Hello All, I recently acquired a Northstar Horizon machine that didn't have any memory boards. I also received a Dynabyte 800589-G 64k dynamic RAM board. Unfortunately, there is no documentation on this board (other than a single set of jumper settings for use in a Dynabyte 5505 computer - http://bitsavers.informatik.uni-stuttgart.de/pdf/dynabyte/404512A_5505_Comp… ) So far, I have the Northstar rebuilt (power supply tested, capacitors reconditioned, IC legs cleaned, etc). I added the components to install a boot PROM on the Z80 CPU card, and I can successfully boot up to a monitor ROM. From here, I can examine and deposit memory and ports. From this, I can determine that I appear to have a problem with the Dynabyte card, assuming I've reverse engineered the jumpers. The jumpers allow me to enable/disable 4 banks of 16k at 0000,4000,8000,C000. I can either have the bands be there on reset, or switch them on/off by setting/resetting bits in a bank select port that can be defined. I can successfully switch on/off banks from the monitor (visible with the LEDs on the board, as well as seeing 00 or a random value instead of FF in the memory location). However, when I try to write any value, I always write FF. Refresh appears to be working, because any FF I deposit will stay there for hours. This is really tough because I don't have a schematic for the memory board. Are there any S100 experts that could point me in a direction to start looking? Port I/O is working (to the serial port as well as to the bank select port). Memory read appears to work, because I get values that are consistent (the RAM will sometimes boot up with random values, and the random value will stay that way in that memory location until I attempt to write anything to it, then it goes FF). It appears to be a write issue. I've enabled a wait state on the CPU in case it was a timing issue, but no difference. I'd appreciate any suggestions on where I should put my logic probe :) Image of the memory board here if interested: http://www.broadbandpig.com/?attachment_id=51 Ian