What happens if I use the I/O address 255 as Input or Output of a PCD3?

FAQ #101252

 

What for is the output 255 used?
This output has two functions. It is present as "normal" I/O address on the I/O bus and can also be accessed there. But additionally, the same address is also representing the hardware watchdog relay (onboard on the PCD3 CPU).

Which restrictions do result in this situation? 

• On the SLOT_15 (with base address 240) you mustn't plug "analog modules" (PCD3.Wxxx) and also mustn't plug any H-modules (PCD3.Hxxx).
• You can't use base address 255 as Input or Output, because it is use for activate the internal relay. This relay is accesible on the "terminal block for supply, watchdog, interrupt inputs".

Additional information

• When you plug an 16 OUTPUT module (e.g PCD3.A460) and you use the output 255 (and you do not use the Hardware Watchog FBox) the output works as usual (however, as it is recommended using the hardware watchog, it is not recommended using the output as "normal" output)
• When you plug an 16 INPUT module (e.g PCD3.E160) you won't be able to read the state of this input (the PCD will always read "0" even if the physical input 255 is high).