[INT] Tibbits That Require an Interrupt Line

Some Tibbits require an interrupt line. For example, on Tibbits #01 (simple RS232 port), #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 the 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 into the control line D:

M1_scheme_int M2_scheme_int

The TPP2 features six "M" sockets with interrupt lines, while the TPP3 has eight such sockets.