Do Modicon PLC M340 support more then 1 client over Modbus TCP/IP


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We have Modicon PLC M340 which is integrated to SCADA over Modbus TCP/IP. We are planing to integrate the same PLC over our SCADA over Modbus TCP/IP. Our query is do Modicon PLC M340 support more then 1 client over Modbus TCP/IP. Please advice.


Lynn August Linse

There are two answers:

1) most PLC will support many Modbus/TCP clients to access NORMAL standard register data.I'd think 4 is likely a lower limit and most will allow dozens of concurrant clients (limited by performance).

2) However, something like UnityPro (any reprogramming tool) will add steps which login to the PLC and block other tools from gaining such potential conflict. This done with vendor functions, NOT 'Modbus' per say.

So that means, you should have no problem pulling normal data from many PLC remote via wide-area-network. However, should some guy in say New York 'log in' to a PLC in Montana, then the guys in Montana would be locked out (or have limited access, etc). So there is a layer of organizational complexity.

Some companies (like GM) have over the years learned that remote access with reprogramming permission is dangerous (an accident waiting to happen), so allow normal data access remotely, but purposely permit programming access only from where the tech could see the machine & any bad results. It is too easy to fat finger an IP and load the wrong program on the wrong controller in the wrong site. Sure, there are 'are you sure?' pop-ups, but if you're frazzled and in a hurry to stop people yelling at you over the phone because production is down or a waste tank overflowing, it is easy to make mistakes.