The 57421 is a Reliance Electric Pulse Tach Module. There is a possibility that this device was also sold by Baldor Reliance and Asea Brown Boveri depending on what years this part was in production.
The 57421 is a Pulse Tach Input Module that was created to be part of the AutoMax Multibus I/O system. The main objective of this device is to be the main connection between a pulse tachometer and the AutoMax system. This is important because it allows the system to be able to track the speed measurements of the actual application.

This module has a 500-volt RMS isolation. It has a 24-bit counter and a comparator and a 100kHz maximum input frequency. It does not have any onboard jumpers. It has an input voltage of 5 V to 12VDC. It has a 16-bit 2kHz internal timer. This module has a total of eight registers that are accessible by AutoMax program tasks. They have the ability to be written to or read from program tasks.

This system has several LED indicator lights. They are labeled CCLK OK, REVERSE, FORWARD, LATCH, CLEAR, and COUNT STOP. This module plugs into the backplane on the AutoMax storage rack using the female card slots. The module can then be secured into place using the rack and faceplate brackets and two screws.