Connecting Modicon 984 PLCs Using RS485

[r6lpq8twg52]

Hi,

Seeking consultation on an implementation matter, and have a question about Modicon Compact 984 communication through RS485:

Three Modicon PLCs are operating on an extrusion machine, employing a Modbus Plus network, looking through the ladder logic, there seems to be a lack of MB+ commands, indicating a viability for a Modbus TCP implementation. The objective is to gain independence from deprecated hardware (416NHM30032 and its analogues) and to develop infrastructure which facilitates further upgrades.

Currently, the prototype configuration involves a computer (Running Wonderware and a DAServer) and a gateway to translate from MBTCP to MBRTU, connecting to a PLC through RS232, this means that there must be three gateways per machine on seven machines. The assumption is that it is preferable to have one gateway per machine which then communicates through RS485 to three RS485/RS232 converters and hence to the PLCs. As the age of the hardware suggests, cost is the limiting factor here. Which option (if any) is the more conventional?

Finally, RS232/RS485 conversion did not succeed for communicating with the PLCs in this instance (Modicon Compact 984-141 and -145) and any knowledge regarding the matter would be helpful. The 232/485 converter works, as successful communication is established between two windows computers. For connecting to the PLC using Proworx (also not operational when using RS485):

- RS232 to RS485 converter: Uotek UT-2201
- Cable pinout: AS W952 012
- USB to RS485 converter: Startech ICUSB422

Each individual component aforementioned performs as desired.

Best Regards,

 

[danw]

If I understand, you're trying to replace Modbus+ with Modbus RTU? Page 13 of Schneider's Modbus Plus Intro guide
https://www.bh-automation.com/Download/Resources/ModBus-Plus-Planning-And-Installation-Guide.pdf
says that RTU has to use a BM85 gateway to get into the Modbus+ network. Assuming that your boxes are capable of ordinary Modbus RTU when they are known capable of Modbus+, then two comments:

1. I hated those in-line RS-232-485 converters that were port powered because the RS-232 port frequently did not source enough power to make them operate reliably. I remember a couple would not work until I added external power (12Vdc?, it was a long time ago). I would use a wallwart DC power supply for power.

Maybe moving from one adapter cable to the other denies the converter the power it needs.

2. Is the RS-485 to USB converter the same one (from working to failed)? Was Putty running a Modbus RTU script? And it worked?

I know I've run into some cheapie USB/485 adapters just don't run Modbus RTU, presumably timing issues. I've always used USB/485 with the FTDI chipset and they've worked for Modbus RTU.

 

[r6lpq8twg52]

Thank you for taking the time to write back.

0. The intention for the protocols is to operate a MBRTU and MBTCP network between the SCADA computer and the PLCs irrespective of the MB+ part. Then MB+ will be removed in phases. The current configuration utilises a BM85 multiplexer, with functionality that can be bypassed with further developments. This issue is a lesser concern at present, but is mentioned in case the proposed system architecture is flawed.

1. Although not displayed in the attached images, a power source is connected to the RS232/485 converter compliant with its specifications (5v).

2. The same RS232/RS485 converter was used for all tests, and the tests were cycled to ensure the operation of the converter device. No MBRTU scripts running on PuTTY. One more test was completed where the computer connects to a G/W via MBTCP and outputs RS485 MBRTU, then connects to the 232/485 converter, then to a 232 to USB converter, receiving data from a Modbus slave simulator (See attached image1 2024-03-27-1916).

3. Good to know that some do work and others do not. The UT-2201 datasheet shows a concerning statement which conveys that the device manages data direction but also ignores handshaking (See attached excerpt 2024-03-27-1918).

Any advice on a specific RS232/485 device the did work with a Modicon PLC, or other devices for that matter? such that it can be tried with the assembly in question.

Regards,

 

[danw]

Looks like you know that the converters work with Modbus.

I have generic Modbus experience; no experience with Modicons so I can't contribute anything further.

 

[r6lpq8twg52]

Thank you for the guidance, and assistance with critical analysis.

It appears that some looting needs to be done on obsolete machines, there are RS232/RS422 converters within the facility that can communicate with the BM85 on the MBRTU side, if authorisation is granted, the next test will involve attempting communication using RS422. Suboptimal, as is the description of the entire status of the machines of concern.

Hope to relay good news soon (three weeks +).