MIDIBass 303 Installation
The TB303's sequencer controls its analogue synth section via a 6-bit digital to analogue convertor and three digital control lines for GATE, ACCENT and SLIDE.
The MB303 midi interface board intercepts these control signals while they are still in the digital domain, to allow the pattern data from the 303 sequencer to be transmitted as midi note output, and to allow an external sequencer to drive the 303 synth section exactly as if it were being driven by the 303 sequencer.
It also has bi-directional connections to the 303's DIN sync signals, so it can either synchronise the 303 sequencer to external midi clock, or generate midi clock output from the 303's internal timebase.
Installation of the MB303 board requires the removal of a 16 pin IC and 3 resistors, plus the replacement of 2 resistors with different values, and the addition of a capacitor to fix a sync bug present in the original 303 CPU that only becomes apparent when you generate midi clock from its internal timebase.
The board has a 16 pin DIL header, two 5 pin connectors and a single 2 pin connector.
The 16 pin header allows the MIDIBass board to take the place of the 4174 latch used in the 303 as part of the digital to analogue convertor that generates the note CV.
The 5 pin connector labelled '303 CPU' has two wires that intercept the GATE control signal, two wires that intercept the ACCENT control signal, and one wire to drive the SLIDE circuit (the SLIDE signal from the CPU is on the 16 pin header, so does not need an input wire).
The two pin connector interfaces with the 303s internal DIN sync RUN and CLOCK signals - these pins switch from input to output mode as necessary.
The other 5 pin connector provides the midi in and midi out connections.
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