Setting up a modbus network on a DL262 Directlogic PLC with Siemens LMV 3 Burner controllers as slaves, eight of them, with OCI 412.10 Interface

[Rsflipflop256]

Good morning fellow sea captains and wizards,

I am being asked to do the above and obtain 4 values from each slave, I know about the MRX and MWX instructions but never used them myself, have all the manuals and am just looking for tips and tricks to set this up as it will be my first time doing it.

Many thanks and mucho apreciao guys and gal.

 

[Steve Bailey]

Pay attention to data structures in the LMV3. If the data you need is included in a data structure, you have to be sure that your Modbus read multiple registers request spans the entire structure. Otherwise the Modbus transaction will fail.

 

[Rsflipflop256]

I need to read 4 items from each LMV, I've been given a V memory location to put it and its data type so no problems with that, Ive got a converter spreadsheet for koyo modbus register addresses. But im not quite sure how to populate the MRX ibox instruction itself.

My responsibility is only to get the modbus network setup and reading those particular 4 values from each LMV3 through the oci interface., 2 of which are BCD/Words and the other 2 are singular bits, bit 11 and bit 0 in direcsoft 6 ladder RLL. Any guidance would be more than welcome.

 

[Steve Bailey]

Do you have the Modbus map for the LMV3? If not I can make a copy of what I have and post it.
Or else tell me which four parameters you need to read and I'll tell you their Modbus address in the LMV3.

 

[Rsflipflop256]

Thanks much! I have the following:

Flame signal: Bcd/word
Error code: bcd/word
Hgps switch status: bit 11
Alarm status: bit 0

Would any others be needed/suggested in a burner control application? Its my first time doing this. Master is a DL262 plc.

 

[Steve Bailey]

Flame signal is Modbus 40014, scaled 0 - 100%
Hgps switch status: Modbus word 40036 or 40037 is listed as "Inputs". These are hard-wired inputs to the LMV3. Bit 11 is listed as "DW Max Gas" in the data sheet I have which dates back to 2004. If "Hgps" can be interpreted as "mercury pressure switch", a pressure switch set to turn on at some pressure measured in inches of mercury, that could be the high level switch while bit 10 labeled as "DW Min Gas" could be the low pressure switch.
Error Code and Alarm status: I see two data structures. One called "Lockout History" is 16 unsigned 16-bit registers. The first of those 16 registers is labeled "Error Code". The starting address is Modbus 400401. The rest of the registers are data related to that lockout. Then starting at Modbus 40417 is another structure for the previous lockout. There are structures for the eight most recent lockouts.
There is another 8-register data structure for "Error History" starting at modbus 40545. The first of the eight registers is the error code, the remaining seven are more data about the error.
Remember that you have to request the entire data structure. You can't request just the error code. You'll have to read all 16 or 8 addresses and sort them out in PLC logic.
You mentioned a burner control system. I was involved in a project about 20 years ago to control multiple LMVs using a PLC to bring them on or off line in sequence to maintain header pressure. As I recall, switching from keypad to Modbus control of each LMV turned out to be a bit tricky, but I can't remember the details.

 

[Steve Bailey]

To clarify what wrote about the Hgps switch.
I wrote that the inputs are either Modbus 40036 or 40037. The page from the LMV3 manual lists the inputs as Modbus 40036, but I have a written a note that says its actually 40037. There is a 2-register (32-bit) gap between the starting address for the discrete inputs and the starting address for the discrete outputs, so Siemens may have assigned enough memory space to accommodate 32 in puts and 32 outputs. You may need to do some experimenting to pin it down.

 

[Rsflipflop256]

Hgps is high gas pressure switch, thanks much for the info!

Hgps and Alarm status are just single bits, while flame signal and error code are words, and the DL series speaks BCD/Hex for some ungodly reason, ive gotta read up on how to actually slap rungs together to read these locations using the MRX ibox I assume.

 

[OkiePC]

The Directsoft setup for MRX and MWX is not as terrible as the RX and WX instructions... but dealing with BCD and Octal addressing makes them not as straightforward as modern PLCs.

 

[Steve Bailey]

I also see a "Current Error Code" at Modbus 40026.
And Bit zero of the discrete outputs is labeled "Alarm". You could monitor that with a hard-wired input to your PLC. That would allow a faster response since you would not be constrained by serial communications timing.
So, it looks like you cold get all the information you need by requesting Modbus 40014 - 40038 in a single "Read multiple registers" command.

 

[Rsflipflop256]

The Directsoft setup for MRX and MWX is not as terrible as the RX and WX instructions... but dealing with BCD and Octal addressing makes them not as straightforward as modern PLCs.

Any tips on that setup and config would be solid gold if you had some.

Thanks again

 

[Steve Bailey]

Here is a sample DL06 program using a MRX instruction to read eleven registers from Modbus device #1 every half second starting at Modbus 40002 and putting the received data to PLC V memory starting at V3000.

The Modbus slave in this case was was a Sure Servo drive which had the ability to customize the Modbus map and put whatever data you wanted into the first sixteen Modbus 4XXXX registers. For your application you will need to assign a unique Modbus ID (slave address) to each LMV and execute the MRX instruction with a different slave address periodically.

 

[Rsflipflop256]

That's something I can chew on, thank you!

 

[Rsflipflop256]

Here is a sample DL06 program using a MRX instruction to read eleven registers from Modbus device #1 every half second starting at Modbus 40002 and putting the received data to PLC V memory starting at V3000.

The Modbus slave in this case was was a Sure Servo drive which had the ability to customize the Modbus map and put whatever data you wanted into the first sixteen Modbus 4XXXX registers. For your application you will need to assign a unique Modbus ID (slave address) to each LMV and execute the MRX instruction with a different slave address periodically.

Is the slave address something to configure also on the LMV?

 

[Steve Bailey]

Yes.