Bit_Bucket_07
Member
this is one of the MAIN stumbling blocks for technicians who habitually use FORCES while trying to troubleshoot a system ... suppose that their line of reasoning is something along these lines:
OK ... we've forced the lamp/pump/valve/etc. to be ON in the field – so that means that the rest of the logic SHOULD respond to that ON status ...
if they haven't had quite enough training along these lines, then they fail to realize that their reasoning is being misled – simply because there is STILL a ZERO in the output's bit/box – even though the output in the field (the actual device) is being turned ON ...
in simplest terms – the PLC's logic will NOT respond to the forced status of the output bit/box as expected – which can lead to a time and money extensive "chase of the wild goose" ...
on the other hand, when you force an INPUT, the forced status (either ONE or ZERO) will (generally) be reflected in the rest of the program's logic ... this can be very confusing to bleary eyes at 3:00 in the morning – and the machinery is down ...
takeaway: INPUTS and OUTPUTS behave differently when they are forced ...
for those who want more details – start here ...
http://www.plctalk.net/qanda/showthread.php?p=537414&postcount=1
and skip ahead to paragraphs 38 through 43 for a quick start ...
party on ...
Excellent description of the phenomenon, Ron.
Of course, it doesn't apply to all PLCs. Am I mistaken in remembering that the PLC-5 and SLC-500 didn't handle forces in this manner? I don't recall encountering this prior to working with the AB CLX models, but that could just be a failing of my memory due to oldtimer's disease.
I know that asynchronous IO updates were new to the CLX models.