1771-DS to 1756 Options

schwoates

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Join Date
Feb 2016
Location
Washington
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8
Hello everyone,

Getting ready to upgrade a PLC-5 system to ControlLogix for a client and there is an existing 1771-DS Latching input card used. It is used in paired mode with two photo-eye's detecting an inverted aluminum can. 1 PE is looking at the side of the can to create a Gate and the other PE is looking from above to detect the bottom of the can for the Pulse. If the can is inverted (can opening facing up towards the PE) during the gate, the PE will pulse and the input will latch to keep the filler from trying to fill an inverted can.

The general consensus is that the 1771-DS is being used because of input delay time during the days when the PLC-5 was the best out. I'm wondering what the best option for 1756 would be to replace this. So far I've come up with using a 1756-IB16IF and simply latching a fault bit in the controller if both inputs are on at the same time. Seems easiest and the 14 us nominal Off to On delay seems mighty quick if you ask me. I've also read about the 1756-IB16ISOE. I'm wondering if this is something I should even bother with or if the 1756-IB16IF will be plenty sufficient.

Thanks for the input!
 
Hello everyone,

Getting ready to upgrade a PLC-5 system to ControlLogix for a client and there is an existing 1771-DS Latching input card used. It is used in paired mode with two photo-eye's detecting an inverted aluminum can. 1 PE is looking at the side of the can to create a Gate and the other PE is looking from above to detect the bottom of the can for the Pulse. If the can is inverted (can opening facing up towards the PE) during the gate, the PE will pulse and the input will latch to keep the filler from trying to fill an inverted can.

The general consensus is that the 1771-DS is being used because of input delay time during the days when the PLC-5 was the best out. I'm wondering what the best option for 1756 would be to replace this. So far I've come up with using a 1756-IB16IF and simply latching a fault bit in the controller if both inputs are on at the same time. Seems easiest and the 14 us nominal Off to On delay seems mighty quick if you ask me. I've also read about the 1756-IB16ISOE. I'm wondering if this is something I should even bother with or if the 1756-IB16IF will be plenty sufficient.

Thanks for the input!

The latching logic is quite simple in the ControlLogix .. but it was also quite simple in the PLC5.

Is there a lot of I/O racks? Did they run them at 230Kbits instead of the default 56 Kbits? It certainly worked but there may have been other options.

From the info you have listed, the 1756IB16IF sounds like a good solution, and using logic to latch.

However, 14 us is only the input delay. You need to scan fast enough (Total ControlLogix scan time), and have your IO updating fast enough, to reliably detect the upside down cans. How long is your pulse if the can is upside down?

I have a 1756-L73 running simple ladder logic with 2 ms scan time, a single rack of IO (all local) that could handle your situation. I'm not so confident if you have 10 racks of ethernet I/O and are doing a bunch of other stuff, like driving HMI alarms, doing motion control, etc.
 
The new system will include local I/O, 16 Powerflex 525/755's on ethernet, and a Flex rack on ethernet so I'm not sure what the effect on scan time would be. We have a similar system that has average scan time of 2 ms.

I'm not sure how long the pulse is but the cans are moving ~1200 cans/minute. I'll have to figure out a way to determine the length of the pulse because if I know this and the scan time and input delay, I'll know if the IB16IF will work.
 
The new system will include local I/O, 16 Powerflex 525/755's on ethernet, and a Flex rack on ethernet so I'm not sure what the effect on scan time would be. We have a similar system that has average scan time of 2 ms.

I'm not sure how long the pulse is but the cans are moving ~1200 cans/minute. I'll have to figure out a way to determine the length of the pulse because if I know this and the scan time and input delay, I'll know if the IB16IF will work.

1200 cans per minute is 1 can every 50 ms. If the cans have some space between them, the time will be lower.

Grabbing some numbers out of the air - 20 ms total time for comfort. 0.014 ms for the input (not a factor). 2 ms scan time. Backplane IO speeds are under 2 ms. Even ethernet at 100 mbits unscheduled, IO .. I think .. gets priority over the drives.

2 ms for scan time, 2 ms worst case local io, maybe 5 ms on ethernet ... I can't see how you are going to be over 10 ms ...

I don't see any barriers. Your speed is comfortably high compared to your requirements .. unless I am missing something.
 
Thank you for your assistance! I guess I've just been mostly over thinking the whole thing as this seems to be a simple answer. I don't expect any issues but if I have any, I'll let this forum know the solution that we come up with.
 

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