I've been trying to repair a Horizon RAMdisk 4000 and have run into a snag.
Using the minimem debugger, I can turn on the card and access the 8k/32K ROS (Ramdisk OS) chip at 0x4000-0x57ff. When I write even bytes to this space, I can read the byte back. When I write to the odd bytes, it always reads back as "00". I have tried both a working 8k and 32k chip, and set the jumper to 8k/32k with no change in symptoms.
In the rack space from 0x5800-0x5fff, both odd and even bytes work as expected. This 2k space is not part of the ROS 8k/32k chip; it is banked in from the rest of the on board sram chips. As a test, I placed the ramdisk in the Geneve, formatted it with the OS, and it reads/writes data just fine. (The geneve does not require the 8k/32k ros).
I reviewed the construction manual but haven't come up with any ideas or clues. It seems to me that a stuck address line would cause an odd read/write to overwrite an even address, yet that doesn't happen either.