The 1772-LH Processor Interface Module is an industrial communication module that belongs to the 1772 Series of mini PLCs that are manufactured by Allen-Bradley/Rockwell Automation. This module comes with 2 sockets (a 15-pin socket and a 50-pin socket), a 4-position mode selector switch, and 2 indicator lights. The 1772-LH Processor Interface Module is used to establish the communication between the processor, user I/O, and the industrial terminal. It is situated in the left-most plastic slot in the 1772-LA processor chassis. The 2 indicators of this interface module are the “DC ON” indicator and the “RUN” indicator. The 4-position modes of the 1772-LH Processor Interface Module include the PROG, TEST, RUN, and RUN/PROG functions.

The 1772-LH Processor Interface Module fits both the PLC-2/20 and PLC-2/30 processors. The 15-pin socket of the module provides linking facilities to the processor and programming terminal. The 50-pin I/O chassis socket can be used for linking the PLC-2/20 or PLC-2/30 processors with the I/O system. The “DC ON” front panel indicator helps to check whether the appropriate power is applied to the power supply module of the system. The “RUN” indicator illuminates when the module select switch is in the "RUN" or the "RUN/PROGRAM" position. The 4-position mode select switch can be found in the front panel of the processor interface module. The key can be removed when the mode select switch is in any of the above-mentioned positions. The “PROG” position is used when entering the program instructions and the “TEST” position is used to test program operations under simulated operating conditions. The “RUN” function scans and executes the program that the users have stored in the memory.