Two A-B controllers in on chassis

currently we are using 1756-L55 controller connected to data-link radio modem through serial port. we are upgrading this controller panel with new hardware and 1756-L71 controller. since L71 doesn't have serial port for radio modem connections. we want to retain L55 controller for radio communication and add new L71 controller in the same chassis for controlling the other process. is it possible to have L55 & L71 in same chassis and share the tags for Read/Write access between these controllers. please enlighten me on this. Thank you.

Hello lrahman
i never tried it with L71 and L55, but i think it should work.
What i did was have 2 L72 in one chasis, that works.

Then you have 2 different projects, one for each controller.
The communication between one controller and the other is done with produced/consumed (through the backplane of the chasis).


Special considerations has to be taken with the I/O cards, and the PointIOs connected to the system. Normally a I/O card is "owned" by one of the controllers, so you cannot directly read/write to the same I/O card from both controllers. But one controller can "own" the I/O card and communicate with it, then pass the data via produced/consumed to the other controller.

Thank you sir,
i cannot do away with L55 as it has Serial port for communication with radio modem. this radio modem gets the data from another remote PLC L55 in central control room. as you rightly said IO will be owned by L71. we will keep L55 only to have data read/write to radio modem through serial port.
in this case we will have L55 project and L71 project running in these two controllers respectively. but i am not sure whether produced/consumed tags will work with two different versions of projects.

Just thinking that there are few gateways that would interface with a serial port (depending on the protocol you're using, though for radio modems, I seem to remember DF1 and Modbus were the most common). If that's not an option, you can use a 5069-L processor with a serial card in it.

Rockwell has some weird limitations (not weird, they're well thought out to make you spend more) where some platforms have things and miss others (5069 has a dedicated serial port card, but not a network card and vice versa for Control Logix).

cardosocea said:

Just thinking that there are few gateways that would interface with a serial port (depending on the protocol you're using, though for radio modems, I seem to remember DF1 and Modbus were the most common). If that's not an option, you can use a 5069-L processor with a serial card in it.

Rockwell has some weird limitations (not weird, they're well thought out to make you spend more) where some platforms have things and miss others (5069 has a dedicated serial port card, but not a network card and vice versa for Control Logix).

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Thank you sir. Using gateways in-between may require read/write tag format changes compatible with gateways used. this requires program modifications on both side i mean remote PLC and local PLC. Our remote PLC communicates to multiple slave plc through radio modems one at a time. since all PLCs are L55, we are using generic read/write tags in master remote for data handshaking with slave local plc. Thanks for your suggestion for 5069-L. I shall explore this. thank you again sir

lrahman said:

Thank you sir. Using gateways in-between may require read/write tag format changes compatible with gateways used. this requires program modifications on both side i mean remote PLC and local PLC. Our remote PLC communicates to multiple slave plc through radio modems one at a time. since all PLCs are L55, we are using generic read/write tags in master remote for data handshaking with slave local plc. Thanks for your suggestion for 5069-L. I shall explore this. thank you again sir

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What type of radios are you using? Surely they're speaking a specific protocol (like I mentioned, the ones I worked with were DF1 and Modbus), if it's DF1, you should only need to modify the program in the new L71 and not have to touch the other radio/PLC links.

i am not sure whether produced/consumed tags will work with two different versions of projects.

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Yes, they absolutely will. A 1756-L1 running v2 firmware can exchange Produced/Consumed Tags with a 1756-L81E running v33 firmware, and any mix of controllers and firmware in between.

If you have trouble exchanging tags between two ControlLogix in the same backplane, then the problem is 99.999% sure to be a configuration issue. They have always worked, and will continue to work for the foreseeable future.

lrahman said:

currently we are using 1756-L55 controller connected to data-link radio modem through serial port. we are upgrading this controller panel with new hardware and 1756-L71 controller. since L71 doesn't have serial port for radio modem connections. we want to retain L55 controller for radio communication and add new L71 controller in the same chassis for controlling the other process. is it possible to have L55 & L71 in same chassis and share the tags for Read/Write access between these controllers. please enlighten me on this. Thank you.

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It should work fine. If the radio modem tags are well-defined (you have not added any lately) you can do produced/consumed tags between the L71 and the L55. Tags from the radio would be PRODUCED in the L55 if you need them in the L71. Tags to the radio would be CONSUMED by the L55 or calculated in the L55.

I had 4 1756-L63 in the same 17 slot rack so that they could do produced/consumed tags - 4 arrays of 100 DINTs produced by each PLC that were updated every 40 ms. (so each PLC consumed 1200 tags) So you won't have problems with produced/consumed bandwidth.

It MAY be a pain to keep the programming software for the L55. I don't think it is supported on any windows computer newer than Server 2008 or Windows 7 (not sure about that). Version 16.023 appears to be the latest available. You might want to keep an older computer to do the programming

thingstodo said:

If the radio modem tags are well-defined / Tags from the radio would be PRODUCED in the L55 if you need them in the L71. Tags to the radio would be CONSUMED by the L55 or calculated in the L55.

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What do you mean by "radio modem tags"

Firejo said:

What do you mean by "radio modem tags"

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The tags read over radio modem, and written to the radio modem, via the serial port that the OP needs to keep

.. or did I read something wrong?

Thank you all
its nice here that L55 and L71 can share data via produced/consumed tags concept in both directions irrespective of firmware and rslogix versions. we have maintain L55 and its firmware/rslogix version for some more time until we upgrade our Data-Link SRM 6100 radio modems to SRM-6330 which are based on ethernet links. Thank you very much indeed.