4-wire Modbus ASCII net to keyup the radio


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Leroy Brown

We have 3 devices (2 GE Horner slaves and CAC 5600 RTU) talking Modbus ASCII over 4 wire RS485 network. Client wants to add a radio (MDS1000) for communication with the SCADA system. How to key the radio? As I understand, CTS is required. Our RS485 has Tx- Tx+ Rx- and Rx+. Is there any RS485/RS232 converter suitable for that task?

Robert Smyth

Connecting Modbus to a radio can be difficult, even if you did have the RTS signal to key-up the transmitter. I know nothing about the MDS1000, but you need to check if it ensures no noise at the receiving end or it provides the leader timing required for Modbus. The problem is that a modbus frame starts with a leader of three characters in duration when no characters are sent! Hence you need to be sure the software radio transmitter it will key-up your radio early to give you this leader. If the receiving device receives any characters or noise in this time period prior to the first character it will reject the frame. So, you need a radio modem that supports modbus. Otherwise you may find that other protocols will work over the radio but not modbus. Robert Smyth EngWare Pty Ltd Voice: 0411 202 514 Fax: 02 9222 1457 E-mail: [email protected]
Check out www.bentek.ca. We've used their SL900 radio modem with Modbus ASCII over RS232, but it has RS485 inputs as well. It can be configured in 'End-to-end I/O' mode, which we used and didn't need to use CTS/RTS lines. All we needed to wire were the RS232 Tx, Rx, and GND wires. It may be what you're looking for. Don Zunti, P. Eng. Delco Automation Canada

Jerry Miille

The Miille Applied Research Multii-Port module can do this for you. This module is a "port multiplier" for either Modbus or Allen-Bradley protocols. You can change baud rate, protocol (ASCII or RTU), signal levels (RS422/RS485/RS232), handshake options and set RTS/CTS delay times individually for each port. The web site is http://www.miille.com or contact me offlist for additional information. Jerry Miille Miille Applied Research Co., Inc. Houston, Texas 713/472-6272 [email protected] http://www.miille.com