Intelligent Modbus Gateway Required

Hi,

I have a situation at my plant, where I need to interface my existing PLC with some field devices Via Modbus Interface. I can not change the program of existing PLC, but I have Modbus slave addresses and register details that are required to communicate with field devices. I want to connect PLC with a gateway that can read the Modbus requests from PLC for each slave (via a single Modbus RTU Communication Port) and poll it intelligently to new addresses via ethernet interface. Attached is a snapshot. Can someone refer how can I achieve this gateway functionality?

Welcome to the PLCTalk forum community. That is a fascinating challenge !

What is the physical type of connection from the PLC to the data collector; RS-232 ? RS-485 ?

The Red Lion DataStation is what I would reach for by default for this kind of application. I believe that the "Modbus Gateway" driver allows you to define multiple Slave address devices on the same physical serial port.

But I'm not an expert in using that specific driver.

Physical connection from PLC to gateway will be RS-485.

Gateway acting on the one side as Modbus RTU Slave and on the other as Modbus TCP master exist on the market, but I doubt that there is one that does that conversion of addresses that you request.

A raspberry pi running custom software could do it ...

The Red Lion DataStation does exactly the sort of thing your diagram describes: connects a block of one data address to block with a different data address.

Does the PLC read data from the Ethernet-connected devices, or write to them, or both ?

I have not used the "Modbus Gateway" driver in Crimson 3, but it appears that it might do what you need.

Hi Ken,

Thanks for the suggestion. I will surely look into Redlion Data Station. For you information, PLC will be doing both read and write tasks via Modbus RTU.

I immediately thought of the Data Station Plus until I saw that there were 16 slaves. I bet you'll run out of 'ports' on the serial side real quick.

I think this is the situation:

PLC in the past has read the requested addresses from multiple slaves in a sequential fashion but now the slaves have changed and the new slaves have different register addresses for the 'same' data:

So the idea is for the PLC master to issue a polling command to a gateway whose registers are mapped like the old slaves. Yes, but . . .

Problem is, I don't think Data Station Plus or any device I know of has multiple slave node ID's on any given single RS-485 port. Typically each RS-485 port will have its own unique Modbus slave ID. Data Station might have a couple 485 ports, but it's not going to have sixteen 485 ports that can be individually given a Modbus slave node ID number.

But if such a gateway exists, then the Master side of the Gateway runs Modbus/TCP over Ethernet and polls each slave node ID in turn, mapping its received data to registers associated with the slave's node ID.

This is probably doable with a Data Station Plus for as many serial ports as it has, since I am unaware of any function that allows a given serial port to have identity of more than one Modbus RTU slave node.

ICPDAS has a bunch of Modbus boxes. I'd call them and see if they have a 'multiple serial port (8? 16?) gateway box where each port is a Modbus slave ID.
https://www.icpdas.com/

Or maybe someone's come up with a 'gateway' box that sniffs the serial side, and is smart enough to reply to any slave's request as long as it's a 'registered' slave and to reply as the polled slave. The master side appears relatively easy, just sequentially polling the new slaves and re-mapping the data for the serial side.

Another thought is Miille Applied Research who has done Multiple RTU master boxes that regulate the master throughput to a slave (maybe slaves plural?). They might have come up with a solution for this situation.
https://www.miille.com/miille-company-information.htm

Of course, I could have misinterpreted the whole scheme, too. Wouldn't be the first time.

I took me awhile to recall where I'd read it and figure out a search term - 'sniffer'. Not that you need a sniffer, but the thread was about a serial sniffer.

Check post #7 on this control.com thread requiring multiple RS-485 ports - jschulze is the resident Modbus pro on control.com and he mentions the Red Lion Flex Edge DA70A with three RS-485 comm sleds added. So maybe Red Lion is a player here.

https://control.com/forums/threads/sniff-8-rs485-modbus-rtu.48925/

Dan,

I think that "multiple Modbus RTU Slave IDs on the same physical serial port" is exactly what the "Modbus Gateway" driver in Crimson does. You see in my screenshot above that I was able to put 16 "PLCx" devices even on the RS-232 port.

I have a DSPSX and an RS485/USB dongle, but lack experimentation time.

I wish Red Lion had some better driver-specific documentation. Crimson has the User Manual and the Reference Manual but neither of them have good driver-specific protocol, command, or feature documentation.

Hi Danw,

You got it right. I have one port and want to implement sniffing on that for 16 slaves. I appreciate your quick support. I am checking details with Redlion regarding the exact functionality I intend to implement.

Ken Roach said:

You see in my screenshot above that I was able to put 16 "PLCx" devices even on the RS-232 port.

Click to expand...

I did not understand the graphic representation, until you made the connection. Thank you.

Small Modbus TCP and serial cabable PLC between would work.
Of course you need to code TCP master to this gateway PLC and make address mapping to different addresses on coding.

Hi Lare,
But how Can this serial capable PLC read 16 different Modbus slave requests on one serial port?

DayV said:

Hi Lare,
But how Can this serial capable PLC read 16 different Modbus slave requests on one serial port?

Click to expand...

One after one. As every slave have different slave number, you only need to sequence you reading / writing to PLC serial port on you plc master logic program.

Lare said:

One after one. As every slave have different slave number, you only need to sequence you reading / writing to PLC serial port on you plc master logic program.

Click to expand...

Post #1: "I can not change the program of existing PLC"