Modbus RS485 Unilogic and Toshiba Invertes

[Lenon Patrick]

Hello friends.
I am using a USC B5 TA30.
This PLC is communicating with 4 frequency inverters in RS485.
I managed to establish communication but it is very unstable, the problems are as follows:
-It does not update in real time, it takes a few seconds because I need to be updating the routine call manually;
-All the inverters are configured correctly and work individually just by changing the cable between them, but when I connect them all there is no command of two or more (each with their own address);

Some data that may be relevant:
-All equipment is functional;
-They are Toshiba inverters of the VSF15 and VF-FS1 lines;
-The distance between the components is small and I'm working with 9600 Baud Rate, Parity None, Stop Bits 8;

I think the problem is in some configuration in the call of routines related to time or ModBus configuration of the UAC-CB-01RS4 cards, the RX / TX LEDs are blinking and very fast as usual in OK communication.

I already have an HMI in the same communication working on the other UAC-CB-01RS4 card, it also has a little bit of instability, but it reads and writes the data on the PLC.

can anybody help me?
Thank you very much and I wish you well!

[Isakovic]

Try to increase "Every Period" time, that helped me in similar situation.

I don't know if it is still a thing in UniLogic like in VisiLogic, but also break rung 5 into more rungs. Cramped rungs used to make issues.

[Ausman]

Don't forget that modbus via 485 is not inherently fast.  If you are wanting things to happen in "real time" then you are going to be struggling using 485.  Even if you limit timings to the bare minimum, you are still not going to get "real time" operaton, there will always be delays.  As Isak says, 100ms is not likely to be anywhere near a long enough time.  It further compounds issues if your system does a retry on a failed attempt.  Total times on all attempts to one ID have to be allowed for, and then a tiny bit more to allow buffers to fully clear.

"The distance between the components is small".  Does this mean you have all the VFDs in the same enclosure?  Bad move if you do.  Any plc needs to be shielded as much as possible against likely interference from them.  Aside from your timings problem, it is one of the first things to look at on a system that has intermittent odd issues and runs VFDs.

cheers, Aus