Questions from my inbox

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Why wasn't the 8051 circuit used in home microcomputers? And why can't you connect a ZX Spectrum to a modern display without modification?

Occasionally you readers write me interesting questions and I think it's a shame to answer just one by email, because more people may be interested. So I'll try an irregular format for such public responses.

The first question was sent by Mr. Tomas:

I was intrigued by your article, where a new hope for connecting parking cameras to my ZX81 was revealed.
The question here is, would it work (generally these TFT monitor type things with PAL input) on an unmodified ZX 80/81/Spectrum, i.e. as is, without reworking for s-video etc.
Thanks in advance for your opinion/advice, it would help me a lot.

Such a signal is connected to the antenna input, from there it goes to the input filter, where it must be tuned to a channel with the same transmission frequency, and if so, it passes on, and there the TV reconstructs it back to PAL.

Which is also why the picture is not as good quality – because it is modulated first to the 36th UHF channel (603.25 – 607.75 MHz), with that frequency it is transmitted over the cable to the TV, and there it is demodulated back to the original PAL, and it is this process that causes the loss and interference.

The PAL monitor wants just that video signal. It removes the whole complex chain of modulation and demodulation, but you have to take the unmodulated signal out of the ZXS/ZX81. It's not complicated, but it does take some intervention.

For those interested, here's instructions for outputting video from ZX81, or here if you have a ZX80 or Timex. It's a bit more challenging to edit, as you need to amplify the video signal a bit with a transistor.

With the ZX Spectrum, you simply disconnect the modulator and feed the video output directly to the antenna output. For step-by-step instructions, see here (ZX Spectrum Composite Mod), or here (ZXS 48k video). Admittedly, the antenna output is a bit different than the standard RCA connector for video („CINCH“), but it can be used. Or you can simply add a second connector:

The second question was sent by Mr. Jan:

why wasn't the 8051 microcontroller used in single board computers? Technically it is a Hardvard architecture, but even the manual says you can use one RAM for code and data by combining the /PSEN and /RD signals. I assumed I would get a von Neumann processor with a similar instruction set and speed to the aforementioned 8080, which has (may have) extra on-chip ROM for the monitor, serial port, timers, and one free I/O port. Basically the entire Omen Alpha on one chip (except memory). Did the 8051 come too late for single board osmibits? Was it too expensive? Wasn't the RAM available fast enough or big enough? Or is there some other treachery that makes such use impossible?

In the first place, the x51 came too late (only in 1980), so by the relevant time, designers had already learned practices with ordinary CPUs. At that time, not only the 8080, 6502 or Z80 were available, but also the 8086 (1978) or 8088 (1979). In the same year as the 8088 came the Motorola 68000. So, in my opinion, the most likely reason is that the x51 series came too late to make an impact on the history of osmibits.

It's not that no microcomputers of this type existed – they did, but there were significantly fewer of them. Quite often the x51 was used, for example, in game consoles.

But the biggest drawback, and the main reason the x51 didn't become widespread in home computers, was that while it allowed external memory, there were no instructions that could use it as an operating memory. With the 6800 / 6502 / Z80 etc there was only one memory space. The x51 microcontrollers worked directly only with internal RAM and a rich set of registers, but only had the MOVX instruction to access external memory (in the case described, MOVC could also be used) with indirect addressing, comparable to, for example, if you had only the MOV M,x and MOV x,M instructions to work with the memory contents in an 8080 processor.

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Martin Maly

Martin Maly

Programmer, journalist, writer and electronic hobbyist. Vintage CPU lover. Creating new computers with the spirit of 80's.