Never heard of it as a generalized term for PLCs. I have heard it used to describe redundant hot backup sytems using PLCs though. In a Synchronous system, 2 CPUs are looking at I/O continuously, although only 1 of the CPUs is active. If that one goes down, the 2nd CPU is already processing the same I/O information and executing the program, so it takes over immediately (AKA Bumpless Transfer). In an Asysnchronous system, both CPUs may be running, but when the primary dies the secondary must scan the I/O status before it can establish control.
"Venditori de oleum-vipera non vigere excordis populi"
In Siemens PLCs synchronous/asynchronous describes system functions/function blocks behaviour. Synchronous means that function needs only one call to return results. Asynchronous means that function needs to be called several times before returning results.
I agree with your explanation, jraef. Asynchronous and synchronous terms are commonly applied to communications and backup scenerios.
The backup processor doesn't always "process the I/O" or execute the program. In some systems, the I/O is scanned and stored only, until a fault is detected in the primary processor. THEN the backup immediately begins executing its program. This may cause a slight delay (one or two scan cycles) in backup. It's useful if the backup program is different than the primary. Sometimes, at least.
I'm sure different vendors use the terms in their own way, as well.
