Allen-Bradley 1785T-PMPP-1700 ProSoft Communication Module

Description

The 1785T-PMPP-1700 is a specialized communication interface module manufactured by ProSoft Technology for the Allen-Bradley PLC-5 platform. It allows a PLC-5 controller to act as a Master on a Probus (PMPP) or specific proprietary peer-to-peer networks.

This module is primarily used in legacy industrial environments to bridge the gap between Allen-Bradley control logic and third-party devices or older networked subsystems that utilize the PMPP protocol.

Availability and Procurement

• Condition: Refurbished (Standardized Industrial Testing) or New Surplus (Legacy Stock)
• Warranty: 12-month technical replacement warranty
• Lead Time: 3–7 business days (Global shipping via DHL/FedEx)
• Pricing: Variable based on firmware revision; contact our technical sales desk for a formal quote.

Technical Specifications

The module sits directly in the PLC-5 1771 I/O chassis and communicates with the processor via the backplane using Block Transfer Reads (BTR) and Block Transfer Writes (BTW).

Core Functionality and Integration

The 1785T-PMPP-1700 is a “coprocessor” module. It handles the heavy lifting of the communication protocol so that the PLC-5 processor doesn’t have to manage complex bit-string manipulation.

1. Block Transfer Communication

To move data between the module and the PLC-5, you must use Block Transfer (BT) instructions.

• BTW (Write): Sends configuration and command data from the PLC-5 to the module.
• BTR (Read): Retrieves status and received network data from the module to the PLC-5.

2. Protocol Flexibility

The module can be configured to operate as a Master or a Slave. In most deployments, it is used as a Master to poll field devices like power meters, variable frequency drives (VFDs), or weighing scales that communicate over RS-485 using the PMPP protocol.

3. Dual Port Operation

With two independent ports, the module can manage two separate networks simultaneously. For example, Port 1 could be polling a line of drives via RS-485, while Port 2 provides a diagnostic RS-232 link to a local PC or HMI.

Engineering Insights: Deployment and Maintenance

As an architect familiar with PLC-5 legacy migrations, I recommend focusing on these areas:

1. Hardware Jumpers

Before sliding the module into the 1771 rack, check the physical jumpers on the side of the board. These determine the electrical interface (RS-232 vs RS-485).

• Warning: Setting the jumpers incorrectly for an RS-485 network can cause communication “noise” or even damage the port if there is a significant ground potential difference between devices.

2. Programming Complexity (The 64-Word Limit)

Standard PLC-5 Block Transfers are limited to 64 words. If you need to move more data, the module uses a “paging” mechanism.

• Requirement: Ensure your ladder logic is robust enough to handle the handshake bits for paging. If the logic is too slow, you may experience data “tearing” where part of the data is from one scan and the rest is from another.

3. Configuration via Configuration File

Unlike modern “Plug-and-Play” modules, the 1785T-PMPP-1700 requires a specific configuration block sent during the first scan.

• Best Practice: Always store a copy of the configuration block (the BTW data) in a protected data file in the PLC-5 (e.g., N10:0). If the module is replaced, the PLC can automatically re-configure the new unit upon startup.

4. LED Diagnostics

The front panel LEDs are your first line of defense:

• COL (Collision): Indicates RS-485 bus contention.
• ERR (Error): Flashing red indicates a configuration mismatch; solid red suggests internal hardware failure.
• TX/RX: Should flicker rapidly; a solid light or no light indicates a wiring or baud rate issue.

FAQ

Q: Can this module be used in a 1771 Remote I/O (RIO) rack?

A: Yes, provided the RIO adapter supports Block Transfers. However, I/O updates will be significantly slower than if the module were in the local chassis with the CPU.

Q: Does it support Modbus?

A: No. This specific model is for the PMPP protocol. For Modbus, you would typically look for the 1785-MCM or ProSoft 3100-MCM module.

Q: How do I configure the baud rate?

A: Baud rate and parity are not set via jumpers; they are part of the configuration block sent from the PLC-5 via a BTW instruction during system initialization.