GE PQMII Data Map To Redlion 1

[iwo]

Hi ALL,

I have a GE PQMII,Redlion Data Station & GE LM90 30 PLC

I am trying to map data out of the PQMII to Redlion using Modbus-RS485 and then using Ethernet (TCP/IP) to map data from Redlion to GE LM 9030 PLC.

I know Modbus uses hexadecimal and PLC Registers are in Decimal

I need a technical procedure on how to configure/program

Thank you
Chris
Ok Tedi Mining Limited
Papua New Guinea

 

[danw2]

Modbus is NOT plug-n-play; it is always a project. It is too complex to provide details, especially when considering the combinations of the 50,000 or 100,000 Modbus-enabled devices.

The PQM II Communications manual is here:

It is worth the read.

The PQM-2 is a Modbus RTU slave only. In your architecture, the Redlion has to be the Modbus RTU master. I believe that the RedLion can operate simultaneously as a Modbus master (fetching data from the PEM-2 and saving it in local registers0 and a Modbus slave, which the PLC can then read as a Modbus/TCP master/client. You might have to check with Red Lion on how to configure such. If not, then the Red Lion would act as a Master to read from the PQM-20 over RS-485 and write to the PLC over Ethernet, each over a different hardware bus.

Some suggestions on Modbus RTU:
Most RS-485 is half duplex, 2 wire. 2 wire is a falsehood, RS-485 should operate with a 3rd wire, a signal ground. Some devices use the chassis ground which can create a common mode ground loop over distance between the devices. An RS-485 isolator will solve that.

The data lines known as A and B or (+) and (-) vary is functionality from vendor to vendor. A should wire to A, B to B, or (+) to (+), (-) to (-), but sometimes the vendors have it opposite one another. Different divisions within the same company can opposite one another. If the drive lines are wired backwards, communications will not work, but the drivers will not be damaged. Swap the driver line wires on one end and it fixes the problem.

The serial settings on both ends need to be identical. Note that some devices do not recognize serial or comm setting changes until power is cycled to the device. Avoid the combination of no parity/2 stop bits, many devices can not support two stop bits. RTU is 8 data bits by definition. Modbus ASCII is not an option on the PQM-2.

RJ-45 ports are sometimes used for serial (485/232) connections. There are commercial adapters to terminal screws.

Do not use slave ID 0 (zero). Slave zero never responds to a query.

Modbus in general
register/addressing - the one offset.
Some devices start register/addressing at zero. Others start at one. So the holding register at (4)0109, might actually be at 0108.

The leading numeral in register references is not part of the Modbus message, it is a convention used so that humans (and some software) can distinguish between memory areas served by different function codes. For instance, Function code 03 reads holding registers in the (4)xxxxx memory area. Function code 04 reads input registers in the (3)xxxxx memory area.

Some master/client use the leading numeral to designate which Function Code to use. Others require defining which Function code and then (typically) only want the indexed value (minus the leading numeral).

Modbus only carries binary bits, it doesn't know what the bits mean or how the bits are interpreted. Interpretation is the job of master/client. Data format is extremely important.

Data formats
Modbus does not define data formats, the implementer/vendor does. The documentation should say what the data format is. It usually does but yesterday I ran into one that doesn't.

If your slave uses floating point, it is undoubtably IEE 754 formatted. There are 4 different byte orders but I've only ever seen two in popular use. Somewhere in the software there's a setting for whether a floating point value is interpreted straight or swapped byte order. If the wrong choice is used the data value will be wildly wrong, like 156.32 will be a ridiculous value like 1.26842274 E11.

Integers frequently are assumed to have a multiplier that the interpreter supplies, for instance 156.32 will transmit as 15632 with an assumed multiplier or 0.01.

Generic master/client
I really recommend using a PC based generic Modbus master, something like Modscan 32, Modpoll or Simply Modbus to 'prove' a connection between the master and slave, because of the number of settings that can hinder a working connection. There are few settings in the generic masters. There are other freebie generics out there.

Start by reading a known value (non-zero) from a register. Make sure that the value is what you expect, to confirm that you're not reading one register off. Unused registers are frequently zeroed out, which makes zero a bad value to expect.

Most PLC vendors seem to have a documented example of setup as a master or slave somewhere on the web.

Dan

 

[iwo]

Hi Dan,

Appreciate the detailed explainations-it really helped me work my way around

Chris