Volex TTX2000 S Teletext adapter

[Guesser]

I've finally got around to writing an article about the TTX2000 S teletext adapter for the ZX Spectrum that I bought in 2017, describing how the hardware works and a (currently incomplete) commented disassembly of the ROM.

https://zxnet.co.uk/spectrum/ttx2000s/

If anyone has a TTX 2000 in their collection I would be very interested in getting internal photographs, and dumps of any different ROM versions too.

[Guesser]

I mentioned it in a couple of other places, but I got hold of an earlier ROM version, and have determined that a bunch of code in my ROM is redundant left over stuff from the old firmware.
With that knowledge I've tidied up my disassembly and I believe I now have a fully commented disassembly of the issue 1.2 firmware: https://zxnet.co.uk/spectrum/ttx2000s/d ... /volex-1.2

The next job is to work through the earlier issue 2.3 ROM and figure out how much of the telesoftware code was implemented. It appears that the code which was replaced in the issue 1.2 ROM implements the CEEFAX Redefineable Telesoftware Format, as used on the Acorn teletext adapter. (And the version of the OEL adapter for Amstrad machines, the TTX2000 A)

The code is blocked off behind an error message, and a number of what appear to be jumps are generated incorrectly in RAM with $3C (inc a; nn nn) instead of $C3 (jp nn nn)!

Further investigation required

[Guesser]

Well I've dug further into the code and I was wrong about the jump instructions. You could say I "jumped" to the wrong conclusion

It's definitely RTF though!