Sinclair ZX81
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(pictures copyright by M.A.Grundke)
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sinclair
ZX81 Personal Computer
USE ONLY APPROVED AC ADAPTOR
PATENT PENDING
MADE IN UK |
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ZX81 ISSUE 3 board overview:
the usage of few and cheap components made the ZX81 the first successful 'mass-produced' computer. On the lower right, you can see the foil connectors to the keyboard. Above them, the two 2114 RAMs (IC4A, IC4B), and still behind, the machine's expansion port. Moving left from the RAMs you come across the BASIC ROM (IC2), the CPU (IC3) and the SCL (IC1). On the board's left, the RF-modulator, with DC in-, MIC and EAR-connectors below.
The EAR and MIC-ports were used to read and write from & to a cassette recorder. Right below there is the voltage regulator with its large, metal heatspreader. Note that the machine is not shielded in any way |
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board silkscreening, part 1 (board's lower side, it's really red):
sinclair
1981 ©
ISS 3
PCIB |
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board silkscreening, part 2:
1981 ©
ISS 3
SINCLAIR ZX81 |
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board silkscreening, part 3:
sinclairBehind it, the two keyboard-connectors (with cables removed; by the way, these cables were the most sensitive parts in most Sinclair microcomputers. But sadly, they never learned...) |
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| The ZX81's Zilog Z80A. It was a very popular CPU these days, especially because its was Intel 8080-compatible and there was CP/M for it (there never was CP/M for the MOS 6502-series, for example). In the ZX micros, the CPU is used to create graphic output to reduce production costs to a minimum. The drawback is that when outputting a TV picture, the CPU would effectively run at about 900 KHz, a tremendous slowdown (thus called the 'SLOW mode'). To make full speed available to programs, the so-called 'FAST mode' was implemented, in which display output was disabled and the CPU could process code at full speed |
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| This 'Ferranti ULA2C210E 8301' ('ULA' means 'Uncommitted Logic Array') is what Sinclair calls 'Sinclair Computer Logic' (or short: SCL). Refering to the manual, 'the SCL coordinates CPU, RAM and ROM' |
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| The two pcs. NEC µPC2114LC-1R (1Kx4) NMOS-RAMs, for 1 KByte RAM. Later machines (like the TIMEX Sinclair 1000) made use of CMOS RAMs, which consumed less power than RAMs from the NMOS production process. As mentioned earlier, the internal RAM is disabled, as soon as there's memory present at the expansion port. These memory expansions were available with up to 64 KBytes - but
because of the BASIC ROM occupying 8K of the memory map, only 56 KBytes were effectively usable |
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| The ZX81's ROM, a Motorola ZCM38818P (the 'P' standing for 'plastic package'), containing the 8K Sinclair BASIC. Compared to the ZX80's ROM, the ZX81's ROM had several improvements, such as a sub-interpreter for floating-point arithmetic. Notable about the ROM area is that it cannot be disabled, like in other Z80-designs (e.g. to access RAM lying 'beneath' it - reason for the 56 KByte RAM limit mentioned earlier) |
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| The machine's expansion slot ('bus connector'). Notable is that when you used it for memory-expansion, the internal 1 KByte was disabled. And, these days, 1 KByte was a great deal of RAM |
Also see the TIMEX Sinclair 1000, a ZX81-clone manufactured by TIMEX Corp.