I have a Beijers (Mitsubishi) E-615 HMI communicating Modbus RTU to 8 Control Techniques
servo drives. It functions well except if I try to communicate to more than 1 drive on a screen the update rate slows down to 3-5 seconds! If there is only 1 drive being accessed on a screen the update is Fast! Any thoughts?

 

The cabling may be poor or may not be terminated. Try terminating the line. Because of this one or more slave devices may not be responding at first or never at all, so the master would timeout and retry. A second thing that may happen is your master command may be reflected back through the line because of poor termination. A temprory solution that would speed up a little could be reducing the request timeout of the HMI.

 

Just a thought, but....

The speed issue may be due to how the HMI reads data. Does it read all data all the time or does it only read data for the active elements displayed? If it only reads data for displayed graphics then the response time will be directly related to the number of displayed graphics.

 

My suggestions:

1) your HMI is likely sending requests too fast - some devices have crude timers to disable serial port during "other slaves" request/response. I have seen this in many products - for example I can poll 1 slave 10 times a second, but as soon as I switch to more than 1 slave, the number drops to 4-5 because every "other" slaves starts a 150msec timer to disable their serial port while my target slave response. Your solution here is to experiment (if possible) with a delay time between the last RESPONSE and the next REQUEST. You may find a simple 10 or 20 msec delay will solve this problem. Without this delay, likely what is happening is the "next slave" misses part of the first few bytes, so your 3-5 seconds is the "slave timeout". But the time this 3-5 seconds has gone by, all slaves are on-line, so a 2nd retry of this slave works. See if you have any time-out stats in your HMI - I'll bet you are seeing a near 50% timeout rate. If the HMI doesn't have an explicit delay, see if it has some RTS/CTS radio modem settings you can use to mimic this - for example a 10 msec RTS setup time will delay a 2nd request at least 10 msec.

2) Some products which non-Modbus native protocols do Modbus in BASIC modules or other means which cause Modbus to be (ooh sorry) slower than their native protocol. I have heard this complaint about the small PLC from a certain big PLC company. It supports Modbus, but when in multi-drop, the Modbus performs much slower than is reasonable. They no double want people to stick to their own protocol. This IS NOT LIKELY your problem, but it is something to be aware of.

best regards

Lynn August Linse, [email protected] IA Firmware Specialist, Digi Int'l (www.digi.com)