Peer-to-Peer Modbus TCP: Can it be done?

Published on Winter 2007 Modbus-IDA News

Yes — with Comtrol’s DeviceMaster UP gateway.

Polling, master/slave — the Modbus model. So, how about peer-to-peer Modbus/TCP? Sounds great, doesn’t it?

The Modbus TCP/IP specification infers that it’s possible, but can it be done and how? Comtrol answered that challenging question by developing a peer-to-peer Modbus TCP application for raw/ASCII data on its DeviceMaster UP gateway.

Comtrol implemented this by allowing the receive channel (device to the PLC) and the transmit channel (PLC to device) to be configured independently as either Master or Slave. This allows the DeviceMaster UP to operate in the standard Modbus TCP Slave and Master device modes, but also in true peer-to-peer Modbus TCP communication in Dual Master Write and Dual Master Read modes.

What are the benefits of this capability?

No more polling: Data received from either serial or Ethernet devices can be written directly into PLC memory with no PLC data requests.
Decreased receive latency: No more polling-rate-induced latency issues.
Decreased PLC overhead: By removing the need for polling, you can use your PLC to control more devices.
Decreased Ethernet bandwidth

The Comtrol device also offers easy-touse interfaces designed to simplify configuration and PLC programming and increased flexibility. Each serial or Ethernet device interface on the DeviceMaster UP can be configured independently.

The following scenarios put the advantages of this new capability to good use:

Data received from a barcode scanner, RFID reader, or weigh scale can be written directly into PLC memory with minimal latency. The PLC only needs to monitor a sequence counter for new data.
Easily control read/write devices, such as printers, from a slave Modbus/TCP PLC.
Control time-critical devices such as servos or robotic arms by being able to transmit commands and receive responses with minimal latency.

Comtrol’s DeviceMaster UP gateway also includes the patent-pending DualConnectPlus technology. This provides dual connectivity between your serial or Ethernet device and a PLC and/or application at the same time. The filtering and data extraction engine can filter a string, barcode, and

RFID data and extract those UPC/EAN barcode and EPCglobal RFID tag parameters so you don’t have to.