Sinclair ZX81, Timex 1000 & 1500

[Homer J Simpson]

[David Hess]

No ZX80?  For shame!

[IanJ]

Back in the early 80's I borrowed a ZX81 off a pal at school (I was 16) and there it started for me........Within a year or so I had my own ZX Spectrum.

Great days!.

Ian.

[Howardlong]

I can’t remember whether it was the ZX80 or ZX81 (or both) used an undocumented feature of the Z80 where the I register would appear on one half of the address bus when the auto-incrementing DRAM R (refresh) register appear on the other half.

It used this in combination with the RAM as a video buffer to address the program ROM and use it as a character generator.

I remember reverse engineering the circuit because I couldn’t figure out how they’d managed to achieve video with so little logic.

There was some tiny code run in the horizontal blanking period to manipulate the I register to go to the next scan line. During the active horizontal period the processor executed NOPs just to auto increment the R register. The only time available for the processor to do any work was during the vertical blanking period.

If you were a bit too clever, you could manipulate the I register to point to a different bit of ROM and therefore create apparently higher resolution graphics out of the random bits of stored machine code.

It was intriguing stuff!

[rsjsouza]

I was more familiar with the Brazilian clone TK85
https://en.m.wikipedia.org/wiki/TK85

[newbrain]

I can’t remember whether it was the ZX80 or ZX81 (or both) used an undocumented feature of the Z80 where the I register would appear on one half of the address bus when the auto-incrementing DRAM R (refresh) register appear on the other half.

If an alpha particle did not flip some neurons, it was documented somewhere: so much that I used it in the written test for "Combinatorial and sequential systems" at the Uni as a crude output port using an octal type D FF clocked by the /RFRSH signal rising edge (a bit fuzzy on signal names...).
The professor did not know or remember about the "feature", and I initially got a very low score.
When I showed him the documentation -paper, of course, at the time- he took the correction graciously, and ended up being the supervisor for my master thesis.

[Howardlong]

I can’t remember whether it was the ZX80 or ZX81 (or both) used an undocumented feature of the Z80 where the I register would appear on one half of the address bus when the auto-incrementing DRAM R (refresh) register appear on the other half.

If an alpha particle did not flip some neurons, it was documented somewhere: so much that I used it in the written test for "Combinatorial and sequential systems" at the Uni as a crude output port using an octal type D FF clocked by the /RFRSH signal rising edge (a bit fuzzy on signal names...).
The professor did not know or remember about the "feature", and I initially got a very low score.
When I showed him the documentation -paper, of course, at the time- he took the correction graciously, and ended up being the supervisor for my master thesis.

It may have been in later years, but my discussions directly with Zilog techs at the time was that it wasn’t a supported feature. That’s not to say it wouldn’t work in 99.999% of the time!

[IanMacdonald]

For a while I worked opposite the Timex factory where the Sinclair computers were made. Apparently the staff were supposed to faultfind and repair any boards from the production line which didn't work. Instead, what they often did was to throw the faulty board out of the window and onto the flat roof of the building. 

When contractors eventually went up there for building repairs, a huge pile of ZX Spectrum boards was found. They were given away, and we got a few. Many turned out to be still repairable despite lying for a year or two in the Scottish weather. Of course there were no cases, just the boards.

The ZX81 used static RAM, but the Spectrum used early dynamic RAM which required a -5v rail. This was generated by a little onboard inverter. If the inverter failed your entire RAM bank drew overcurrent and fried itself.  Since RAM chips were not exactly cheap in those days, that little inverter was the cause of a lot of grief. The later RAM chips did away with that arrangement.