Fast Input Modules

Does the PLC scan rate limit the response time on a fast Input module such as the Allen-Bradley 1746-ITV16. I am thinking of using this module in stead of high speed counters for an application with counters in the PLC logic. The module is fast enough for my application, but will I lose counts if my scan time is to slow?

No. High speed counters operate independently from the scan. They have their own logic that writes directly to memory.

keithkyll. I do not want to use the high speed counter. I want to use a digital input module (with a fast response time) and count the "input" toggles with a couter in the ladder logic. Will my ladder scan rate cause me to miss counts if it is slower then the input "toggles"?

In that case, yes. The scan sequence is read inputs, solve logic, set outputs, housekeeping (self test), read inputs, etc. You will only see 1 count per scan max.

That's what I figured. Thanks

My answer is based on general PLC knowledge, not specific to AB. That module may work differently, so wait for an AB expert to verify.

I think that module will trigger a 'hardware' interrupt. You just need to define the program file that will run when an interrupt occurs. This is done using the 'Advanced' button in the hardware config section os Logix500. I don't think you will do any better than 1 msec response, though.

Like the other Keith, i don't have any info directly in front of me right now. You will need to look into this a little further.

Keith

Greetings Pletchj ...

and welcome to the forum ...

here's a tip that might help you get EXACTLY the answer you need ... tell us more (actually ALL) about your system ... the basic idea is that you are asking specific questions about specific pieces of hardware ... it's hard for us to answer those (except in general terms) when we don't know anything specific about how your system is set up ...

if we knew more about the actual requirements for your system, then we might be able to make better recommendations about how to solve them ... what is your processor's scan time? ... how many counts per second do you need to make? ... how long will the input signal be off? ... how long will the input signal be on? ... things like that are important ...

one more thing: have you checked into the IIM (Immediate Input with Mask) instruction? ... that might (or might NOT) be helpful ... I'd also look at the STI (Selectable Timed Interrupt) feature ... again, that might not work for you - but it's something that you need to look at ...