modbus Modbus AB Micro850 with Schneider Altivar Drive

[willings 0]

Hi all, I am trying to control and/or read values from Schneider Altivar12 VFDs. Via Modbus on a AB Micro850 PLC (using the Connected Components Workbench Software). There seems to be two ways to do this with the Micro850, either with messaging (using the msg_modbus instruction function block) or with the Modbus mapping feature. I have a list of the communication parameters (see attached fileATV12_Communication_parameters_BBV51917_1.4IE05b.xls ATV32 Communication parameters_a1.2ie03.xls) But I haven't had any experience with Modbus and I am not having any success. There is not very much documentation about how to do this, If anyone can help me who has done this or knows how to do it I would appreciate it.

 

[kaiser_will 65]

Modbus is one of the oldest controls communication protocols.  When I configure two different devices to communicate via Modbus, I experiment with Modbus registers at both ends.  Change values and verify reading at other end.  

I have run into issues before with devices in that one has to shift the Modbus address up or down a register to get to line up.  There aren't many tools I have round for sniffing Modbus packets.

[willings 0]

Thanks, that is the case, the AB Micro850 used a six letter code e.g 400001,  But the Schneider drives use codes like 16#2199 = 8601   So I am not sure how the two codes relate to each other?

[kaiser_will 65]

It is possible the Schneider VFD displays Modbus addresses as Hex, and the A-B PLC displays Modbus register addresses as Decimal.

8601 dec <=> 2199 hex

So keep in mind with Modbus that addresses can be DEC or HEX at one end, and understand the stacked memory registers of both devices.  Modbus is the communication pipeline and all possible memory registers, be they bits or words, is one complete stack.  The device manufacturer designed that device for all inputs to have a section of the stack, all outputs to have a section of the stack, etc.

Attached is a Modbus Memory Map I created for a Wago PLC - ICP DAS HMI project.  I read the literature, researched Modbus communication and experimented with writing and reading many tags at both ends.  Note that the ICP DAS HMI registers are off from my Wago PLC, the stack is shifted at one end.  If you can monitor memory registers at both ends, you can clear registers on both ends, toggle bits or write data to register words, monitor the other end, and take notes as you go.

Once you understand the Modbus memory stack and how both devices utilize the stack, you are home free.

There are some free Modbus tools (ICP DAS Modbus Master Tool is one).  http://www.icpdas.com/products/PAC/viewpac/download/viewpac/download_utility.htm

Lighting Control Tag List.xlsx