Hi , Does anyone experienced with DF-1 Half-duplex message-based communication? I'm gonna using Radio Modem to transmit data from Master PLC to Slave PLC , and the Slave also shall be able to transmit data to Slave . Is there any guideline on doing these?It's better to have a sample code and guideline to establish the communication link and configuring the PLC. The controller is Compact Logix .

Thank you......

I have never used AB's special modes for doing this stuff. I have used Aerocomm's modems extensively for doing wireless DF1 without any noticeable problems in regular old full duplex mode. They have a range of several hundred feet in a plant with lots of RF around or miles outdoors. They are sold through Mouser (www.mouser.com) for $110 a piece. This is just about the best price point I've been able to find. The PLC's won't even realize that the radio modem is there.

Word of warning: buy the 900 MHz frequency hopping models. That is one of the things that attracted me to them. Frequency hopping is much better at dealing with interference in practice. The lower the frequency, the longer the range and the greater the ability to "punch through" walls. Higher frequencies generally allow you to use more bandwidth at the expense of reduced range, but serial protocols don't have that problem. At 900 MHz, a 1/2" steel plate is still mostly transparent. At 2.4 GHz, even a couple layers of drywall becomes a significant barrier to signals. In addition, 2.4 GHz is very close to the frequency that microwave ovens use. For serial communications, there is no advantage whatsoever to 2.4 GHz.

What you don't want to do is to buy the RS-485 version of these modems and attempt DH-485. It won't work and AB specifically advertises that. DH-485 is very timing sensitive and the delay of a radio modem is enough to make it fail.

Hi thanks for your prompted reply ,

I already purchase the esteem radio modem , it is 2.4Ghz frequency . Do you know how to do message-based logic? My deadline is closing soon , hope can get any solutions here .

Thank you very much

On a somewhat related note, a couple years ago I was out shopping for land-line cordless phones. They had 900 MHz models, 2.4 GHz models, and even 5.8 GHz ones. The salesman asserted that the higher the frequency the longer the range, and since we have a big backyard we had better choose from the (most expensive) 5.8 GHz models. So did he have it dead wrong? I would expect voice transmission to be pretty low-bandwidth.

I have done several scada projects using 900MHZ spread spectrum modems. 900Mhz, 2.4Ghx and 5.8 Ghz all end up being line of sight for long transmission distances. Typically you are limited on power as the frequency goes up. I know that 900Mhz RF modems can only have 1 watt of power for transmission to comply with FCC regulations.

You guys sort out which of you is asking the question, Controlian or B Hong.

Since Controlian posted the original post, I will answer that yes, I have experience with CompactLogix DF1 Half Duplex message based and standard polling communications. So do many other members of the Forum.

You're going to have to post more about how much data you are transferring, and how often. Posting the number of slave devices and the exact manufacturer and model of radio modems you intend to use.

There is a very good and very thorough book from Rockwell regarding telemetry applications using Rockwell controllers. Download or purchase a copy of the SCADA Application Guide, publication AG-UM008.

The problem with doing this the first time is that there are lots of little details in terms of configuration. The PLC baud rates, modem baud rates, connectors...all of these can drive you up the wall unless you go after it all systematically. Since there are fewer complications if you initially get everything working on a PC (laptop), I recommend getting the radio link initially set up with PC's and then gradually introducing the PLC's into the problem.

Step #1: Use 2 PC's. Make sure you can communicate with the radios. This simply avoids any issues with the PLC's initially. Simply set up Hyperterm to communicate via COMx and verify that everything works...when you type on one end, it should appear at the other. Use whatever diagnostics are in the modem if possible as well. Set up everything with a common baud rate (usually 19.2 kbps is as fast as you can get from most AB PLC's) and 8N1. Turn all handshaking off...you can optionally use it later but initially get rid of it so that you don't have to put up with the problems this creates.

Step #2: Switch one of the PC's for the PLC. At this point, you should be able to talk to it as if you have a serial port plugged directly into CH0. Usually at this point you need a DB-9 to DB-25 cable or converter. This just verifies that you got all the cabling and configurations correct.

Step #3: Do the same thing again with the other PLC. Now you verified that everything is working correctly everywhere.

Step #4: Write a simple "Read" MSG block. Trigger it with a timer say once per second. Watch the lights on the PLC's, the modems, and monitor the ER & DN bits on the MSG block. If you got everything working correctly above, you'll be all set.



The free space path loss equation (basically, ideal conditions and line of sight communication) is:
32.4 + 20*log(frequency) + 20*log(distance)

At least within the ISM bands (900 MHz, 2.4 GHz, 5.8 GHz), the FCC has almost entirely frozen the maximum power output at 1 watt.

However, you can trade off coding (including spreading in spread spectrum systems) for increased range...it's the old speed vs. distance problem. The bandwidth of the BAND is 26 MHz @ 900 MHz, 83 MHz @ 2.4 GHz, and up to 125 MHz @ 5.8 GHz if memory serves correctly. If you calculate the bandwidth/spreading advantage, it turns out that in spite of the increased bandwidth, lower bands still translate into longer ranges for the ISM bands, if you are pushing bandwidth limits. If the modulation format is not spectrally efficient (most phones aren't because they don't have to be), the opposite usually happens.

Also, higher power obviously increases distance. However, the FCC really hasn't made any room to play with higher/lower power. It is pretty much fixed. One significant difference though is that achieving higher output power is more difficult as frequency goes up.

And, antenna gains also help. As frequency increases, for the same physically antenna size, the "aperature" or gain of the antenna increases (assuming of course that the antenna design is not bandwidth dependent). This is a huge difference if for instance the only area you have to work in is the "bomb bay" of a laptop.

Finally, any electrically conductive object that is less than 10% of the wavelength is effectively transparent...LOS still applies even if it is no longer optical LOS...as frequency increases, walls become more opaque.

Uncompressed telephone quality voice is 64 kbps. Applying "simple" compression gets you down to about 16 kbps. CELP is 4kbps. Some very advanced voice compression algorithms achieve acceptable quality down in the range of <1 kbps.

Please reply to me Controlian ,
This is a water telemetry system , there are 16 slave device and 1 master station , Slave shall be able to transmit the data to Slave . The dats is sent when the specific condition occurs ( Tank level deviates 0.5Feet).

Example : Tank level of Device Slave 1 deviates 0.5feet , it shall send the data to the Master device(for monitoring only) and to MCC(Slave2) to control the pump start or interlocking.

The Radio Modem is Esteem195es , currently i'm just using Radiolinx FHS for mock-up testing . Esteem will be the actual products in this project.

Please suggested me if you have any solutions on these , i am new to AB PLC , deadline coming soon.....