Various ZX81 Items on SellMyRetro.com

[gozzo]

When this other one arrives, I should then have equipment which can hopefully (!!) handle all sizes/voltages.! I have several eproms I would like to make a dump of for backup purposes!! As and when I can work out the circuit of the FDC,maybe it would be possible for a ULA to be made (like the zx ula 2)to replace all or most of the logic so others could build a clone easier....?????

[sirmorris]

Certainly. With a correct schematic it would be possible to re-create the logic in a suitable CPLD.

C

[gozzo]

One big problem is that one of the chips has it's type markings rubbed/scratched off, maybe deliberately (seen that done before..) I do have a chip tester thingy which can supposedly identify unknown logic chips but I don't really want the hassle of having to desolder one unless I really have to!! It looks and appears to be connected similar to one of the others so for the moment, I'll assume its the same... I'll try draw out the circuit first as 'physical' connections, then work out the logic details afterwards when I'm sure of the chip type!

[1024MAK]

One big problem is that one of the chips has it's type markings rubbed/scratched off, maybe deliberately...<snip>

If the marking is hard to see because it looks to have been poorly printed, or rubbed off, try using different lights at different angles (including different colours), as sometimes this helps to see any remaining marking.
As you say, sometimes it is possible to work out which logic chip it could be from reverse engineering the circuit schematic. Also if the device can be made to stop in a static state, you can use a meter, logic probe or 'scope to record the logic levels at the pins. Combined with the information from the reverse engineered circuit schematic this should allow you to work out the type, or get you close .
Of course if the chip is a programmable type or a custom IC then you could be stuck .

Mark

[gozzo]

*** IT WORKS!!! *** - Well, almost! I connected it up with a rampack on the back, but without a disk drive, switched on and waited..... then disaster? - random patterns all over the screen "Aargh - it's killed me ULA!!" - unplugged everything and retried my zx81 on its own - works ok (PHEW!), looked at the edge connector and saw it was a bit tarnished so squirted it with switch cleaner,reassembled everything (minus a drive still)and gave them all a wiggle to help remove the tarnish, switched on,crossed my fingers, and waited...and waited....BINGO! "0 REM COMPUSA FDC-100 v1.5" on the top line, as the manual says should happen!! Now to try it with a drive - a possible problem is that it was designed for 5 1/4 inch single or double density drives, I don't know if it will work with 'modern' 3 1/2 inch types - I have read somewhere there can be differences in how pin 1 (I think?) may be used, usually a 'ready' signal but some use it as 'disk change' ... correct me if I am wrong!)
And my EPROM programmer has now arrived for me to make a dump of the ROMS...but it arrived with the driver disk smashed to bits ... F*** it!! They are sending me a replacement, luckily, just have to wait a bit longer to use it!!!

[sirmorris]

BBC Micro users have been debating the '3.5"' issue for years, you can find lots of useful information about using modern drives with old computers over at the stardot forums.

HTH

C

[1024MAK]

Some time ago I posted lots of general information on floppy drives at Atari-Forum http://www.atari-forum.com/viewtopic.php?f=74&t=19148
Others have also posted lots of info on 3.5in double density and high density drives at Atari-Forum as well.
To see most of the info, you will need to register and log-in.

Or if you wish, I can post some info on this forum .

Mark