Every MINC system came with an IBV11 controller card to connect the IEEE488 (GPIB/HPIB) bus. Unlike the MNCxx modules, the IBV11 was a generic controller – you could also get it in a non-MINC system, and there was a manual documenting how to program it, as well as a maintenance print set including schematics. No need for reverse engineering any longer! Find the manuals here: https://bitsavers.org/pdf/dec/qbus/IBV11/
In the MINC, the CSR and vectors are here:
IBV11 CSR 171420 VEC 420/424 BR 4
Note that these CSR addresses are different than the IBV11 UG mentions (16015x). The vectors are the same though.
Information sources
Besides the manuals linked above, we also used the XXDP VIBB listing and many text books on GPIB.
Missing information
None identified at the moment.
Implementation status
The latest implementation works well with MINC BASIC and the bus commands implemented by it. It has been used to interface several instruments, ranging from early-80s HP equipment using the R2D2 style, to 90s Philips/Fluke using the pre-SCPI style, to 00s Agilent using the standard SCPI style.
One open question is if parallel polling works; we have been unable so far to test because none of our test instruments supports it.
Interface hardware
It is possible to interface the FPGA directly to the bus, using only some resistors for signal conditioning and limiting the impact of transients on the FPGA. A simple interface is shown here:
From experience with several test setups, we’ve learned that this will probably work for small setups – ie, max two instruments on the bus and limited cable lengths. The interface is still out of spec for the FPGA, but it will likely work even so, although we cannot rule out damage to the FPGA I/O circuits.
Far better of course to add proper interface driver circuits, and the most obvious choice are the SN75160 and SN75161 ICs from TI. For a long time however those were not in stock with any of the usual suppliers, and TI listed the status as something like out-of-production or obsolete. But apparently that decision was reversed in some shape or form after considerable feedback from the marketplace, and their website now shows the parts as being in production, and at least Digikey has ample stock.
Tests on the breadboard are promising, and a PCB has been designed – along the PCB that was already there for the direct connection scenario. I’ll share the design when it has been verified – in other words, when the proverbial slow boat from China arrives and I’ve had a chance to test the boards.