Allen-Bradley 1771-WS PLC-5 Weigh Scale Module

The Allen-Bradley 1771-WS, also cataloged as the 1771-WS Weigh Scale Module, operates as a dedicated hardware component for converting load cell signals into accurate digital weight data within PLC-5 I/O chassis platforms.

Hardware Specifications

Backplane Bus Communication Velocity and Density Scaling

The 1771-WS leverages the native Allen-Bradley 1771 backplane bus communication velocity to execute deterministic block transfer reads (BTR) and block transfer writes (BTW) directly interfacing with the PLC-5 processor. This localized processing model ensures high-speed I/O density scaling by offloading the complex 16-bit analog-to-digital weight calculation parameters from the primary CPU. Firmware flash compatibility restrictions apply based on the series revision, maintaining stable communication profiles across legacy 1771 universal I/O chassis architectures without degrading backplane network bandwidth.

Frequently Asked Questions

Q: What is the maximum number of load cells that can be driven by a single 1771-WS module?

A: The internal excitation power supply of the module delivers 10 VDC and is rated to drive up to four parallel 350 ohm full Wheatstone bridge load cells simultaneously.

Q: Does the 1771-WS support hot-swapping while the 1771 I/O chassis backplane is energized?

A: No. The 1771 architecture requires the chassis power supply to be fully de-energized prior to inserting or removing the module to prevent transient electrical damage or disruption to backplane communications.

Q: How is calibration data retained if backplane power is lost?

A: Calibration parameters, including zero and span targets, are transmitted via block transfer writes from the PLC-5 processor logic and stored in non-volatile memory or managed directly via the controller application code upon system initialization.

Field Installation Guidelines

• Chassis Alignment and Keying: Ensure the plastic keying bands on the 1771 chassis backplane connector match the designated slot positioning for the 1771-WS module to prevent incorrect hardware insertion.
• Shielding and Grounding: Terminate all load cell cable shields at a single, clean instrumentation ground point. Do not ground the shield at both ends to prevent the formation of ground loops that inject electrical noise into the 0 to 30 mV analog signal line.
• Wiring Separation: Route low-voltage load cell signal wiring through dedicated conduit paths separated from high-voltage AC power lines or variable frequency drive (VFD) motor cables by a minimum distance of 300 mm.
• Terminal Block Engagement: Verify that the field wiring arm is fully seated and securely latched onto the module frame prior to system power-up.