Omron CJ-Series Unit Numbering

rlao

Member
Join Date
Jun 2015
Location
Australia
Posts
4
I have next-to-no experience working on Omron CJ-series PLC's so bear with me. When I'm assigning Unit Numbers to Omron PLC modules via CX Programmer, does it only affect the memory addresses that these modules will use?

Also, how much leeway do I have in having modules share Unit Numbers? I was under the impression that each module has to have a unique Unit Number, but some PLC backups I have browsed have the Analog and comms modules sharing the same Unit Number, like here: http://i.imgur.com/OhSgMLK.jpg.

Is this configuration possible because the I/O modules and comms modules use such vastly different areas of the PLC memory, that there is no risk of memory overlap even if Unit Numbers are shared?

Yet the Omron ETN21 manual says: "The unit number is used to identify individual CPU Bus Units when more than one CPU Bus Unit is mounted to the same PLC." - which implies the Unit Number must be unique.

I have a rather large set of Omron PLC's in use over several sites that I need to add Modbus TCP functionality to. While doing this, I am trying to minimize changes to the existing PLC code, but on some sites it looks like I may have to modify some of the Unit Numbers of the comms modules. I want to know how much freedom I have in doing this without breaking the existing functionality.

From reading the Omron manuals, it seems the only thing I have to look out for when changing Unit Numbers of existing modules is updating any related symbol or variable addressing to reflect the updated Unit Number. Does this sound correct?
 
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At the end of the day some modules can have the same number - it depends entirely on the module type. For bus units they have to all have individual numbers. For special I/O units they all have to have different numbers. The bus units and special I/O units are different and you can have say, an Ethernet module with number 1 and a special I/O module with number 1.

If you open a new project in CX-Programmer and set up an I/O table with say an Ethernet module then a serial card SCU31-V1 the Ethernet module is automatically allocated address 0 and the serial module is allocated address 1 - it is up to you what you do with them of course and will depend on the setting of the DIP switch. Then set up a CRM21-V1 card in mode 0 and it will use address 0 but 2 card allocations in memory - then an AD081-V1 analogue card and it will automatically suggest address 2 then a DA08C analogue card and address 3 will be suggested. Quickest way to learn what is a bus unit and a special unit and how you can address them.

Why would you have to move them around? There are plenty of numbers to allocate for bus and special I/O cards. I would just be using a new address and moving nothing. The biggest issue you would have is the setup for the cards is stored in data memories and automatically allocated when you do an online setup for each card. You would have to re-write all the card setups.
 
Thanks BobB. Your clarification on the distinction between Bus and special I/O units has helped me a lot in my understanding.

Regarding shuffling the existing modules around, I have a function block from the Omron website that implements Modbus TCP on an ETN21 module. At the moment, the preference is to have all ETN21 modules on all PLC's use the same unit number so the same function block can be applied to all PLC's to save engineering effort.

Most PLC's I'm working with don't have an ETN21 module, which is fine as we can add one and configure as we please. A few PLC's do have ETN21 modules existing so we're seeing if it is feasible to change their unit numbers to match our new code additions, without breaking existing functionality.

However, as you say, we may end up having to rewrite card setups if we do shuffle the existing ETN21 modules, so we're probably better off modifying our new code on a per PLC basis.
 
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I would to be honest - no big deal. Takes 5 seconds. Quicker than changing the numbers and setup of other cards.
 

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