The Allen-Bradley 1775-S4A is an industrial control system input/output (I/O) scanner-programmer interface module for use with PLC-3 processors. This module has an I/O capacity of 2,048 inputs and of 2,048 outputs, with 4 I/O communication channels and with 1 RS-232-C communication channel in each module. Note that the RS-232-C interface is used for report generation and ladder diagram programming, with the ladder diagram recorded using a data cassette recorder or data cartridge and printed to a printer via the Channel C interface, force inputs and outputs, enter and monitor the data table values, and operate the function of the PLC-3 LIST. Installed on the 1775-S4A module are 4 LED status indicators for the I/O communication channels, the backup connector, the channel connector, and the terminal arm swing to connect the I/O channels to each other for peer-to-peer and backup communication purposes.

For every I/O adapter, the 1775-S4A module has nominal I/O scan times of 6 to 6.5 ms for 4 channels, 6 ms each for both 3 channels and 2 channels, and 5.5 to 6.5 ms for 1 channel. It also features communication rates of 110 baud to 19.2 kbaud for the RS-232-C channel and 57.6 kbaud or 115. kbaud for the I/O channel. When the 1775-S4A is installed on a PLC-3 processor, which occupies only 1 slot in the processor’s main chassis, make sure that its location has a temperature within a 0 to 60 °C range when in use and a -40 to 85 °C range when it is in storage. Note that the 1775-S4A module is a vital component of every PLC-3 processor.