A || &plusmn;5%
| +12 V || 0.04 A || 1.2 A || ±5 %
|-
|-
| &minus;6 Vdc || 0.0 A || 0.025 A || &plusmn;16%
| −6 V || 0.0 A || 0.025 A || ±16 %
|}
|}


'''There must be a minimum 1.5 A load on +5 V for the &minus;6 V rail to be present at all.'''<ref name="trjr" /> A lightly loaded machine that measures no &minus;6 V is behaving normally.
The −6 V output is present only when +5 V carries at least 1.5 A.<ref name="trjr2135" /> The input is 120 V AC nominal (104 to 127 V), 60 Hz, 0.65 A maximum at 104 V.<ref name="trjr2135" />


Protection is split between the two stages:<ref name="trjr" />
{| class="wikitable styled-table" style="width:85%; text-align:center;"
 
|+'''IBM PCjr protection (Technical Reference p. 2-137)'''
{| class="wikitable styled-table" style="width:85%;"
! Where !! Protection !! Rating or trip point
|+'''IBM PCjr &mdash; protection'''
! Where !! Type !! Trip
|-
|-
| Transformer input || Non-resettable fuse, plus thermal / over-current || Not user-resettable
| Transformer input || Non-resettable fuse; thermal / over-current || 5 A slow-blow
|-
|-
| Power Board, +5 V over-voltage || Fuse || 6.3 &plusmn; 0.7 Vdc
| Power Board, +5 V || Over-voltage, by fuse F1 || 6.3 ± 0.7 V
|-
|-
| Power Board, +12 V over-voltage || Fuse || 14.4 &plusmn; 1.4 Vdc
| Power Board, +12 V || Over-voltage, by fuse F1 || 14.4 ± 1.4 V
|-
|-
| Power Board, +5 V over-current || Resettable || about 3.9 A
| Power Board, +5 V || Over-current, resettable || 3.9 ± 0.25 A
|-
|-
| Power Board, +12 V over-current || Resettable || 2.2 &plusmn; 0.9 A
| Power Board, +12 V || Over-current, resettable || 2.2 ± 0.9 A
|}
|}


The over-current protection resets by '''removing the fault condition for at least five seconds and then re-applying power'''.<ref name="trjr" /> Over-voltage protection is by fuse, so a Power Board that has been through an over-voltage event may need more than the fault cleared.
To reset the over-current protection, remove the fault and keep the power off for at least 5 seconds before switching on again.<ref name="trjr2135" />
 
A machine fitted with all the internal features has 400 mA of +5 V and nothing on +12 V or −6 V left for the I/O connector.<ref name="trjr2135" /> Brutman reports that heavy or numerous sidecars stretch the supply and cause erratic operation, and that IBM's power sidecar uses the same external transformer.<ref name="brutman" /> The system unit has no fan; the only fan is on the diskette drive.<ref name="brutman" />
 
== Documentation ==


A fully configured machine &mdash; modem, memory expansion and diskette drive fitted, but no sidecars &mdash; leaves roughly '''400 mA of +5 V''' for sidecars.<ref name="brutman" /> That is not much, and a heavy sidecar load produces flaky operation rather than a clean failure. IBM's Power Expansion Attachment adds capacity using a second transformer of the same type.
* [[IBM PCjr Technical Reference]]: the November 1983 hardware and BIOS reference.
* [[:File:IBM PCjr Hardware Maintenance and Service (September 1983).pdf|IBM PCjr Hardware Maintenance and Service]] (September 1983): advanced POST, the Problem Isolation Charts, removal and replacement procedures and the parts catalogue.<ref name="hms" />
* [[IBM Personal Computer Family Service Information Manual]] (SA38-0037-00, 1989): chapter 2 covers the PCjr.<ref name="sim2" />
* [[IBM PCjr BASIC Reference (June 1983)]].


== General Maintenance ==
== General maintenance ==


Cleaning, keyboard and infrared care, sidecar handling, diskette drive care and what to check on the Power Board are in [[IBM PCjr Maintenance Guide]].
Cleaning, keyboard batteries, the diskette drive and Power Board checks are in [[IBM PCjr Maintenance Guide]].


== Troubleshooting ==
== Troubleshooting ==


POST beep codes, the PCjr error code set, keystroke loss during diskette access, sidecar power problems and infrared interference are in [[IBM PCjr Troubleshooting Guide]].
IBM's advanced POST, the POST error table, the Power Board voltage checks and the customer error codes are in [[IBM PCjr Troubleshooting Guide]].


== Capacitor Replacement ==
== Capacitor replacement ==


Tantalum failure on the system board and electrolytic ageing on the Power Board are in [[IBM PCjr Capacitor Replacement Guide]].
What IBM documents about the PCjr's capacitors, and what it does not, is in [[IBM PCjr Capacitor Replacement Guide]].


== Gallery ==
== Gallery ==
<gallery mode="packed" heights="220">
<gallery mode="packed" heights="220">
File:IBM PCjr with display.jpg|IBM PCjr with the IBM 4863 display
File:IBM PCjr with display.jpg|IBM PCjr with the IBM PCjr Color Display
File:IBM PCjr keyboard.png|The 62-key cordless keyboard
File:IBM PCjr keyboard.png|The 62-key cordless keyboard
File:IBM PCjr motherboard.jpg|IBM PCjr system board
File:IBM PCjr motherboard.jpg|IBM PCjr system board
File:IBM PCjr expanded with sidecars.jpg|IBM PCjr with several sidecars attached
File:IBM PCjr expanded with sidecars.jpg|IBM PCjr with several sidecars attached
File:IBM PCjr internal teardown.jpg|Inside the system unit; the Power Board is the second stage of the supply
File:IBM PCjr internal teardown.jpg|PCjr system unit: diskette drive and the two cartridge slots
</gallery>
</gallery>


Line 146: Line 149:
<references />
<references />


== Related Pages ==
== Related pages ==


* [[IBM PCjr Maintenance Guide]] &middot; [[IBM PCjr Troubleshooting Guide]] &middot; [[IBM PCjr Capacitor Replacement Guide]]
* [[IBM PCjr Maintenance Guide]]
* [[IBM PC (5150)]] &mdash; the business sibling
* [[IBM PCjr Troubleshooting Guide]]
* [[IBM PCjr Capacitor Replacement Guide]]
* [[IBM PC (5150)]]
* [[IBM Cassette BASIC]]
* [[IBM Cassette BASIC]]
* [[IBM PCjr Technical Reference]] &mdash; IBM's own reference, hosted here
* '''IBM documentation hosted here:''' [[IBM PCjr Technical Reference]] &middot; [[IBM PCjr BASIC Reference (June 1983)]]


{{Navbox-IBMComputers|state=collapsed}}
{{Navbox-IBMComputers|state=collapsed}}


[[Category:IBM]]
[[Category:IBM]]

Latest revision as of 20:39, 30 September 2026

IBM PCjr
IBM PCjr (4860) with the IBM PCjr Color Display (4863) and the 62-key cordless keyboard
Specifications
ManufacturerIBM (built by Teledyne in Lewisburg, Tennessee)
TypeHome personal computer
ReleasedMarch 1984 (announced November 1983)
DiscontinuedMarch 1985
Intro priceUS$669 (64 KB); US$1,269 (128 KB with diskette drive)
Units soldAbout 270,000
CPUIntel 8088 at 4.77 MHz
Memory64 KB on the system board; 128 KB with the 64KB Memory and Display Expansion; IBM lists expansion to 512 KB
StorageOptional 5.25-inch 360 KB diskette drive (one only); cassette interface; two ROM cartridge slots
DisplayBuilt-in video: Motorola 6845 CRT controller and IBM Video Gate Array; RGB direct drive, composite video and TV connector
SoundTexas Instruments SN76496N (three tone channels and noise), 8253 timer, cassette audio
Dimensions97 × 354 × 290 mm (3.8 × 13.9 × 11.4 in)
Weight3.71 kg with diskette drive; 2.61 kg without
OS / FirmwareCassette BASIC in ROM; Cartridge BASIC; PC DOS 2.1
Model no.4860 Model 04, 4860 Model 67

The IBM PCjr (type 4860) is IBM's home computer of 1984. It uses the same 8088 processor as the IBM PC (5150) but is a different design: its video is built into the system board and uses main memory as the display buffer, it has a three-voice sound chip, ROM cartridge slots and an infrared keyboard, and it expands through side-mounted "sidecar" modules instead of PC expansion slots. IBM announced the PCjr in November 1983, shipped it from March 1984 and discontinued it in March 1985 after selling about 270,000.[1]

History

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The PCjr was sold at US$669 with 64 KB of memory and US$1,269 with 128 KB and a diskette drive. Teledyne built it for IBM in Lewisburg, Tennessee.[1] Criticism of the machine centred on the original keyboard, which had small calculator-style keys.[1] IBM replaced it with a version that is electronically identical but has bigger keys with the legends on the key tops.[2][3]

Models

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IBM PCjr models
Model Memory Diskette drive Launch price
4860-04 64 KB None US$669
4860-67 128 KB 360 KB 5.25-inch US$1,269

IBM's service manual names the two models 04 and 67.[2] The 128 KB machine has the 64KB Memory and Display Expansion card fitted.[1][4]

Architecture

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Processor and memory

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The PCjr uses an Intel 8088 at 4.77 MHz.[1] Early boards have the 8088 in a socket; later boards have it soldered in, and some use an AMD D8088.[3] The PCjr has no DMA controller, so the processor handles diskette transfers itself.[3][5]

The system board's 64 KB of RAM uses eight 64K × 1, 150 ns dynamic RAMs (Motorola MCM6665AL15, Texas Instruments TMS4164-15 or equivalent) and has no parity.[4] It sits at the bottom of the address space. Without the 64KB Memory and Display Expansion, the same 64 KB also appears in the next 64 KB of address space. With the expansion fitted, the expansion card takes the odd addresses and the system board the even addresses across the bottom 128 KB. The 6845 and the gate array refresh the RAM and arbitrate between display and processor cycles.[4]

This RAM is both the display buffer and the 8088's main memory. The processor reaches it either directly from address 00000 or through a 16 KB window at B8000, with a page register choosing which part of RAM the window points at.[6] Brutman gives the practical cost: a 128 KB PCjr in a standard CGA mode leaves 112 KB for programs, and 96 KB in the 16-colour modes.[3]

ROM and cartridges

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The system ROM is 64 KB at the top of the address space, built from 32K × 8 modules (Mostek MK38000, TMM23256P or equivalent) with no parity, 250 ns access and 375 ns cycle time. It holds the BIOS, the diagnostics and Cassette BASIC from F0000 to FFFFF. The two standard application cartridge areas are E0000 and E8000, and D0000 and D8000 are reserved for further cartridges.[7] Each cartridge is wired for a fixed region, and two cartridges that use the same region usually lock the machine up. Inserting or removing a cartridge resets the machine.[3]

Cassette BASIC is in the system ROM, and Cartridge BASIC supplies the Advanced BASIC features from ROM, so neither takes user memory.[5]

Video

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The colour/graphics subsystem uses a Motorola 6845 CRT controller and IBM's Video Gate Array (LSI5220), a CMOS gate array that generates the memory timing (RAS, CAS, WE), the direct-drive and composite-colour outputs and the status signals.[6] The gate array has a 16-entry palette. The ROM character generator is an MCM68A316E or equivalent, 2 KB, 350 ns, pin-compatible with 2716 and 2732 EPROMs.[6] Outputs are RGB direct drive, composite video and the connector for the optional IBM Connector for Television.[6]

The PCjr has the CGA modes plus 320 × 200 in 16 colours and 640 × 200 in 4 colours, and a 160 × 200 16-colour mode.[3] IBM classes 80-column text, 640 × 200 4-colour and 320 × 200 16-colour as high-bandwidth modes that need the 64KB Memory and Display Expansion, so a machine without the expansion cannot use them.[6]

Sound

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An analogue multiplexer, controlled by bits PB5 and PB6 of the 8255, selects one of four sources: the Texas Instruments SN76496N complex sound generator, the 8253 timer, cassette audio and audio from the I/O channel. The SN76496N has three tone channels and a noise generator, each with its own attenuation. The output is 1 V peak to peak through a 4.8 kHz low-pass filter, and goes to the external-amplifier connector and the television connector; an external speaker needs its own amplifier.[8]

Keyboard

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IBM describes the cordless keyboard as a low-profile, 62-key detached keyboard with full-travel keys in a typewriter layout, plus a function key and cursor keys. On the original keyboard the keys are unmarked for their alternate functions and an overlay carries the descriptions.[9] It runs on four AA batteries. An 80C48 microcontroller stays in a power-down standby mode until a key is pressed, then scans, sends the scan code and goes back to standby, so the keyboard has no on/off switch.[9][2]

The link to the system unit is infrared: 11-bit words with odd parity, biphase-encoded with a 440 µs bit cell, each logical 1 sent as a 40 kHz burst.[9] The optional keyboard cord carries the same data. Plugging it in disconnects the batteries and the infrared transmitter, and its −CABLE CONNECT signal disables the system unit's infrared receiver, so other infrared devices no longer interfere.[9]

Unlike other IBM PC keyboards, the PCjr keyboard has phantom-key detection. When three or more keys are pressed together in an invalid combination, it generates a hex 55 code and ignores all the keys pressed at that moment; BIOS ignores the code.[9] Scan codes match the PC and XT at BIOS level, and each break code is the make code plus hex 80.[9]

Expansion

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The system board has internal connectors for the 64KB Memory and Display Expansion, the internal modem and the diskette-drive adapter.[2] Only one diskette drive is supported.[3] On the right-hand side is the I/O expansion bus for sidecars, which is not the PC's I/O channel, so PC and XT cards do not fit.[2][3] IBM's launch options include the Parallel Printer Attachment, which is a sidecar, and the internal modem, which fits inside.[3][10]

With no internal modem, the external serial port is COM1 but uses the COM2 address and interrupt (2F8h, IRQ 3); with the modem fitted, the modem becomes COM1 and the serial port COM2.[3]

Power

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IBM's supply is a 33 W, three-voltage, two-stage design. The first stage is an external transformer with a single-fuse-protected, extra-low-voltage AC output. The second stage is the Power Board, a card mounted vertically in the system board, which converts that AC into three DC levels. The power switch is on the Power Board at the rear.[11] Brutman reports a short 33 W Power Board and a later long 45 W version.[3]

IBM PCjr DC outputs (Technical Reference p. 2-136)
Rail Minimum load Maximum load Regulation
+5 V 1.5 A 3.6 A ±5 %
+12 V 0.04 A 1.2 A ±5 %
−6 V 0.0 A 0.025 A ±16 %

The −6 V output is present only when +5 V carries at least 1.5 A.[11] The input is 120 V AC nominal (104 to 127 V), 60 Hz, 0.65 A maximum at 104 V.[11]

IBM PCjr protection (Technical Reference p. 2-137)
Where Protection Rating or trip point
Transformer input Non-resettable fuse; thermal / over-current 5 A slow-blow
Power Board, +5 V Over-voltage, by fuse F1 6.3 ± 0.7 V
Power Board, +12 V Over-voltage, by fuse F1 14.4 ± 1.4 V
Power Board, +5 V Over-current, resettable 3.9 ± 0.25 A
Power Board, +12 V Over-current, resettable 2.2 ± 0.9 A

To reset the over-current protection, remove the fault and keep the power off for at least 5 seconds before switching on again.[11]

A machine fitted with all the internal features has 400 mA of +5 V and nothing on +12 V or −6 V left for the I/O connector.[11] Brutman reports that heavy or numerous sidecars stretch the supply and cause erratic operation, and that IBM's power sidecar uses the same external transformer.[3] The system unit has no fan; the only fan is on the diskette drive.[3]

Documentation

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General maintenance

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Cleaning, keyboard batteries, the diskette drive and Power Board checks are in IBM PCjr Maintenance Guide.

Troubleshooting

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IBM's advanced POST, the POST error table, the Power Board voltage checks and the customer error codes are in IBM PCjr Troubleshooting Guide.

Capacitor replacement

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What IBM documents about the PCjr's capacitors, and what it does not, is in IBM PCjr Capacitor Replacement Guide.

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  • IBM PCjr with the IBM PCjr Color Display
    IBM PCjr with the IBM PCjr Color Display
  • The 62-key cordless keyboard
    The 62-key cordless keyboard
  • IBM PCjr system board
    IBM PCjr system board
  • IBM PCjr with several sidecars attached
    IBM PCjr with several sidecars attached
  • PCjr system unit: diskette drive and the two cartridge slots
    PCjr system unit: diskette drive and the two cartridge slots

References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 National Museum of American History, "IBM PCjr Microcomputer", object 2002.0090.01. Announcement and availability dates, prices, manufacture by Teledyne in Lewisburg, Tennessee, discontinuation and units sold.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 IBM, Personal Computer Family Service Information Manual, SA38-0037-00, July 1989, chapter 2, "4860 PCjr", pp. 2-1 to 2-3, hosted as IBM Personal Computer Family Service Information Manual. Models 04 and 67, unit specifications, interface locations, the power supply, both keyboard versions, and the keyboard as a non-repairable FRU.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 Mike Brutman, "PCjr Hardware Notes", 2000, updated 2010. Community reference. Graphics modes, cartridge address regions, the missing DMA controller and its effects, the diskette drive and fan, DOS 2.1, the COM port numbering, the replacement keyboard, and the short (33 W) and long (45 W) Power Board versions.
  4. ↑ 4.0 4.1 4.2 IBM, IBM PCjr Technical Reference, revised edition, November 1983, "64K RAM", p. 2-17, hosted as File:IBM PCjr Technical Reference.pdf.
  5. ↑ 5.0 5.1 IBM, IBM PCjr Technical Reference, November 1983, "Hardware Differences", pp. 4-12 to 4-13, hosted as File:IBM PCjr Technical Reference.pdf.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 IBM, IBM PCjr Technical Reference, November 1983, "Video Color/Graphics Subsystem", pp. 2-44 to 2-50 and 2-64 to 2-65, hosted as File:IBM PCjr Technical Reference.pdf.
  7. ↑ IBM, IBM PCjr Technical Reference, November 1983, "ROM Subsystem", p. 2-19, hosted as File:IBM PCjr Technical Reference.pdf.
  8. ↑ IBM, IBM PCjr Technical Reference, November 1983, "Sound Subsystem", pp. 2-87 to 2-88, hosted as File:IBM PCjr Technical Reference.pdf.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 IBM, IBM PCjr Technical Reference, November 1983, "IBM PCjr Cordless Keyboard", pp. 2-101 to 2-105, hosted as File:IBM PCjr Technical Reference.pdf.
  10. ↑ 10.0 10.1 IBM, Personal Computer PCjr Hardware Maintenance and Service, first edition, September 1983, hosted as File:IBM PCjr Hardware Maintenance and Service (September 1983).pdf. Options list, p. 1-4; parts catalogue, pp. 6-7 to 6-13.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 IBM, IBM PCjr Technical Reference, November 1983, "System Power Supply", pp. 2-135 to 2-137, hosted as File:IBM PCjr Technical Reference.pdf.
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