Prosoft MVI69-MCM CompactLogix Modbus

[mrmss]

Hi,

I'm trying to integrate an MVI69-MCM module into my project. I need to be a Modbus slave and I need to send and receive data. I'm really confused about how the Modbus addresses map to an Address in the PLC.

Anyone have an example they could run me by?

Main area of confusion I think is with the Backplane Read Register. Following the example I end up with an address of 500 but nothing I change in my Modbus Master simulator shows a change in the PLC. I changed this address to 300 and I can see values I change being written into the PLC, however if I write certain registers I end up clearing a different register - which implies I have an overlap.

I have my Backplane Write Register set to 0 and 200 values, this works perfectly fine.

A sample project with some documentation would be great help.

 

[mrmss]

Got this going now, tip - use the Web chat on Prosoft website - its absolutely awesome...even if Sylvia is actually a man (which they must be, surely)...I dont care

It is really really confusing...I had already read the 160 page manual three times but I'm just not clever enough to work it out on my own - tech support via Web chat set me right in no time.

Starting with the Prosoft Configuration Builder Backplane settings. The Read Register Start and Write Register Start have to be multiples of the Block Transfer Size (which can only be 60, 120 or 240).

Preferred defaults (which for some reason arent the programmed defaults);
Read Register Start : 240 # Starting register in virtual database to write to processor
Read Register Count : 240 # Number of registers to write to processor
Write Register Start : 0 # Starting register in virtual database to read from processor
Write Register Count : 240 # Number of registers to read from processor
Backplane Fail Count : 11 # Number of consecutive backplane transfer failures before halting communications
Error/Status Block Pointer : 3000 # Number of register to store error and status
Initialize Output Data : No # Initialize the database file on startup
Block Transfer Size : 60 # Data size for BTR/BTW 60, 120 or 240

The trick is, to write to a Coil (0xxxx range) from the master, your first Modbus address is ReadRegisterStart*16, so in my case the first Writtable Coil address is 3841.

3841 (in Master) = ReadData[2].0 (in RSLinx)

It goes to ReadData[2] (i.e. not [0]) because I put my Output Offset (for Function Code 1) at 2.



Writting Holding registers is just the same (4xxxx range). From the master, your first Modbus address is ReadRegisterStart, so in my case the first Writtable Holding register is 40241.

40241 (in Master) = ReadData[40] (in RSLinx)

It goes to ReadData[40] because I put my Hold Offset (for Function Code 6) at 40.



Oh and don't forget the Modbus out-by-one rule, first Modbus register is e.g. 10001 but in the PLC this would be 0.

Clear as mud?

 

[Dravik]

lol, i've setup the SLC version of the same module. Yeah.. took a bit to figure out

 

[tomalbright]

First Prosoft Modbus module I worked on, back when they produced one product, I called the company and asked for the "application engineer who wrote the impossible to understand manual". They put me through to him. We got it straight, using some rungs listed in the manuals, with corrections made. Trying to do it myself? At that time, it was rough.

Now they have the manuals straight, still, the best thing to do is use what's on disk. The examples they supply are tried and true, and trust me, repeating that headache now is one I don't want to try again.