A1SX81 - Max. simultaneously ON: 60% (at 26.4 V DC)

[Yevgeny 0]

Hello!!! I need to develop an QnA project using several A1SX81 modules (32 digital IN). But I've found such a strange detail in catalogue description: "Max. simultaneously ON: 60% (at 26.4 V DC)" What does it mean? Have anybody used that module? Can there be some problems with it?

[Colin Carpenter 10]

I've done lots of jobs with QnA and A1SX81 modules. They work fine. I have no idea what that statement means.

[Yevgeny 0]

Thank You! That sounds great! By the way I just have found the document ANS_PLC_35.pdf ( IB (NA) 66541-G ). There the diagram is shown that describes "Number of ON signals" dependency from Ambient Temperature. It shows that Number of signals at 43 C degree starts decreasing and at 55 it becomes 60% So can you tell what are the conditions of your project?

[Colin Carpenter 10]

They are all installed in panels in UK summer time conditions, which can get pretty warm .... maximum temperature would be 35-40 Centigrade, I would think. I have no idea what that bit means (and I also found it in the manual), certainly it implies that you can only have a certain number of inputs ON on a card at any one time, which seems pretty worrying if true, in fact it would make the PLC programme very insecure as it means you could never rely on the inputs. I'm sure it must mean something else, but I'm not sure what. All I can say is that I have NEVER had a problem with these cards not showing an input when it should have been doing so.

[Yevgeny 0]

Thank you for your answer! My application won't have temperatures greater than 43 Centigrade, so I hope using A1SX81 will be successfull :)

[The Cat 0]

Sorry to be a little late into the discussion. The derateing of modules comes about because the equipment itself is rated for use in ambient temperatures across a range, often 0-55C. When turned on, the contribution of the heat dissipation generated by the modules internal circuitry (attenuating resistors, etc)combined with the ambient temperature raises the local temperature of the components used towards their maximum functional limit. Thus, the higher the ambient temperature the fewer circuits that can be active (and dissipating heat) before the working temperature of the module goes outside the specified operating temperature for the components used. Reducing the number of channels that are useable as the ambient rises is a compromise for having a high number of channels available in the module when used at lower temperatures. The lower the ambient (forced or unforced) the more channels that are available.

[Colin Carpenter 10]

Hello Mate, What I still don't understand, though, is that all of the digital input modules give a "% value at a certain voltage", depending on the voltage that the inputs are designed to work at, say 60% at 26.5 VDC. Presumably this must relate to a fixed temperature point of the module? For example, at 55C, only 60% of the inputs should be on, or it will cause the module to "go critical". Presumably, at, say, 10C, overheating's not a problem and you can have them all on? Wonder what temperature the spec refers to?

[panic mode 246]

Derating is used to reduce heat dissipation inside module (just like with wire or cable etc.). A1SX81 is high density module (32-point) with inputs designed for low voltage (turn on at 8V). Although they still can be used at common 24VDC, this means that part of it (LED which is part of the input optocoupler and it's series resistor for example) will be stressed more (hotspots) if the input voltage is increased to such levels. They probably don't use many discrete components since that would take too much space. This also means higher sensitivity to ambient temperature. In many cases inputs are used in pairs (sensors for cylinder extended/retracted etc.) so this is not a problem. If you look into specs of 64-point modules, you will see something like max 50% on at the same time. I was using 16, 32 and 64 points without issue (ambient temperature in low 30degC, a bit more inside cabinet). This part of spec is only what they guarantee. Most people don't run their PLCs on 26.4V. Actual voltage input sees is usually not even 24V (reed switches, photoeyes etc all have some voltage drop which is in the order of 1-2V). Rember, dissipated power is proportional to square of voltage (or current). If you think this could be a problem, choose lower number of points card, choose lower input voltage, regroup I/O, lower temperature of the cabinet etc. If you are using lot's of contact type of inputs, space is at premium and you want 100% rating, consider multiplexing for example (check out A1S42X for example).