PLC with RF and MODBUS


Thread Starter

Mark Hill

Greetings all;
I have a small project which require 10 simple PLCs, (each with 4 analog inputs, 2 digital inputs) that must communicate via RF.

I'd like the base station portion to support MODBUS.

Any and all hardware suggestions will be greatly appreciated.

Mark Hill
[email protected]

Garrett, James P


Check out Motorola's MOSCAD system. We have had very good service with ours. It can support as many RTUs as you need, and using MODBUS is not a
problem. We use both 900mhz MAS frequencies as well as 150mhz VHF. Reliability is outstanding!

Jim Garrett
Sr. IT Analyst
Baltimore Gas & Electric Company

Andrew Murashko

Take a look at Fisher Controls' Remote Operations Controller (
They make ROC306 and ROC312 PLCs, that meet your requirements. RF modem is external unit made, for example, by Dataradio, and connected to ROC over
RS-232 port. This is proven solution for large number of small RTUs (e.g. gas wells).

Andrew Murashko, [email protected]

Robert Philliips City of Wichita Falls C

The simple was just killed when you mentioned RF.

Might use "brick" type plc interfaced to RTU.
Suggest you look at Zetron RTU. Believe they are in England. In US$, a standard host that supports modbus is $1000 while remotes with 4 Analog
inputs and 8 digital out and 8 digital in list for $750. You will need to add radios and antennas.

Robert Phillips
How about a Modicon Momentum PLC (din rail mount) w/ an Ethernet Port. Then use BreezeCom Wireless Ethernet Modems. SST, no license, about 3Mbit throughput, 5 miles or more with directional antennas. Does require line of sight. Under $2K /sight.

Jerome W. Lavoie

I suggest Black Box radio modem MDR100A-R4 and YAGI antenna MDR152-0100-R3 and RS485 interface MD3310-R2. (2 of each required for each remote link.) This will give you RS-485 RF comms up to 20 miles in a line of sight application, requiring on FCC license (910 MHZ). You can have up 2 100 of them in the same spread array area without any cross interference, by use of the electronic coding feature. If cost is an issue, look into Shiele-Entrelec PLC's they are very simple, but effective little PLC's with FREE graphical programming/ monitoring software in standard ladder logic presentation. I have found that for small applications, the entire Shiele-Entrelec PLC costs less than just most typical I/O hardware would. Jerry Lavoie Automation Engineer Nucor Steel Corporation
Current versions of the MicroLogix 1200 and MicroLogix 1500 controllers support configuration of their serial ports for Modbus RTU protocol for
data access.

They do not support all Modbus command codes, only the common ones for RTU data extraction. For example, you cannot upload and download a MicroLogix program while it's serial port is in Modbus mode (thus the COMM DEFAULT button feature).

The reference manual for the MicroLogix 1200 and 1500 instruction set (1762-RM001A-US-P) pages E-9 through E-12 contain reference information on
setting up MicroLogix memory locations to correspond to Modbus registers. This manual is available from

The first releases of MicroLogix 1200 do support Modbus RTU, but support was added to the MicroLogix 1500 in firmware revision 4 in April, 2000. Your RSLogix 500 software will also need to be version 4.10 at least to support this new function (please, no flames, I don't want to have to explain the concept of "time" again). Firmware upgrades are available at no charge from

If your hardware and software support Modbus RTU and it is configured properly, the problem likely lies in the Modbus timing settings for your
master system, which I have found to be the trickiest part of many SCADA projects with inexpensive modems. Page E-12 of the reference manual describes the handshaking delay settings.

Good luck,

Ken Roach
A-B Seattle
[email protected]

Bob Peterson

I believe that the serial ports in a ML1500 can be configured as a Modbus RTU slave port. I don't think it can be configured as a Modbus master, or as a Modbus ASCII port.

Bob Peterson