Rs 485

hey guys

I am in a status which I am confused in bits and pieces. Hoping you can help me on these.

I have a unit called Elca which has both Modbus and RS485 comports. I need to connect this device through Compactlogix PLC to change only one variable while monitoring couple of variables.

So I either will use PRMVI69-MCM using modbus comport or HMS Anybus Communicator using RS485 port. Modbus comport is in use with control panel which behaves as Master. So I thought I cannot use it as there is already a master. So I purchased HMS Anybus Ethernet Serial Gateway instead.

When reading things on the net trying to get the idea how RS485 works I am in pieces which put me into situation that i did things wrong. At some point it was saying rs485 is only an electrical circuitry and it still needs a communication method to work with. Even if use RS485 port will still modbus be in use?

RS485 is an electrical standard for transmitting information. It doesn't care what protocol is used.

Modbus is a protocol, a definition of how bytes transmitted are used to convey information, especially requests and replies for information from a PLC.

If you want to provide a link to the manual for the 'Elca' we can help interpret and explain.

Attached file is the register map for modbus, but also from page 14 it gives the comports info of the device that can be used. At the moment X105 is used by control panel device. only port is x104 which can be used as rs485/rs422.

Below is url that has a little info about the device

http://www.elcasrl.com/aluminium_anodizing/full_interface_to_external_plc.html

The "do not use" statements seem to rule out the use of the X105 and JP101 ports.

I think the key to the use of the remaining port, X104, is in the statement, "The port configuration can be changed to meet client’s needs by entering function “Com Settings” of the “Options” section in the main menu" which is not part of the .pdf file.

I'm guessing that the function of the X104 port is defined in the Com settings, which must be more than the usual baud rate, parity, etc.

The diagrams showing multiple units in a daisy chain would infer that each unit functions as a Modbus Slave.

Note that the available COM registers correspond to the unit's operating mode. You'll have to constantly monitor/control the wRunMode register (offset 2) for manual/remote/ramper/or APC mode. (page 7)

Biasing might be accomplished through resistors in the 'other' RS-485 device (the Modbus master), but termination is usually done at each end of an RS-485 link.

Modbus usually uses register addressing, but this unit might use 'byte' addressing.

On page 4, it shows

a FLOAT, fVolt, Output voltage at offset 8
FLOAT, fAmpere, Output current at offset 12
FLOAT, fTankTemper, Tank temperature (°C) at offset 16

The offset refers to the memory location with respect to zero (for holding registers, register 40000)

A floating point value is 4 bytes or 2 conventional Modbus registers.

If the offsets are truly indexed as byte offsets then the unit might put the values for fvolt at 40008, fampere at 40012 and fTankTemper at 40016.

But if it's conventional Modbus but uses unconventional terminology for its documentation, then fvolt would be a 40004, fampere at 40006, and fTankTemper at 40008.

Whatever Master you are likely to use is going to use the standard 2 bytes/Modbus register.

So can I use still Anybus Communicator module or should I use PRMVI69-MCM instead and look for a solution to use two masters in the same network?

I don't know.

What communications settings are available that are referred to in "The port configuration can be changed to meet client’s needs by entering function “Com Settings” of the “Options” section in the main menu"?

What are the choices for configuring port X104?

After scrolling options

Baud rate, Parity, Stop Bits, Station number and Port(RS 485 or RS 422)

can be set

I would conclude that the device is a Modbus Slave that uses port X104 as an RS-485/RS-422 Modbus RTU port.

That means that you'd use the appropriate Modbus Prosoft module in Modbus Master mode with the PLC.

Thanks for the information so far...

Lastly if I use prosoft module as master will it not be second master module in the circuit as remote control panel has been already connected as master via port X105?

Yes, The rule is only one Modbus RTU master on a 485 network. There are specialty gateway modules like Miille that can be used to manage two RTU masters.

Your control panel master, whatever that is, will have read and write the data as necessary. It's my impression that Prosoft modules perform as either master or slave, depending on how it is configured.

If use prosoft as slave can I able to change a parameter on other slave while master is commanding same parameter?

If i read your question right if you want to change a device parameter through the prosoft card you will need to do it in a round about way.
You can set the parameter value into one of the read registers in the prosoft slave.
The modbus master will read that register out of of the prosoft card.
Your code should test the parameter data received from the prosoft card against what parameter data resides in the other slave.
If it is different then your code will instruct the modbus master to write the new parameter to the other slave.

Your control panel master, do you have the option to edit it's code?

After the getting idea of RS485 and modbus method from "bernie_carlton" and "danw" I was also in touch with Elca.

Now I got the information from them that Elca unit can support two masters at the same time. One is already connected on port x105 so I can connect my master on X104 as well. They said both ports are fully insulated and independant. It seems it will make my job even easier because i was initially thinking to put all elca units (3) in the same network.

So I will try when I get prosoft module on the PLC rack and see what happens from there.

Thanks for the information you shared with me