Hi All,

I am facing the problem of delay wrt Modbus RS-485 communication between S7-1215C PLC with CM-1241 Module & Danfoss FC102 VFD's. In my Project i have 7-VFD's where those has to be communicated via Modbus Communication (RS-485), i was succeeded to establish the communication between these 2-Devices, but the problem is updating the values in VFD's are getting delayed, so that it is affecting the operation of the system. Here PLC is Master & VFD's are slave. I have configured "MB_MASTER'' instruction in my program and tried executing these instructions cyclically & periodically on both cases i am facing the same problem. But if try to communicate 1or2 VFD's speed is OK, but if i try to communicate more than 2-3 VFD's i am facing a problem of getting delayed. Can anyone help me regarding this problem.

Regards,

Santhosh.HS

Yes, modbus 485 is not the fastest protocol. End of frame is detected by silence... My results wete that indeed with 2 vfd you can get update speeds 50 - 100 ms. You can optimize a bit by shortening the 'inter gap delay' (don't remember) and increase kbps but it won't be fast, 40 ms per station iirc.

Split he bus with max. 2 vfd per bus will do the job. Or check if better protocol is available.

Santosh1989

Hi All,

I am facing the problem of delay wrt Modbus RS-485 communication between S7-1215C PLC with CM-1241 Module & Danfoss FC102 VFD's. In my Project i have 7-VFD's where those has to be communicated via Modbus Communication (RS-485), i was succeeded to establish the communication between these 2-Devices, but the problem is updating the values in VFD's are getting delayed, so that it is affecting the operation of the system. Here PLC is Master & VFD's are slave. I have configured "MB_MASTER'' instruction in my program and tried executing these instructions cyclically & periodically on both cases i am facing the same problem. But if try to communicate 1or2 VFD's speed is OK, but if i try to communicate more than 2-3 VFD's i am facing a problem of getting delayed. Can anyone help me regarding this problem.

Regards,

Santhosh.HS

 Hi

Modbus RTU-485 protocol is not fastest protocol, as number of devices will increase your speed will decrease.

try to increase your Baud rate of communication & try to decrease distance between slaves & master if distance is more than 10-20 meter.

by doing this you can increase response time by negligible hike only, also try to optimize your slave multi-dropping technique also lay your RS-485 cable isolated from power cables it will helps to minimize noise issue.

try it may be useful for network

no other option to  

Hi,

I have using Baudrate of 19200, and distance of VFD's are almost 20-25mts. In my case 7th VFD details will be updated in my PLC at sec (5-6sec) which is not at all useful. But with other controllers with same protocol with the same VFD's communication is comparitively fast, hence i am in serious problamatic situation.

Regards

Santhosh.HS

Santosh1989

Hi,

I have using Baudrate of 19200, and distance of VFD's are almost 20-25mts. In my case 7th VFD details will be updated in my PLC at sec (5-6sec) which is not at all useful. But with other controllers with same protocol with the same VFD's communication is comparitively fast, hence i am in serious problamatic situation.

Regards

Santhosh.HS

 Other controllers meaning non-s7-1200? 5-6 sec. is a bit too much, something goes wrong. Hard to tell what without code or monitoring. Guess retries on wrong config vfd?

If you careful tune comms and sequence my est. is 1 - 2 sec upd with 7 stations. Eg cyclic polling status of vfd's with event driven write priority to the vfd's.

Hello,

your info is about some delay for data. I'll be start investigation from first device, but attention can be looking to exact step for request to 3-rd Drive (probably what cause that delay) and watching TX/RX status on Cm card, or monitor internally SEND/RECV status/ERROR. Info about your defined max timeout for receive data is needed. My idea is divide 2 situations:

1. Send request-ok, receive message - ok in time defined. In this case question is what data you receive, correct or some message about problem.

2. Send request-ok, receive delayed - RX light come after defined timeout, or after some time but inside defined timeout, maybe will not come and another request is initiated.

To this situations I'll be pay attention, investigation.

Duris_I

To this situations I'll be pay attention, investigation.

..to add - how about the terminating resistors?

Hi All,

Please find the attached file of my program with the VFD's, in my case i am "Reading- 4 Data's"(Voltage-416120, Dint, Length-2 ; Frequency-416130, Dint, Length-2 ; Current-416140, Uint, Length-1 ; Kw-416100, Dint, Length-2).

"Writing- 2 Data's"(Control Word-450000, Uint, Length-2 ; Speed Reference-450010, Uint, Length-1).

I am used individual "MB_MASTER" instruction for each registers, and it has been executed one after the other, by "Done" or "Error" bits of each registers instructions.

Baudrate : 19200, Parity : 0, Stop Bit : 1.

Regards,

Santhosh.HS

Hi, try Ola's code:

 https://support.industry.siemens.com/tf//WW/en/posts/communication-between-s7-1200-and-danfoss-fc302-how-to-setup-more-than-one-drive-and-how-to-set-the-commands-speed-etc/206062?page=0&pageSize=10

with minor code tweaks it works fine