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Almost there!

I replaced the HAL with a GAL I programmed and it made absolutely no difference.  That lead me to suspect he MMU (344-0010-B) of being fauklty and I was able to get hold of a replacement MMU temporarily.  I swapped the MMU over and it sprang to life!  The only remaining fault is thet the keyboard input is garbled, however as I have not replaced he Keyboard ROM and the original gets pretty hot, I think I can guess what the problem is.  My attention now is on how to replave the MMU.

Getting warm, in more ways than one

So in my last entry I mentioned that the ICs LS245, 341-0170-A, 74166, 6502 and 341-0150 were getting warm.  This wasnt the full list, the MMU, IOU and ROMs were also feeling a bit above room temperature, but it was subtle.  After looking at the schematic however, I think I now have a pretty good idea of what's going on.  The LS 245 is getting very hot.  It is an octal bus trancever that connects 74166 to the databus, based on it's inputs from the HAL and the MMU.  The 74166 is also getting very hot and it is controlled by the MMU and the IOU.  The ROMs and the CPU are getting hot because t

Garbled screen, but progress

So the first thing I tried was swapping the RAM out and that had no effect.  I then had a look at the EF ROM.  Testing it wasn't too difficvult, and with the air of an Arduino and breadboard I descovered that blocks of the rom appeared blank.  I read the rom a few times and the blank areas moved about.  I put a PCB together with a ZIF socket and soldered the connections to make certain the blank areas weren't a loose connection.  I then downloaded the rom, bought an EPROM and turned my reader into a programmer.  However with the faulty rom replaced very little changed, how


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