Editing Sinclair ZX81

{{Infobox computer
| name         = Sinclair ZX81
| image        = [[File:ZX81.jpg|250px]]
| caption      = Sinclair ZX81
| manufacturer = Sinclair Research Ltd
| type         = Home computer
| release date = March 1981
| discontinued = 1984
| cpu          = Z80A class (Z80A or NEC D780C-1) @ 3.25 MHz
| memory       = 1 KB static RAM, expandable externally
| storage      = Cassette, through 3.5 mm EAR and MIC jacks
| display      = Monochrome, through a UM1233 UHF modulator on channel 36; 32×24 characters
| sound        = None
| os           = 8 KB Sinclair BASIC in ROM
| predecessor  = [[Sinclair ZX80]]
| successor    = [[Sinclair ZX Spectrum]]
| model        = ZX81
}}
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The '''Sinclair ZX81''' is an 8-bit home computer launched by Sinclair Research in March 1981, sold both as a kit and ready-built. It was sold in very large numbers, but its lasting technical significance is that '''Sinclair replaced about twenty 74LS packages from the [[Sinclair ZX80|ZX80]] with a single custom chip''' — the uncommitted logic array that does the video, the keyboard scan, the tape interface and the chip selects.

'''Everything on this page is from Sinclair's own documentation''' — the ''ZX81 Service and Assembly Manual'' and circuit diagram SRC 049, both hosted here — or from a named secondary source.<ref name="zx81man">Sinclair Research Ltd, ''ZX81 Service and Assembly Manual'', section 6 (fault finding), section 8 (component lists) and section 9 (power supply specification). Hosted on this wiki as [[:File:Sinclair ZX81 Service and Assembly Manual.pdf]]. Source for the component list, the capacitor list, the supply specification and Sinclair's own first-line fault checks.</ref><ref name="zx81sch">''ZX81 Circuit Diagram'', drawing SRC 049, Sinclair Research Ltd. Hosted on this wiki as [[:File:Sinclair ZX81 circuit diagram.png]]. Source for pin numbers, designators and circuit positions.</ref><ref name="tv2">"Servicing Sinclair Computers, Part 2", ''Television'' magazine, 1986. Trade servicing article devoted to the ZX81, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: on-load supply figures, board issues, the ULA clock derivation and the modulator drive level.</ref>

== Architecture ==

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''The whole machine, from Sinclair's component list'''<ref name="zx81man" /><ref name="zx81sch" />
! Ref !! Part !! Function
|-
| '''IC1''' || Sinclair Logic IC (ULA), 40-pin || Video, keyboard scan, tape I/O and chip selects. Called "Sinclair Computer Logic" on the circuit diagram
|-
| '''IC2''' || 2364, 24-pin || '''8 KB''' mask ROM — Sinclair BASIC and the operating system
|-
| '''IC3''' || Z80A or D780C-1, 40-pin || CPU
|-
| '''IC4''' || MK4118, 24-pin || 1 KB × 8 static RAM. The position also takes a 4816 2K × 8 part in a 28-pin package
|-
| ''IC4a, IC4b'' || µPD2114LC, 18-pin || Alternative to IC4 — two 1K × 4 static RAMs
|-
| REG || 7805 || The 5 V regulator, on an aluminium heatsink
|-
| TR1, TR2 || ZTX313 || Video output stage; master oscillator
|-
| D1–D8 || 1N4448, 1N4148 or 1S4A || Keyboard matrix diodes
|-
| '''X1''' || CDA 6.5MC || '''6.5 MHz ceramic filter.''' The ULA divides it to the 3.25 MHz CPU clock
|-
| MOD || UM1233 || UHF modulator, channel 36
|}

'''The ROM is 8 KB, not 4 KB.''' The 4 KB ROM belongs to the ZX80; the ZX81's larger ROM is where floating point, SLOW mode and the improved BASIC came from.<ref name="zx81man" />

'''The CPU clock is 3.25 MHz on every ZX81.''' It is derived by the ULA from the 6.5 MHz ceramic filter at X1, and it does not change between board issues. 3.5 MHz is the [[Sinclair ZX Spectrum|ZX Spectrum]] figure.<ref name="tv2" /><ref name="zx81sch" />

=== Video generation ===

The ZX81 has no frame buffer and no display processor. The Z80 itself fetches the character data and the ULA turns it into video, so in '''FAST''' mode the machine cannot compute and display at the same time — the picture blanks while a program runs and flickers when a key is pressed. '''That is designed behaviour, not a fault.''' '''SLOW''' mode interleaves the two at the cost of speed.

== Power ==

An external '''unregulated DC''' adaptor feeds a '''3.5 mm mono jack''' — the same size as the EAR and MIC sockets next to it, which is an easy and expensive mistake to make. On the board the supply goes through '''C3, a 22 µF electrolytic''', to the 7805.<ref name="zx81man" /><ref name="zx81sch" />

'''The supply is unregulated by design.''' Sinclair specifies it that way and the 7805 on the board does the regulating. A high off-load reading on an original adaptor is normal. Sinclair's own requirement is a supply capable of '''not less than 600 mA''', or 1.2 A if a ZX Printer is to run from the same adaptor.<ref name="zx81man" />

Getting the polarity wrong is a recognised way to kill the regulator; it usually dies and spares the rest of the board, which is luck rather than protection.<ref name="tv2" />

== Capacitors ==

'''There are twelve capacitors on a ZX81 and only two of them are electrolytic''' — C3, the 22 µF reservoir ahead of the regulator, and C5, the 1 µF part in the power-on reset network with R15 (220 k) on IC3 pin 26. Everything else is a ceramic disc and is not a service item. This is not a machine that needs a wholesale recap.<ref name="zx81man" />

Full values are on [[Sinclair ZX81 Capacitor Replacement Guide]].

'''The ZX81 has no reset switch.''' The only reset is R15 charging C5 at switch-on, which is why the remedy for a hung machine is to unplug it and wait a few seconds for C5 to discharge.<ref name="zx81man" /><ref name="zx81sch" />

== Board issues ==

'''At least three PCB versions exist, and the component reference numbers are the same on all of them.''' The notable Issue 3 change is that resistor packs RP1 and RP3 are replaced by individual resistors, R35–R42 and R43–R47.<ref name="tv2" /><ref name="zx81man" /> Work from the issue number silkscreened on your own board.

Earlier per-issue "errata" tables attributing missing video pull-ups, overheating and added heatsinks to particular issues could not be traced to Sinclair or to any trade source, and have been removed rather than repeated.

== Storage ==

Cassette, through the EAR and MIC jacks. C10 forms the EAR input network with R33 and R34; C11 and C12 form the MIC output network with R27 and R29.<ref name="zx81man" /><ref name="zx81sch" />

The tape format is Sinclair's own, not the Kansas City Standard, and no loading-rate figure is quoted here because none appears in Sinclair's documentation.

== Expansion ==

A rear edge connector brings out the Z80 bus, the supply and the video signals; it is what the 16 KB RAM pack, the ZX Printer and the various interfaces plug into. '''"RAM pack wobble"''' — crashes and resets caused by the pack moving on the connector — is the single best-known ZX81 complaint, and it is a mechanical fault at the connector, not an electronic one.

== Servicing ==

* [[Sinclair ZX81 General Maintenance]] — component list, supply figures, test points, keyboard tails
* [[Sinclair ZX81 Troubleshooting Guide]] — fault-finding
* [[Sinclair ZX81 Capacitor Replacement Guide]] — the twelve capacitors, by value and type

Documents hosted here: [[:File:Sinclair ZX81 Service and Assembly Manual.pdf|ZX81 Service and Assembly Manual]] and [[:File:Sinclair ZX81 circuit diagram.png|circuit diagram SRC 049]].

== Schematic and PCB ==
{| class="wikitable styled-table" style="width:85%;"
|-
| [[File:Sinclair ZX81 circuit diagram.png|thumb|center|420px|Sinclair's ZX81 circuit diagram, drawing SRC 049]]
| [[File:Sinclair ZX81 motherboard (3327788413).jpg|center|thumb|300px|ZX81 motherboard]]
|}

== Related pages ==
* [[Sinclair ZX80]]
* [[Sinclair ZX Spectrum]]
* [[Sinclair ZX81 General Maintenance]]
* [[Sinclair ZX81 Troubleshooting Guide]]
* [[Sinclair ZX81 Capacitor Replacement Guide]]

== References ==
<references />

[[Category:Sinclair Computers]]