[INT] Tibbits That Require an Interrupt Line
Some Tibbits require an interrupt line. For example, with Tibbits #01 (simple RS232 port) and #02 (universal RS232/422/485 port), the interrupt is used to detect state changes on the CTS line. This is necessary for the RTS/CTS flow control. Obviously, an RS232 port doesn't always need flow control, so the interrupt line may not be necessary. Other Tibbits, such as #08 (Wiegand and clock/data reader port), must be wired to the interrupt line, or they won't be able to do their job.
The INT line, when present on a Tibbit, is always mapped to control line D:
Size 2 Tibbo Project PCBs (TPPs) feature six "M" sockets with interrupt lines, while Size 3 TPPs have eight.