Macintosh SE VIA pin assignments.

VIA Data register A:

7 Input  vSCCWrReq  0 = SCC Wait/Request, channel A or B
6 Output vPage2     0 = alternate screen buffer, 1 = main screen buffer
5 Output vHeadSel   Floppy disk state-control line SEL
4 Output vDriveSel  0 = upper internal floppy disk drive, 1 = lower
3 Output vSync      1 = synchronous model support, channel A
2 Output vSound(2)    Sound volume
1 Output vSound(1)  { (111 = maximum, 000 = minimum;
0 Output vSound(0)    bit  2 = most significant bit )

VIA Data register B:

7 Output    vSndEnb 0 = sound enabled
6 Output    vH4     0 = SCSI IRQ enabled
5 Output    vFDesk2 ADB state input 1 (ST1)
4 Output    vFDesk1 ADB state input 0 (ST0)
3 Input     vFDBInt 0 = ADB interrupt
2 Output    rTCEnb  0 = real-time clock enabled
1 Output    rtcCLK  Real-time clock's serial data-clock line
0 In or Out rtcData Real-time clock's serial data line

VIA Peripheral Control register:

CB2 7-5 Apple Desktop Bus data
CB1 4   Apple Desktop Bus clock
CA2 3-1 One-second interrupt
CA1 0   Vertical blanking interrupt