Allen Bradley 1771-CFM or PLC fast input module?

ckchew666 · Mar 1, 2005

Hi,

We have a customer using PLC-5 system with RIO. From what they complain is the slow speed of the PLC DI (normal), thus they wanted to get the 1771-CFM module as replacement (I'm not sure if the 1771-CFM serve its purpose, furthermore it need a PCO software to configure the cfm module if not mistaken). They need 8 inputs, but CFM only has 4 Digital Inuts. Is there other model we can recommend with 8 Fast DI?

Thx.

Ken Moore · Mar 1, 2005

The 1771-CFM (Configurable Flow Meter) was designed to count high freq. pulses from a flow transmitter. The accumalated pulses are then transfered to the main Program via Block transfer functions. If your application involves high speed pulse counting, this card is ideal. If you are looking for fast updating digital I/O this card may not be what you want.

What is the baud rate of the RIO? Depending on what you're RIO make is, you may be able to increase the baud rate. Just a thought.

If the digital inputs, and the controlled outputs are all in one remote rack, you could consider installing a processor, and making it a stand alone system. The local I/O would be controlled by the new processor, and then data could be passed back and forth to the old processor via message instructions. Just another thought.

gluca · Mar 1, 2005

How fast do the DI need to be?
In other terms, which is the expected frequency update of the inputs?

Jiri Toman · Mar 1, 2005

Introduce an STI (selectable timed interrupt) file in your ladder
program. In PLC 5 you can go to 10 ms with no problem.
Put all critical inputs at the beginning of this file. Use
immediate input IIN instruction to do that. This way you will
insure that all critical inputs are processed every 10 ms.

Gerry · Mar 1, 2005

Is there other model we can recommend with 8 Fast DI?

Have you considered the 1771-IT ? (8-point 24VDC fast Input)

This way you will
insure that all critical inputs are processed every 10 ms.

I don't think you can ensure that over RIO.

ckchew666 · Mar 1, 2005

Hi,

Thx for all the replies. Appreciate all the info given.

BTW, will the 1771-VHSC & 1771-DR play their role in this situation, PLC-5 just have too many cards around and what I normally deal is the normal DI/DO/AI/AO. These CFM,VHSC,IT,DR module do not have a change to even touch it.

In the mean time, i'll check out with customer for more info. Thx.

ckchew666 · Mar 1, 2005

Gerry said:
Have you considered the 1771-IT ? (8-point 24VDC fast Input)

I don't think you can ensure that over RIO.

Hi,

Do you have any idea what is the maximum input frequency for 1771-IT module?

Customer requirement is about 2000Hz (2000 pulses per second).

From the manual 1771-VHSC is a 1M Hz freq module, <---- this is real fast

ckchew666 · Mar 1, 2005

Gerry said:
Have you considered the 1771-IT ? (8-point 24VDC fast Input)

I don't think you can ensure that over RIO.

Let's do some calculation If you don't mind

If the RIO speed is 57.6Kbps. Assuming the efficient is 50% (worst case), the speed is 28.8Kbps. So if I'm transferring 8 DI points with 2K Hz each, I'm using 16Kb of bandwidth. I still have 12.8Kb of bandwidth for others, am I right? COrrect me if I'm wrong

Thx.

Gerry · Mar 1, 2005

ckchew666 said:
Let's do some calculation If you don't mind

If the RIO speed is 57.6Kbps. Assuming the efficient is 50% (worst case), the speed is 28.8Kbps. So if I'm transferring 8 DI points with 2K Hz each, I'm using 16Kb of bandwidth. I still have 12.8Kb of bandwidth for others, am I right? COrrect me if I'm wrong Thx.

My response was to the 10 msec immediate I/O update - too fast for RIO. Consult the system design manual for details on RIO timing.

The 1771-IT won't do 2KHz - you'll need a VHSC or similar. Do they still sell the -DR module? Good luck finding documentation for it - I tried unsuccessfully about 3 years ago.

Ken Roach · Mar 2, 2005

The essential question about this application has not been answered: "What does the customer need to do with this 2000 hz signal ?"

The application might be simply a rate measurement. This is very easy for a 1771-VHSC, 1794-VHSC, or 1734-VHSC.

The application might be a counter application with a fast-acting output after a preset is reached.

The application might have to react very quickly to a single input point that is only on for 1/2000 of a second.

Tell us about the application and I'll bet you will get a lot more suggestions. One of my thoughts is to use little MicroLogix 1000 controllers with their 6.6 kHz high-speed counters, and network them into the PLC-5 for status reporting.

ckchew666 · Mar 2, 2005

Gerry said:
My response was to the 10 msec immediate I/O update - too fast for RIO. Consult the system design manual for details on RIO timing.

The 1771-IT won't do 2KHz - you'll need a VHSC or similar. Do they still sell the -DR module? Good luck finding documentation for it - I tried unsuccessfully about 3 years ago.

Yeah, I found the 1771-DR module user manual. Will 1771-DR better than 1771-IT?

ckchew666 · Mar 2, 2005

Ken Roach said:
The essential question about this application has not been answered: "What does the customer need to do with this 2000 hz signal ?"

The application might be simply a rate measurement. This is very easy for a 1771-VHSC, 1794-VHSC, or 1734-VHSC.

The application might be a counter application with a fast-acting output after a preset is reached.

The application might have to react very quickly to a single input point that is only on for 1/2000 of a second.

Tell us about the application and I'll bet you will get a lot more suggestions. One of my thoughts is to use little MicroLogix 1000 controllers with their 6.6 kHz high-speed counters, and network them into the PLC-5 for status reporting.

Ken,

Updates from our customer, it actually has 8 tank gauging system with pulse output signal. The signals are wired to DI (fast response) & the CPU executing the pulse measurement indication.

Your second suggestion looks interesting, but with network implemented, we are affraid the bottleneck will happen in the network itself.

JesperMP · Mar 2, 2005

ckchew666.

It sounds strange to have such an output signal to measure tanks. Is it a level measurement ?
Pulse signals I have heard of when measuring flow, but not level.

What happened to good old 4-20 mA ?
Rather than going overboard to try and evaluate the fast pulse signals, try to convince the customer to change the measuring devices to 4-20 mA outputs (or something else more normal than 2kHz pulse signals).

Ken Moore · Mar 2, 2005

I agree with Jesper, pulse level transmitters are a new breed to me. If that is the case, many mfgs. sell pulse to mA converters. Then you could use a plain old 1771-IFE or simialar.

Ken

JesperMP · Mar 2, 2005

ckchew666 said:
.. The signals are wired to DI (fast response) & the CPU executing the pulse measurement indication ..

From that, I take that you want the CPU to evaluate the signals programmatically.

At 2000 Hz, you would have to scan the signal every 0.2 millisecond - or faster ! (*). That is impossible in the PLC5 program, even if it was dedicated to this application alone.
Even Selectable Timed Interrupts cannot be faster than 10 milliseconds. And if you programmed Input Interrupts, I dont think it would be possible, but if it is possible it would make the CPU do nothing else but react to the interrupts.

Dedicated counter cards in the PLC, or standalone counter to analog converter, or changing to something else than 2kHz pulse signals is the solution to your problem.

*: 0.2 milliseconds scan assumes that the pulses have evenly distributed pause and pulse periods. If the pause and pulse is not evenly distributed, the scantime would have to be proportionally shorter !

Allen Bradley 1771-CFM or PLC fast input module?

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