Ah totally overlooked the level of the enable signal, being so used to active low signals, but the H at the end of ROMOE should have give me that clue.. I will insert IC17 or a simple inverter on top of one of the closest TTL chips to test that out. Then the other brands will work too I guess.QLObi wrote:Hi gertk,
as far as i see on your picture you forgot to invert the ROMEH ! The EPROMs normaly need an active low signal, but ROMEH is aktiv high ! On all drawings i saw, there is a minimum of this inverter. Even on the orginal ISSSUE 5 drawing from SINCLAIR you will find this option (IC17). In the orginal design only LS TTL chips are used, these a quite slow but work anyway.
Those pesky 23128/23256 roms could be ordered with enable signals on any level you wanted.
Neat trick is also how Sinclair use the address pins and enable signals so it does not matter in what position you insert the 23128 or the 23256, it always works. Both sockets are connected in parallel.
For now I don't mind that I can not use an external dongle.If you put PIN20 of the EPROM to permanent GND, you are not able to free the adress range from C000 - FFFF for some extensions like ICE or the DONGLE of some compilers. If you don't need this, its fine to enable it permanent.
For the SRAM extension I need to generate some extra signals too, I have a bunch of GAL chips which could be used for that and then the inversion of ROMOEH can be programmed too.I am normaly using EPROM with 200ns access time, but i also tested 250ns and they were ok.
There is no issue with the manufacturer, i am using what's available.
One advantage of the board is the short and noise reduced wiring, but i tested the circuit also with a open wiring you had. The only excpetion is, i soldered a 7400 on top of an existing IC to get the inverter.
I am sure it is, thanks for the tips!
If you frequendly change the EPROM the board is easier to handle.
QLObi
Gert
