problem to comunicate plc with altivar 28


Thread Starter


We have plc tsx 3722 sv 5.0, a box tsx pacc01 and a variable speed controller altivar28.
I program plc using pl7 4.2. I want to communicate the plc with a variable speed controller altivar28, using modbus.
When ordering a communication (read_var/ write_var) the variador does not respond and the plc
is waiting for a reply, but there isn't message of error.
*Could somebody tell me what is the problem?.
*Could be the problem in the connection altivar to bus modbus?
We have connected altivar28 to the box of the following way

Box tsxpacc01 Altivar 28
0 Vl [2]------------[2] 0 V
D(A) [4]------------[5] D(A)
D(B) [5]------------[3] D(B)


Franck Lebas

You have a problem of Modbus communication between a master Modbus Micro PLC via its terminal TER port & an a slave Modbus Altivar.

I think that it is necessary to check 2 things:
1) the hardware configuration
2) the programm PL7

Check n°1)
- have you 1 or more slaves Modbus on your network?
If not, could you make the test with only your Altivar.
- have you try to permute the D(A) & D(B)wires?
- what is the equipment which polarize and make the resistance of end of line?

Check n°2)
- could you send to me your application?
- what is the report of the function of communication?

I reminder to you the characteristics of the Altivar28 :
- slave 1 to 31
- speed 9600 or 19200
- Parity none
- write & read of 7 words Max
- 8 bits / 1 start bit / 1 stop bit

Please, could you use my E-mail address <[email protected]> to reply to me.

Tech Support

Franck lebas

But be carreful, i think that the wiring of the Altivar depends of its version:

If your Altivar is < September 2001, your wiring is right, it means:
D(A) = pin 5
D(B)= pin 3
0 volt = pin 2

If your Altivar is >= September 2001, your wiring is bad, because the wiring has been modified and must be:
D(A) = pin 5
D(B) = pin 4
0 volt = pin 8

Could you make a test with this wiring?

Could you give to me the following information:

- Could you send to me your application PLC for that i check it?
- What is the length between the ACC01 box and the Altivar?

Franck lebas

Pascal Tondeux

First you have to verify if your TSX37 PLC uses MODBUS for the communication. It uses Modbus RTU master only if you have configured your PLC terminal communication for this purpose, and if the signal INL/DPT of the mini DIN is grounded, in your case by positioning the good switch of the TSXPACC01 box to "slave position" which in fact forces the configured communication mode.
When the PLC is in stop mode, if this switch is not in the right position, the yellow LED of the PLC flashes indicating UNITELWAY Master pollings. If the switch is in the right position, this led is lighted only when there is MODBUS exchanges (PLC in RUN mode and functions READ_VAR or WRITE_VAR active).
Pascal Tondeux.
The people at Schneider tell me that the wiring doesn't matter. I wonder however if there is a newer driver version in the Altivar 28 after september 2001. I have a project running with 4 Altivars in one RS 485 network, one is running ok the other three give time-out errors occasionally. 5% of the frames give this error.