i just want to know whether Analog I/O scan reading and updates are part of I/O scan sequence of PLC. That is, how do Modicon and Siemens PLC process analog I/O scanning? i have a doubt that Whether PLC SCAN all analog input once in Input scan cycle by reading the memory image of interfacing cards (Modules) and store in PLC internal process image in %IW area. and similarly update analog I/O from process image (%QW) to Module memory during output update cycle.

i have read about peripheral addressing in Siemens PLC, where analog data updated in program cycle. if it is so, then which technique is more advantageous when no of analog i/os are more?

how Modicon PLC scan analog i/o and whether does it support peripheral /direct analog I/O scanning?

please explain

 

The same applies to Schneider PLCs. Simply: read the inputs / solve the logic / write the outputs. The "formal" explanation about state tables and updating state tables and how they function is available from Schneider (with some searching) but use the simple read/do/write.

However, you can, with Unity Software, designate specific I/O to be read and then written for a specified section of program logic. This requires a lot of planning and in my many years of PLC applications support and programming I have never been presented a situation where this was necessary. With PLC/PAC scan times in the single digit milliseconds the logic solutions are likely faster than changes in the analog values.

I have also observed that PLC speeds and languages like Unity with user created function blocks have also made the use of "subroutines" and "jumps" obsolete. Another discussion??

 

Dear Brian

It's ok with Schneider PLC, which scan all input first then program execute and then update. but in case if we required immediate update to controlling valve, PLC will scan input at x time then it will be updated after (x+ PLC scan time). but in case of peripheral addressing PLC read and immediately update the output. i just want to know if peripheral addressing (direct addressing) is more real time than the PLC normal scan sequence, then why the most of PLC do not use the direct addressing method? what are the limitation in that?

what is the schneider PLC scan sequence when executing the FAST task?

please explain.

 

Brian is quite correct. - There is very little need for direct I/O scanning.

The only times it has seen to be used in the past is:
1. The Allen Bradley LN2 processor in it's day had 'Immediate O/P' instruction. This was used to energise a TTL digital o/p on and then back off again immediately. This gave a 120microsecond pulse to a printer - more a fetish than necessity.

2. An Allen Bradley PLC 5 system had a 'PLC I/O module. This was a high speed PLC within a PLC rack and ran asynchronous to the main processor to give millisecond responses with timers set in milliseconds. It was extremely difficult to programme (in Hex)and digital I/O only. Never had feedback from the end user to whether it was necessary or indeed useful. I've never seen such a setup since.

Direct Analogue I/O scanning would seem even more undesirable.
A system I designed last year had one Analogue working at relative high speed. This had to be isolated from the other Analogue signals and have it's own 'fast conversion' module. It's conversion time that matters not I/O scan time.

And don't be fooled by Manufacturers specifications - some give reasonably high speed conversion times, failing to mention a self-calibrate routine which halts conversion for anything up to 150milliseconds when invoked.