modbus

[can]

Hi. I'm trying out the modbus ascii communication between 2 MD888 using RS485. When Master 888 input 1 is on, output 1 on Slave 888 should be turned on. Sometimes the slave 888 will have a delayed response. Any way to improve the response of the system? I've put a 0.1s clock to write and read from the MAster to the Slave currently. Any problem with that?

Thanks

[support]

When the master send a command to slave, and if the slave doesn't respond before time out, or if the slave send back a bad response (wrong LRC), then the master will re-transmit the command up to 2 additonal times. What you think as "delayed" response is probably a response on a resend.

1) How many devices are on the RS485 bus?
2) Are you using only Modbus ASCII protocol or are you switching between protocols?
3) How long is the distance between the PLCs?
4) For longer distance, try to use biasing resistors to put the RS485 in a known state and you may need terminating resistors.

[can]

1) 4 devices namely Md888 master, exp4040, md888 slave, and I7018
2) communication between 888 done in modbus ascii and i7018 using netcmd
3) both plc and i7018 next to each other

Problem with the I7018? any way to work around?

[support]

Why don't you remove the I-7018 and temporarily disable access to I-70xx and see if the communication improves? If so then it means using two protocols on the same network is giving problem.

Try to insert a delay of at least 10ms between each command being set to different slave devices.

Best solution - use F-2424 PLC which has two separate RS485 ports.