AB PLC5 Scan Time Question

What are the factors that vary scan time in an AB PLC5 processor? RS Logix shows the current scan time and a max scan time. The current scan time flucuates by a relatively small amount. The max scan time is usually about double the normal scan time, but I've seen a few with a max scan time 30 times higher than the normal scan time. What influences that?

Also, can anyone shed some light on the best methods for setting up scanner channels. We have one processor with 20 rack addresses on channel 1B and channel 2A and 2B are unused. Is this a good setup? Does it depend on the application? Or would it be best to evenly distribute these racks over two or three channels?

a)The huge difference usually is when you do an online edit. Reset the max and try it.
b)another reason is code that is not continuously scanned. ie conditional subroutines
c)and lastly conditional regressive jumps.

Regards Alan Case

To answer the second part of your question, using more than one scanner channel should only be necessary when you have too many devices for one channel (I think that is 32), or you have a large amount of block transfer modules that would slow down your I/O scan to unacceptable levels.

With just normal discrete I/O, you get about 6 msec scan time per adapter on the channel so with 20 racks you're up to 120 msec, about 8 times per second. This should be fine for most applications.

If you need faster update, split your racks among channels. The only penalty would be when you want to add an adapter channel down the road and you have used all the channels for I/O scan.

Is this a trick question?

A single RIO channel can have a maximum of 16 adapters connected. Depending on chassis size and addressing mode, each adapter may present anything from one quarter rack to four racks (logical racks).

So, you may have 20 logical rack addresses (indicates a 5/60 or 5/80) but you may have as few as 5 adapters.