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{{Infobox computer
{{Infobox computer
| name          = IBM PCjr
| name          = IBM PCjr
| image        = [[File:IBM PCjr with display.jpg|260px]]
| logo          = [[File:IBM PCjr wordmark.svg|150px]]
| caption      = IBM PCjr (4860) with its companion IBM 4863 display
| image        = [[File:IBM PCjr with display.jpg|260px|frameless]]
| developer    = IBM Entry Systems Division, Boca Raton
| caption      = IBM PCjr (4860) with the IBM PCjr Color Display (4863) and the 62-key cordless keyboard
| manufacturer  = IBM
| manufacturer  = IBM (built by Teledyne in Lewisburg, Tennessee)
| type          = Home / consumer personal computer
| type          = Home personal computer
| release date  = March 19, 1984 (announced November 1, 1983)
| release date  = March 1984 (announced November 1983)
| discontinued  = March 19, 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-board (4860-004) or 128 KB on-board (4860-067); expandable to 512 KB and beyond via sidecar memory modules
| units_sold    = About 270,000
| storage      = One 5.25" 360 KB half-height floppy drive (4860-067 only); cassette port; two ROM cartridge ports
| cpu          = Intel 8088 at 4.77 MHz
| display      = Built-in CGA-compatible video with extra 16-colour 320×200 and 4-colour 640×200 modes; RGB DIN, composite video, audio out, RF modulator via IBM PCjr 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 '''SN76496''' 3-voice tone generator with one noise channel, plus PC speaker
| storage      = Optional 5.25-inch 360 KB diskette drive (one only); cassette interface; two ROM cartridge slots
| dimensions    = ~14" W × 11.4" D × 3.8" H (35.6 × 29 × 9.5 cm)
| display      = Built-in video: Motorola 6845 CRT controller and IBM Video Gate Array; RGB direct drive, composite video and TV connector
| weight        = ~11.7 lb (5.3 kg) system unit, without keyboard or monitor
| sound        = Texas Instruments SN76496N (three tone channels and noise), 8253 timer, cassette audio
| os            = IBM PC DOS 2.1 (introduced specifically for the PCjr); IBM Cassette BASIC J1.0 in ROM
| os            = Cassette BASIC in ROM; Cartridge BASIC; PC DOS 2.1
| predecessor  = None — new product line
| dimensions    = 97 × 354 × 290 mm (3.8 × 13.9 × 11.4 in)
| successor    = None — line ended after one year
| weight        = 3.71 kg with diskette drive; 2.61 kg without
| model        = 4860
| model        = 4860 Model 04, 4860 Model 67
}}
}}


The '''IBM PCjr''' (model '''4860''') was IBM's first attempt at a low-cost home personal computer. It was announced on November 1, 1983 and shipped from March 19, 1984. After poor sales and persistent criticism, IBM withdrew the line on March 19, 1985 — exactly one year after launch. Approximately 250,000 PCjrs were produced before the line ended.
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>


The PCjr is broadly compatible with the IBM PC at the BIOS level, but several hardware differences (no DMA controller, different memory map, '''SN76496''' sound chip in place of the speaker, ROM cartridges instead of expansion slots, infrared wireless keyboard, sidecar bus instead of ISA) make it a separate platform in practice. Software designed for the PCjr's "Tandy/PCjr" video and sound modes is also fully compatible with the Tandy 1000.
== 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 ==


The PCjr was offered as two off-the-shelf configurations.
<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 !! Floppy !! Modem !! Original US list price
! Model !! Memory !! Diskette drive !! Launch price
|-
|-
| 4860-004 (Entry) || 64 KB || none || none || US $669
| 4860-04 || 64 KB || None || US$669
|-
|-
| 4860-067 (Enhanced) || 128 KB || 360 KB 5.25" || Optional internal 300 baud modem || US $1269
| 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 ==


The PCjr is based on an '''Intel 8088''' running at '''4.77 MHz''', the same CPU clock as the 5150 and 5160. It does '''not''' have an Intel 8237 DMA controller; all I/O including floppy operations is done by PIO. As a result, keystrokes can be lost while the floppy drive is being accessed (the keyboard receiver also drops scan codes during disk operation, which is a known PCjr limitation).
=== Processor and memory ===


Key chips on the motherboard include:
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>
* '''Intel 8088''' &mdash; CPU (no second-source NEC V20 was officially endorsed for the PCjr).
* '''Intel 8259A''' &mdash; programmable interrupt controller.
* '''Intel 8253''' &mdash; programmable interval timer.
* '''Intel 8255A''' &mdash; programmable peripheral interface.
* '''Motorola MC6845''' &mdash; CRT controller, embedded on the motherboard rather than on a video card.
* '''Texas Instruments SN76496''' &mdash; sound chip with three square-wave tone generators plus a noise channel. The same chip used on the Tandy 1000, which is why the chip and its register layout are commonly called "PCjr/Tandy sound."


The PCjr motherboard does not implement memory parity, in order to save chip count and cost. Bit errors in DRAM are not reported.
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" />


== Memory ==
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" />


The 4860-004 has '''64 KB''' of on-board DRAM. The 4860-067 has '''128 KB''' of on-board DRAM. Additional RAM is provided via sidecar memory expansion modules:
=== ROM and cartridges ===
* '''IBM PCjr Memory Expansion Attachment''' &mdash; 128 KB sidecar (and its variants), commonly stacked to reach 384 KB or 512 KB.
* Third-party memory sidecars from Tecmar (Captain Jr.), Microsoft (Booster), Racore (PC Drive) and others extended the PCjr to '''640 KB''' and beyond.


A known software limitation (the "PCjr cliff") is that some PC-compatible software written before late 1984 fails on the PCjr when it tries to use video memory at A0000h, because the PCjr video memory is mapped at the top of system memory and shadows the area expected by some software.
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" />


== Video ==
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" />


Video is built into the motherboard, based on the MC6845 CRTC. The PCjr offers all of the standard CGA modes (40-column text, 80-column text, 320×200×4 colour graphics, 640×200×2 colour graphics) and two additional modes:
=== Video ===
* '''320 × 200 × 16''' colours.
* '''640 × 200 × 4''' colours.


These two extra modes are commonly referred to as "PCjr/Tandy graphics" because Tandy reused them in the Tandy 1000. Video output is available as TTL RGB (DIN-6), composite video, and via the optional IBM PCjr TV Connector as RF for a domestic TV. The IBM 4863 Display is the matched CRT monitor sold for the PCjr.
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" />


== Sound ==
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 is produced by the Texas Instruments '''SN76496'''. The chip provides three square-wave tone channels (with programmable frequency and 16-step amplitude) and one noise channel. Output is mixed with the PC speaker on the motherboard and exposed on a 1/8" mono audio jack on the rear. Sound from the SN76496 is output to either an external amplifier or, via the TV Connector, to the TV speaker.
=== Sound ===
 
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 62-key infrared wireless unit. The first revision is the famously criticised "'''chiclet'''" keyboard with small flat blank rubber keycaps. IBM replaced it in mid-1984 with a revised keyboard that has full-travel keycaps with printed legends and a similar key layout (still 62 keys).
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" />


[[File:IBM PCjr keyboard.png|center|thumb|480px|The IBM PCjr 62-key infrared wireless keyboard (revised typewriter version).]]
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" />


The keyboard communicates with the system unit via an infrared (IR) link. The IR receiver is at the front of the system unit. Strong sunlight, certain fluorescent lighting and other IR sources can interfere with reception; for reliable typing, IBM also supplied an optional cable that connects the keyboard directly to the system unit.
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" />


The keyboard can be operated in '''Fn-key''' mode (where a secondary legend printed in green/blue on the front face of each key is accessed via the Fn modifier) or normally. Plastic keytop overlays were supplied for several software products to relabel the keys.
== Expansion ==


== Cartridge and Sidecar Expansion ==
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>
 
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" />
 
== Power ==
 
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" />
 
{| class="wikitable styled-table" style="width:85%; text-align:center;"
|+'''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 PCjr exposes two '''ROM cartridge ports''' on the front of the system unit (below the diskette drive). Each cartridge contains up to 64 KB of ROM and is memory-mapped at fixed system locations. Cartridge ROM may also substitute for the system BIOS or BASIC, via chip selects /CS6 and /CS7 routed through the connector.
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" />


On the right-hand side of the system unit, behind a removable cover, is a proprietary '''sidecar bus'''. Sidecars latch onto the side of the system unit and extend it horizontally. Multiple sidecars can be chained. IBM-sold sidecars include the Memory Expansion Attachment (128 KB), the Parallel Printer Attachment, the PCjr Power Expansion Attachment (a second power supply for sidecars), the IBM PCjr Internal Modem (300 baud), and the PCjr Cluster Attachment.
{| class="wikitable styled-table" style="width:85%; text-align:center;"
|+'''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
|}


[[File:IBM PCjr expanded with sidecars.jpg|center|thumb|480px|An IBM PCjr fitted with several sidecar expansion modules along the right side. (Image: Wikimedia Commons)]]
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" />


The sidecar bus is not the same as the 8-bit ISA bus and is not electrically compatible with PC/XT cards.
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" />


== Power ==
== Documentation ==


The PCjr uses an external '''AC power brick''' that connects to the rear of the system unit. The brick supplies regulated DC to the motherboard. Sidecars that require additional current can be powered through the optional IBM Power Expansion Attachment.
* [[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 ==


Battery care (the PCjr does not have a CMOS battery), keyboard IR receiver care, sidecar latching and motherboard inspection are documented 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 limited PCjr error code set, keystroke loss during floppy access, sidecar power problems and the well-known IR keyboard interference issues are documented 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 capacitor failure on the PCjr motherboard and AC adapter is documented 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|62-key infrared wireless keyboard
File:IBM PCjr keyboard.png|The 62-key cordless keyboard
File:IBM PCjr motherboard.jpg|IBM PCjr motherboard
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|IBM PCjr internal teardown
File:IBM PCjr internal teardown.jpg|PCjr system unit: diskette drive and the two cartridge slots
</gallery>
</gallery>


== Related Pages ==
== References ==
 
<references />
 
== Related pages ==


* [[IBM PCjr Maintenance Guide]]
* [[IBM PCjr Maintenance Guide]]
* [[IBM PCjr Troubleshooting Guide]]
* [[IBM PCjr Troubleshooting Guide]]
* [[IBM PCjr Capacitor Replacement Guide]]
* [[IBM PCjr Capacitor Replacement Guide]]
* [[IBM PC (5150)]] &mdash; sibling product (business-oriented PC)
* [[IBM PC (5150)]]
 
* [[IBM Cassette BASIC]]
== References ==
 
* [https://en.wikipedia.org/wiki/IBM_PCjr IBM PCjr &mdash; Wikipedia]. Source for release/withdrawal dates, model prices, sales figures and chronology.
* [https://www.brutman.com/PCjr/ Mike Brutman's PCjr resource]. Reference for keyboard handling, hardware specifics and software notes.
* [https://minuszerodegrees.net/4860/doco/4860_documentation.htm IBM 4860 (PCjr) &mdash; Documentation pointers], minuszerodegrees.net.
* IBM, ''IBM PCjr Technical Reference''. Source for SN76496 register layout, sidecar bus, cartridge interface chip-select assignments (/CS2&ndash;/CS7), and motherboard chip layout.
* [http://nerdlypleasures.blogspot.com/2015/10/the-journey-of-pcjrtandy-sound-chip.html The Journey of the PCjr/Tandy Sound Chip]. Reference for the SN76496 and its reuse on the Tandy 1000.


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

[edit | edit source]
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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