DHRIO Flex IO Analog Input Issues

[holtjac2]

Hey guys,

We recently picked up a new customer that just finished a controls upgrade with some other systems integrator company.
They upgraded to an L81 controller but still have some old 1794- Flex IO racks scattered throughout the plant. These Flex IO racks are still communicating with the PLC through DH+ using a 1756-DHRIO card in the PLC Chassis.

All the IO is responding great other than the analog inputs. We can start and stop motors using the Flex IO we can also receive Discrete Inputs.
for some reason the analog ins arent receiving any data. Our messaging block isnt erroring out. but our DHRIO card say "recoverable" but now information on why.

They said that the analog inputs (1 1794-IE8 on each Flex IO rack, slot 0) stopped working when the came back up from a power outage and that the PLC code shouldnt have been touched throughout the down period.
Ive double and triple checked the MSG block configuration, Swapped the card, swapped the Back Plane terminal block thing where the card plugs into, and still nothing. ive verified a i have a 4-20ma signal into the card as well.


any ideas would be much appreciated. were going to hopefully move them to ethernet instead of DH+ but parts are a ways out.


Thanks guys!

 

[tarik1978]

Have you checked the power supply to these cards?

 

[Ken Roach]

Welcome to the PLCTalk forum community !

The 1756-DHRIO module, as the name suggests, supports both DH+ legacy peer-to-peer messaging, as well as A-B Universal Remote I/O (RIO) Scanner mode. The two channels can be configured independently for either protocol.

>our DHRIO card say "recoverable" but now information on why.

Are you using either channel for DH+ ? I think you might be seeing a nuisance error message that appears when you have a DH+ channel and the module is in the I/O tree. The proper way to set up an all-DH+ DHRIO module is to either leave it out of the I/O tree or Inhibit the connection to it, because that's used only to transfer RIO data. When you're using one channel for DH+ and the other channel for RIO, I think that a nuisance error is actually unavoidable.

>Our messaging block isnt erroring out [...] double and triple checked the MSG block configuration

DH+ and Block Transfer messaging with DHRIO is actually a tricky subject, because the backplane buffers available in the ControlLogix operating system haven't changed in 20 years. If you have more than 16 MSG instructions in your whole program that use DHRIO modules, you probably need a buffer management routine.

If your logic re-triggers the Block Transfer MSG instructions automatically, I encourage you to modify it so that you can trigger them one at a time and be completely certain that you're getting a /DN bit.

>came back up from a power outage

The classic logic issue with A-B analog cards in general, and which includes the 1794-IE8, is that most of them need a Configuration data block sent after a power cycle.

Some users program the 1794-IE8 to send a configuration BTW alternating with the Input data BTR all the time.

But the right way to do it is to send a configuration BTW only when you need to change the configuration, or when the Power Up bit is true. That occurs, of course, after a power cycle.

One mistake I've made is when I'm working with different analog cards and I forget where the /PU bit is in the data map. In the 1794-IE8, it's Bit 15 of the last 16-bit Word, and on some other modules it's Bit 0 or Bit 15 of the first Word.

https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1794-um002_-en-p.pdf

Another thing to check on is the DC power for the analog modules and the RIO adapter, to be sure they come up at the same time. You could try piggybacking in a +24V supply just for that analog module, so it's powered up before the 1794-ASB. The fact that it's in Slot 0 and the discrete I/O works correctly suggests that's not a problem, though.

Again, welcome to the PLCTalk forum !

 

[joseph_e2]

Another issue I've seen with Flex IO Analog inputs is when the field device powers up before the Flex chassis, or the chassis is power cycled while the field device is still powered on. There's a tech note about it (access level: TechConnect):
https://rockwellautomation.custhelp.com/app/answers/answer_view/a_id/40829


That note is for the 1794-IE12 or 1794-IE8XOE4, but something similar could apply to your -IE8.


If an additional device is connected to a previously unused channel while the module is powered up, it can cause all channels to not work. They propose several fixesL
Don't connect/disconnect a device while the field device is powered
Don't power cycle the Flex chassis while field devices are powered
Wire a zener diode in parallel with the input channel and add a current limiting resistor in series with the field device. They have details on sizing those but also state that their values may have to be tuned for each field device.

 

[holtjac2]

Ken, youre a life saver. Thank you.
Using your post and the link you provided I wrote the configuration into the card and it seems to have worked.
I will keep this thread updated on new info/issues.

Thanks again to everyone that replied.