After searching for quite some time for a GPIB interface I could afford (without severe buyer's remorse), I came across a National Instruments GPIB-232CV-A quite cheap. This is an RS232-GPIB converter. Now, by itself it's good enough for most of my purposes, but it could be a lot better: this adapter only supports addressing one device, set by DIP switch. In fact, all of its functionality is set by switches, and it just tosses data from one interface to the other.
However, the manual claims that it uses National's NAT4882 GPIB ASIC. This chip handles all of the GPIB transactions and control systems, and is not programmable, so this is not where multiple address support is missing/disabled. My guess of the architecture of the adapter is that it contains a microprocessor which moves data between a UART connected to the serial port and this NAT4882. The processor, as far as I can see, wouldn't do very much. I think the most complicated thing it would have to do is mediate the DMA connection to the ASIC.
Considering all the hardware is there, and the real functionality is in the ASIC, I am considering attempting to reversibly replace the built-in processor with an AVR running custom code. It should have no problem tying into the UART and the NAT4882 control lines, and I don't even have to bother with DMA if I don't want to, because that can be disabled in a control register. Can anyone think of a reason why this would be a lot harder than it seems? I've got a lot of projects going on simultaneously and don't really want to bite off too much.
NAT4882 datasheet available from
here
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