RF Modem for AB DH-485


Thread Starter


Our facility is fully networked. All PLC's are tied to a server using either Allen Bradley DH485, DH+, or ethernet.

But one PLC is contained within a "mobile" machine. It's normally used down at one end of the plant, but every now and then, it's wheeled to the other end of the plant. It's never at the other end of the plant long enought to worry about data acquisition - I'm only worried about data acquisition when it's in its more permanent position. But remembering to plug the PLC back into the DH485 network seems to be a chore that never gets done.

So I'd like to place a duplex RF modem close to the machines permanent location (about 10 to 20 feet away), and another duplex modem directly to the PLC's RS232 port. Thus whenever the machine is in it's "home" position, it will communicate over the DH485 network.

To accomplish this feat, the RF modems would have to be completely transparent, and require no initialization strings, no handshaking etc. In other words, what goes into modem # 1, comes out modem # 2, and whatever goes in modem # 2, comes out modem # 1. It has to be duplex, because DH-485 can't supply any sort of handshaking signal to perform TX/RX control. The modems have to be capable of operating at 19.2Kbaud.

If you know of any product available to perform this task, please advise.
There are 2 more details that need to be specified before getting an answer

1- what is the maximum time delay that is tolerated in each direction ? The reason for this question is that the RF channel has a finite bandwidth (and most of them quite narrowband for that matter) which constitutes a lowpass filter with a low cutoff frequency response. Due to this, the RF channel introduces a delay in the signal path in both directions. If you started
with a wired installation where whatever your PLC was putting on the bus was seen at the same moment by the other end, and whatever was replied by the network propagated back to your plc in very little time, now you might see delays in the (at least) millisecond range if not more (some transparent mode links need a few bytes time to turn from transmit to receive being half duplex over the air and full duplex only over the wires, this could mean even more delays ) for a roundtrip. There are RF links out there which work
transparently but there are no physically achievable RF links that work instantaneously .

2- what is the link supposed to do under error conditions ? Cables either transmit the signals or are broken and don't transmit , at least at the slow speeds we're talking about. RF links don't exhibit such a clear cut behaviour , there are always moments of severe interference (the question is not if but when and how often) . Think of a digital cell phone, the person at the other end is always able to tell when you're calling from your cell. When burst errors occur, RF modems attempt to correct them with FEC, slowing down the data rate, doing retries, etc . This all of a sudden changes the minimum delay from #1 to something open ended, usually a retry
time out . After a number of retries have failed the link times out , however 200 milliseconds later (or a half hour or year) everything might be
fine again . This is different from cable , how would your PLC react to this slowdown/error and recover from it when the link recovers? and when the link doesn't recover ?

Best Regards,
Matt Tudor , MSEE
Elmar Technologies
http://personal.lig.bellsouth.net/~mariusrf consulting
http://www.rfdatacorp.com spread spectrum radio telemetry

William L. Mostia, Jr

I would suggest that you talk to:

Data-Linc Group
David Mhoon
[email protected]

They make RF modems that plug right into a AB rack.

Bill Mostia
William(Bill) L. Mostia, Jr. PE
Principal Engineer
WLM Engineering Co
Independent I&E Consultant
P.O. Box 1129
Kemah, TX 77565 USA
E-Mail: [email protected]
Proxim uses the OpenAir standard which is _not_ full duplex over the air. There will be delays albeit short most of the time . They are forced to
use a form of handshaking over rs232 (they support hardware and software handshaking) . The original request was for a full duplex link with no handshaking required.
Very few RF links are full duplex due to the impossible requirement of transmitting and receiving simultaneously . This must happen on different channels or else the rx would hear its own tx only. Due to spectrum allocation regulations the 900MHz and 2.4GHz ISM units can't do this (there isn't enough channel separation for the duplex filter to work-the transmit frequency too close to rx frequency saturates the rx front end). As a consequence they can be made to look and feel almost like full duplex over the rs232 wired interface but really they're taking turns at transmitting
and receiving , with a lot of handshaking going on over the air and internal buffering for the packets .
There are inherent delays associated with the packet buffering so transparent is more accurately only "one way transparent" and incurs one way trip delays of 1-25ms . This might not be acceptable for an application that doesn't have handshaking.

Matt Tudor, MSEE
Elmar Technologies

Matt Tudor's comments about the transparency of RF links to various protocols are well received.

Allen-Bradley DH-485 is a protocol which cannot be transmitted over modems for the reasons Matt wrote about; trip delays, error correction, etc.
The DH-485 protocol requires such strict timing between transmission and reception that it's tough to run it through the Windows hardware layer, let alone over a modem.

DH-485 runs over twisted-pair copper or over fiber optics, but it cannot be conducted over leased-line, dialup, or radio modems.

Data-Linc Group investigated special ways to do DH485 over 900 MHz spread spectrum a few years ago, but found it impractical. I have seen a fancy microwave system built by the German military for mobile video still fail to conduct DH-485 reliably, where simple twisted-pair wiring will.

To put a mobile machine into a DH-485 network, you need to convert DH-485 to the full-duplex ansynchronous Allen-Bradley protocol called DF1, which is full-dupex, allows for transmission delays, and can easily be transmitted over radiomodems.

If you have a SLC-500 rack handy in a stationary position, you can use the 1747-KE module, which is relatively inexpensive compared to the tabletop
equivalent, the 1770-KF3. Either of those will behave as a node on the DH-485 and will connect to the serial port of a 5/03 or higher controller
via DF1. You assign the KE or KF3 the same DH485 node number as the processor previously had, and it will pass packets to the controller and it
will be able to communicate just as it was before.

In the overall function of the project, this is a lot of money and hardware to take away the need to plug the unit back into the DH-485 network. I'd try to devise some other way to remind the equipment operators to plug the system back in.


Ken Roach
Rockwell Automation / Seattle
[email protected]

Jerome W. Lavoie

BlackBox MDR100A-R4 910 mhz 20-mile radio modem (no fcc license required) MDR152-0100-R3 YAGI Antenna w/ 100 foot cable MD3310-R2 RS-485 input module for above modem. You would need 2 of each of the above. PS: Black Box gives 45 days to try it out and return at no cost. - Jerry Lavoie Automation Engineer Nucor Steel Corporation

Ben Brechtel

The "Communicator" (at www.controlchief.com) is the only RF device that directly exchanges data with the backplane of a PLC. It can eliminate potential bottle necks of serial connections, and is set up and utilized in RS logix by linking an I/O module. The module is allocated an input and output image table to Tranceive information (words) between one or multiple PLCs. This product has been developed with Rockwell and is successfully used to reliably and safely implement control with RF.

Scott Rusterholz

I would recommend you contact Sick Optics. They make a very reliable industrial modem. We use them on our AS\RS cranes at distances up to 300'.