First post, by mac57mac57
I have been working on an older 286 AT system which I acquired on eBay: an NEC PowerMate 286. I have posted several times previously about my efforts to restore this machine. I am happy to report that it is now fully up and running and in use for its originally intended purpose, which was to provide an older, slower and EGA/CGA test bed for my DOS VE text editor (see www.inverary.net/ve).
Along the way, looking to gain some UMBs for the system, I purchased a TexElec LoTech 1MB RAM card (https://texelec.com/product/lo-tech-1mb-ram/). This is an 8-bit ISA RAM card with 1 MB total on it, controlled by 16 DIP switches. Turn a DIP switch on and the associated 64KB bank of RAM is enabled. With 1 MB total on the card, the entire address space of an 8088/8086 based XT class machine is covered. With 16 DIP switches, you can enable RAM at any 64 KB boundary you want and use it. This sounded appealing to me. I had free 64 KB ranges in these machines and wanted to place RAM into them for use as UMBs. After much searching around, I came upon TexElec and their LoTech 1 MB RAM card.
This is an 8-bit ISA card, and I wanted to use it in a 16-bit 286 AT system, and even more specifically, a 16 bit ISA slot (there are no 8-bit ISA slots on the machine). I wrote TexElec and asked if they thought this would work. Their response was not encouraging... the card was designed for XT-class systems, not AT-class systems and there was no guarantee that it would work in an AT system. Further, if it did, the RAM access times would be horrifically sloooooow - all in all, not recommended. However, after more reading about how 8 bit ISA accesses work on a 16-bit ISA busses, along with observing that the system had arrived with an 8-bit ISA card in it, I decided to take the risk. I ordered a LoTech 1 MB RAM card and waited impatiently for it to arrive.
It arrived a week or so ago and I configured it for 128 KB of base RAM (at the time, it only had 512 KB of lower RAM - this brought it up to 640 KB) and 64 KB of UMBs at each of A000:0 and D000:0. I coupled that with the UMB_DRVR.SYS UMB driver (available from https://archive.org/details/UMBDR513_ZIP) and fired it up. Instant success! Power up RAM tests found and passed 640 KB of RAM, converted the 64 KB at D000:0 to UMBs and then did something quite unexpected: since the 64 KB intended for UMB at A000:0 was contiguous with the preceding 640 KB of base RAM, it EXTENDED the base ram to 704 KB!
I have used a variety of diagnostic packages to test the RAM on the LoTech board since then. It is solid. I have run software that requires pretty much all of the base RAM and it all has worked cleanly. The accesses to the RAM on the LoTech card must be incredibly slow, but relative to human perception, it does not seem noticeable.
So, the answer to the question "can I use an 8-bit ISA RAM card in a 16-bit 286 AT-class system?" is a resounding "yes". Like all things related to DOS and the PC architecture, your mileage may vary depending on the memory layout of your machines, but in mine, it has worked well and is an amazingly useful solution. I only wish that a 16-bit equivalent of this card existed!
