Allen-Bradley 1771-DL Gray Encoder Input Module

The Allen-Bradley 1771-DL Gray Encoder Input Module is an 8-channel Gray-code input card for PLC-5 backplanes that accepts a Gray-encoded absolute encoder signal and exposes a binary word to the controller for position feedback. This 12–24 VDC-powered module supports an 8-bit Gray input with a digital resolution of 1 part in 256, making it suitable for rotary encoders that output Gray code. The module uses high-true logic with open-collector characteristics and expects logic levels of about 0–2 V for a low and 10–27 V DC for a high; each input bit sources current from the input side, typically around 6 mA at 10 V and up to 15 mA at 27 V. An input filter time delay of up to 1 ms helps suppress glitches on fast-moving encoders, while the backplane draws roughly 120 mA at 5 V. In normal operation, the 1771-DL decodes the eight Gray-code signals into a binary word that the PLC-5 processor can use for precise shaft-position indication. Typical applications include reading absolute encoder position for motion control, indexing, and automation tasks where high-resolution feedback is required in a PLC-5 environment.