80-Bus News


November–December 1982, Volume 1, Issue 4

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A report on the Microcode Control battery backed RAM board.

B. M. Farrelly

Whilst looking for an EPROM board which I could afford, an ad. offering a 32K battery backed RAM board which would also accomodate single rail EPROMs caught my eye. It was too good a chance to miss, especially as the manufacturer was willing to supply a bare board at a reasonable cost. I wasn’t sure what the battery backed RAM would be used for, but no doubt I would think of something.

A cheque was despatched, and before long the board arrived, complete with manual. Parts were ordered from a certain mail order firm, which had better remain annonimous, and after several weeks, and not a few phone calls, TWO parcels arrived. The bad news was that two deductions had also been made to my Access account, perhaps I should have bought a made up version.

For my money I got an 80-BUS compatible double sided board. It is silk screened and solder masked, and is made to a high standard. The only reservation I would have is that the supply rail tracks are a bit light, but to be fair it does not seem to be causing any problems. There were a couple of minor errors in the silk screening, but nothing serious.

The documentation was a different story though. It seems I have heard that cry before!! One useful feature is a “How it works” section, which other producers would do well to follow. Not so good were the examples of wiring the link options. They were WRONG!!!

Construction was simple enough, anyone who has built a Nascom will have no trouble here. One nice feature was that even though DIL resistor arrays were specified, extra rows of holes had been left on the board so that ordinary resistors would fit. A small saving but worth having. I also liked the idea of having all the 80-BUS lines available – even those that are not used go to holes, making mods. easier.

The board will hold 16 memory chips, which may be 6116, 2516 or a mixture of both, depending on whether RAM or EPROM is required. Battery backup is provided by an on-board PCB battery, and each memory IC may be battery backed or not using the link options, but for obvious reasons EPROMS should not use this facility. The board may be configured as one 32K, or two 16K pages, in 4K blocks. The blocks need not be consecutive, and if two pages are selected the addresses may overlap, but if so be sure not to enable them both together. Options are set by wire links. The “off the peg” board uses special, no solder, push-in connectors for this. Since I did not have any, wire wrap pins were used instead. Working out what to link to where is not too difficult so long as no notice is taken of the examples given.

Paging is different from the normal Nascom/​Gemini method in that the board may be enabled on reset regardless of which page it has been set to. If this is done however the page control bit works in reverse i.e. to turn the board off the appropriate bit must be set.

One important point to note is that, although the board responds to RAMDIS it does not generate it, a shortcoming in a board which claims to support EPROMs, but I can see the problem on a board with interchangeable RAM and EPROM, of deciding whether to originate RAMDIS or respond to it. Since my system has 64K of RAM, well it doesn’t yet but it will have, a RAMDIS output was essential, so a slight mod was needed. The only other problem was that the board would not run at 4MHz without waits. The makers claim 6MHz but maybe I have a slow chip somewhere. No matter, a little thought and some more surgery, and I now have an EPROM board which will also support battery backed RAM.

In conclusion if you want a low cost EPROM board you could do worse, and if you need battery backed RAM this is the board for you.

I know what that RAM will be used for. A permanent record of which programs are on what tapes, preferably automatically updated every time a program is saved. All it needs is someone to write the software!

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