Regardless of what machinery is being controlled and what the user's view of the system looks like on the desk (whether it is a web-based system or a control mimic) I am seeking some opinions from the rest about what the most appropriate response time maxima should be. I have found, followed and read a few links and so far the best advice gives three zones (0.1 seconds, 0.2 to 1 second and 1.1 seconds to 10 seconds) with advice on the style expected for each of these zones.
What I am looking at would be some quotable figures that would apply to an embedded (perhaps PLC based) system being conrtrolled through a centralised server based farm of terminals.
Some parts of this conglomorate system gives responses in the two seconds region (by being given a little more priority for the sub-system traffic). Other parts are operating with a latency, between commanding a plant action and seeing confirmation of the action having occurred, of four to 8 seconds. This does not feel so good to operate.
In operating with the programmers interface to a PLC I was able to confirm that toggling the bits that caused the actions to occur and seeing the response status from the plant was seen within 3 scans (28ms per scan). This was for the operation of a small, fast, solenoid valve. With the central user terminal system this same action was at least 4 seconds and almost took 8 seconds on occasions (first scan sets the output, one scan while the device changes state physically, the third scan returns the new state of plant - approximately 84ms end to end of the change of state).
This indicates to me that the central system is much too loaded and could do with a boost in capability or an off-loading of some of the work that it is carrying out.
So, how swift should I specify the plant change of state response be displayed on the users terminal? This figure would be more interesting if it was also backed by some good document links. In the mean-time I am going to see if I can write a justification to support demanding less than 0.25 second response to any change of state of the plant.