DLMIDI circuit board

The DLMIDI is an adapter for connecting MIDI (musical instrument digital interface) synthesizers and keyboards to a PDP-11. Rather than build an entire serial interface for this, I just used a regular DL11 (Unibus) or DLV11J (Q-bus) SLU port and made an adapter which does level conversion (MIDI uses a 5 mA current loop) and provides the baud rate generator for the 31.25 kbps MIDI data rate. Fortunately both the DL11 and DLV11 provide for outboard baud rate generators, and the DL11 provides TTL level serial in/out pins on its 40-pin header. The DLV11 doesn't, so the DLMIDI card has a MAX232 level converter.

The circuit is based on one I hand-wired on a prototype board in Fall, 1990, where I connected my roommate Ed Lansinger's Yamaha DX100 synthesizer to a PDP-11/34a as part of my senior project (a program which did a really bad job of trying to produce machine-generated counterpoint), with the addition of the MAX232 and 7805 regulator to support the DLV11. I never got around to trying the DLMIDI out on a DL11, but on the DLV11, it worked only for receiving (the hard part!) but not for transmitting (which is easy). I always meant to haul out my scope and find out what was going on (it's sending out something but it's just not right, so I suspect it's a problem with rise/fall times or something basic like that), but it just didn't seem that important. Especially because I added support to Ersatz-11 for the MPU401 MIDI interface on most ISA sound cards, so that my old software runs fine under emulation, without necessarily even needing an outboard synthesizer.

Anyway it connects via ribbon cable, either to the 40-pin connector on a DL11 (EIA or 20 mA version, doesn't matter since the DLMIDI provides its own level converters), or to the 10-pin connector on a DLV11J (etc.). Power comes from the SLU.

I would finish debugging this if I thought anyone cared.

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