Xtend 900 - Rs232 Polling


I have a Base modem connected to the Master PLC which will read and write to 3 Remote Modems Connected to 3 Distinct PLCs.

2 of the remote Modems are within LOS of the Master Base Modem, but the 3rd Modem has NO LOS with the base modem, but does have LOS with one of the Remote Modems.

Im polling each via RS232, Im using polling to make sure data gets thru one at a time with no conflicts. Im using MODBUS protocol, please take this into account.

I need to know what parameters, and Pins to set so that the remote modems can re-transmit , so the modem that had no LOS with the Base modem, can answer back all the way to the Base modem. Is this possible? If not… what other configuration will you suggest?

*I Im using solar cells to power this units, so battery is a big issue for me. I need to poll at the most Once every 10 seconds, preferable more… but I need al modems to reliably respond to the requests.

Modems been used: XT09-PKI-RA , total 4 modems.

What you will do is place the radios in user defined mode (switches 5 and 6 up). (Switches 1 - 4 are for RS232 (1 on 2-4 off)).

Manually set the radios as fallows:


Remote 1

Remote 2
DT 1
MD 5

Remote 3

Remote 4
DT 1

This allows the base to send to all remotes and all remotes to send back to the base. This also allows remotes 1 - 3 to act as a repeater to get the data to remote 4.

Thanks, I already Programmed the modems, Im just making my rs232 Null modem cables.

  1. I noticed on remote 3, you went straight ahead for My 5 and not my4, is there a special reason for this?
  2. This configuration is Polling? or is it repeater, or are they the same all together?

3)Also y have only 4 modems, including the Base modem.
So I can go right ahead and scratch out the Remote 3 config you told me, and replace it with the Remote 4 (Modem with No LOS with base, but LOS with another remote) modem. Correct?

Thanks in advance for your support

Nope, no reason at all. In reality, the source address just has to be unique.

That is for a repeater network. Polling is where the Polling master allows for a node with a matching address a time slot in which it can send the data. Once that time slot is over, the node with the next address is allowed to talk.


I went ahead and used the Repeater Mode, but had no success. Data was Coming in and out thru the master plc, but intermittently 5-10 minute Lags.

I switched to Polling, and instantly had a better result. Data comes out great, fast, 1-10 seconds, but here it gets tricky:

I have a PLC Master Modbus using rs232, connected to xtend modem via rs232 too. The master PLC Reads/Writes to 2 Slaves PLCs using 1 radio each, and 1 Slave for data logging, Also using rs232 ports.

When the master PLC is set to WriteModbus Commands… It works flawlessly… BUT. When I run the ReadModbus Commands… it doesn’t Read… but continues to write.

Will this have anything to do with the Buffer Size on the Modem side?

Why will it Write, but not read to/from Slaves?

Does Polling Mode, work even if a Remote modem has no LOS with the Base Modem, but does have LOS with another remote modem? will data go thru?

No the buffer size of the xtend is about 2K so that should not be an issue. Normally Modbus is a simple plug in and let it go type of app for these radios. Now the repeater function does add additional latency and decreased through put that does need to be accounted for in your timeout setting of the modbus. Did you remember to adjust for that?

No I don’t think that repeater mode and polling mode will work with each other as you are setting the same command to different functions.