PAL16R4
GRiD part number: 300135-002

Used as UART and GPiB setup register.
Logic mplement 4 independently addressable registers.
Have 2 address lines and 1 data line
Address lines are connected to A1 and A2 on mainboard
Data line is connected to D0 on main board

Registers 0,1,2:
* baud clock source: internal (from 80130) or external (DE19S connetor inputs)
* RS-422 or RS-232 mode
Register logic is ripped from GRiD patent US4571456 Fig.12
So far it is observed that there flags are initialised in CCPROM.
No info yet what happening while CCOS and user apps working.
Init sequence and state are little different between defferent CCRPOM versions:
1101 rev3 main board (EEPROM 300142-04/300143-04)
1101 rev4 main board (EEPROM 300250-04/300251-04)
1109 main board (EEPROM 300373-04/300374-04)


Register 4 control GPiB SC (System controller) pin for SN75162B IC
to add more felixibility on for GPiB bus.
So far it is observed that this flag set to Active in CCRPOM and
not used anywhere else. No info yet what happening while CCOS and user apps working.

Use logisim evolution to open .circ file and see possible internals