VIA Control register "controls" the Control lines of via. Control lines are:
CA1
CA2
CB1
CB2
All control lines act as interrupt inputs or as handshake Outputs.
For vectrex anlog parts (and for now) the lines CA2 (ZERO) and CB2 (BLANK) are most interesting.
CA1
The CA1 line is connected to the second joystickport of the vectrex, more specific to button4. For joystick activity the controlline is not used. However, the lightpen device and the 3d Imager make use of that control line!
The lightpen for example "triggers" CA1 when it sees some "light" :-). Bit 0 controls the interrupt behaviour of the CA1 line. Usually the bit is (left ) to 0. If this bit is a logic 0, the CAl interrupt flag will be set by a negative transition (high to low) of the signal on the CAl pin. If bit 0 is a logic 1, the CAl interrupt flag will be set by a positive transition (low to high) of this signal. The Imager 3d (at least in Narrow Escape), sets this bit to 1 to receive the "signal" from the index hole.
CA2 - ZERO
Responsible bits are bit 1-3:
110 = LOW output (translated to vectrex analog - ZERO is enabled)
111 = HIGH output (translated to vectrex analog - ZERO is diaabled)
CB2 - BLANK
Responsible bits are bit 5-7:
110 = LOW output (translated to vectrex analog - BLANK is enabled)
111 = HIGH output (translated to vectrex analog - BLANK is diaabled) Although blank is "usually" controlled by the shift register (output)!
Together in vectrex terms:
(assuming bit 0 and 5 (interrupt controls for CA1 and CB1) are zero)
%110x110x = $cc means ZERO enabled, and BLANK enabled
%111x110x = $ec means ZERO enabled, and BLANK disabled
%111x111x = $ee means ZERO disabled, and BLANK disabled
%110x111x = $ce means ZERO disabled, and BLANK enabled