BY JEREMY POLLARD

Device servers are typically used with a PC, not a PLC. And a "virtualized" serial port is used to interface the PC to the end device. However, Rockwell Automationís PLCs cannot send serial commands out of the Ethernet port, so the only way to interface with any serial device is through the intrinsic serial port on the processor. You could use an off-board I/O card; however, the added cost and effort may not justify the end. 

New devices are Ethernet-based out of the gate. Depending on the device, you may be justified in spending the extra money. In legacy applications, however, you may not have the option. This is where Real Time Automationís Messenger comes in. Messenger is a hardware device with firmware that transfers serial data over the selected network to a target PLC.

There are various hardware configurations available that interface with different PLC and network types such as DeviceNET. I tested Messenger with two RS-232 ports with Ethernet.

The device comes with the manual on CD, which is very organized and easy to follow. The manual, however, does not make it easy to understand how to set up some of the parameters. But the great thing about this device is how easy it is to interface a barcode scanner or any other serial device to the PLC over Ethernet, and get the data into the PLC.

The device comes defaulted to a fixed IP address on the Ethernet, a built-in web server for configuration, power supply and serial cable for device connection, if needed. I used Port 1 since the manual stated that Port 0 was 232/485 selectable, and didnít mention which was the default. The manual also stated that it is jumper selectable on one page, and dual inline package switch selectable on another. Neither was visible, so Port 1 was the safe bet.

Once I changed the IP of the Messenger (DHCP is not supported) to the network where my PLCs are, I began the configuration process. The major concern here is whether your device is data terminal equipment or data communications equipment, which will determine the cable you use - null modem or straight through.

Nothing other than the network settings need to be altered when using the Messenger unit. Configuring the PLC requires entering the IP address of the PLC, which is very easy.

Serial device configuration has always been a bit difficult. There is no help file, so having the manual handy is important. Data can be obtained from the device in various ways: start and end delimiters, number of bytes, or on time. Most serial protocols use start and end delimiters, but I can see the use for a fixed message length. Your method will be determined by your device. The barcode scanners used a start and stop delimiter, whereas the laser measurement device could use a fixed message length.

Regardless of the selection, the number of transmit bytes should be set to the number of bytes in the data if you are not using delimiters, or to the maximum data length that the destination location can handle (maximum file size, for instance).

There is built-in buffering of the serial data that can lead to latency, which may be undesirable. Real Time Automation can send a firmware update to put the newest data in first, which will be very handy when your device sends data too quickly for the PLC to assimilate.

The communication setup allows for two-way communications between the PLC and the device, which is very cool. Changing data in the PLC data table can trigger the movement of data to the device. This is a polled activity by the Messenger, and no unsolicited messaging is supported.

I discovered that you have access to a diagnostic screen to help with the troubleshooting. By simply setting the PLC IP address to zero, you can put the device in loopback mode and monitor the incoming data from the device. I highly recommend starting here for cabling and port setup. Once that is done, you put the PLC IP address in, select the data type (string or integer), indicate where you want to put it, and you're in business.

You have to zero out the first integer word or the string length parameters in the PLC before the Messenger will transmit another data packet.

Once the device is set up and working, it couldn't be easier to use. There is no excuse for not getting legacy data into today's automation systems over Ethernet.


Jeremy Pollard is a 25-year veteran of the industrial automation industry. He has worked as a systems integrator, consultant and an educator in the field. Jeremy can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .