The PIC interface is the heart of the digital circuitry of the ARS2. This interface was not included in the original plan for the ARS2 and was developed during implementation, hence, it's a little messy. The I/O is organized after a few dedicated functions and a generall data bus:

• RA0-RA5 - reserved for 6-bit R2R network
• RC4, RC5 - reserved for USB
• RB0-RB7, RC1-RC2, RC6-RC7 - bus I/O
• RC0 - reserved for Gate (legacy)

The bus I/O is divided into two categories; generall data lines (multiplexed) and function-dedicated enable lines. RB4 is generally used to communicate any analog signals to the pic, RC7 is so far unused, RB6-RB7 are sometimes used for In-Circuit programming, RC6-RC7 could potentially be used for RS232 communication (MIDI) (depending on the status of a silicon bug in the PIC18F2455). The dedicated vs. generall data lines are marked in the schematics as "Ded" and "Gen" along with some other internal markings.

At the first designstage, the R2R together with Gate was designed to form a single CV/Gate pair with Op.Amp outputs on both. Since more analog outputs were desired the development of the Quad S&H was started and integrated, though without changing the original (legacy) support for a single CV/Gate pair.

The "PGM"-box is only a symbol for the In-Circuit programming which can be omited (just connect the cables seen on the other side of the box) or implemented (see microchip's notes on the PIC18F2455).
A word of warning; Interfacing a delicate analog instrument such as a synthesizer with something as dirty as a computer can yield problems, as i myself have experienced. I have yet to find a satisfying solution to this problem.

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