Sending modbus command

I have been trying to send modbus commands to a plc with xbees in transparent mode without success, I can send commands from computer to computer with the xbees successfully using a modbus test program, and from computer to plc with cable. (everything works until I plug in the xbees) has anyone got any suggestions???

What firmware version are you using, and what baud rate?

As regards the plc circuitry, does Q7 of the Unofficial FAQ help?

Thanks for your reply john, I’m actually using series 2 xbees xb24b version 1247 and 1047(Znetcoordinator at and znet router end device at), the plc I use will only run at 9600 and i am using off the shelf serial interface board. I am only sending 11 byte packages.

To begin, if you are using the Series 2 radios, it is recommended to begin your development using the ZB firmware (latest 2x64).

When using ZigBee, it is important to realize how transmissions are sent and the timing involved.

By default, the coordinator will broadcast messages to remote devices. The remote devices (router or end devices) will then unicast their replies to the coordinator. The boradcast transmissions can be quite slow, while the unicast transmission is much faster.

Due to these timing issues, you must modify the timouts on your Modbus to better accomodate. Additionally, some Modbus protocols are more timing dependent than others. Modbus ASCII is fairly liberal when it comes to timing, so if you use that protocol, you will have the best chance at success.

If this is simply a point to point link, you can speed up communication between the coordinator and the router, by setting the DH and DL of the coordinator to match the Sh and SL of the Router. This will cause a unicast transmission to be sent from the coordinator to the device with the corresponding SH and SL parameter.

Moved this to the correct (ZNet 2.5) forum fyi…