Modicon 984 4-20mA input rollover problem

[TConnolly 13]

I have a modicon 984 using 875-002 4-20mA input. Sensor is a pressure transducer. When the pressure is at 0 the input word from the card tends to roll over backwards to 9999 when the input current drops just below 4.0 mA, ie 3.999 mA reads 9999. This of course creates a bogus pressure reading. To prevent this the transducer has to be calibrated with a slight zero offset, which creates other problems. Has any one else ever encountered a similar issue and how did you resolve it?

[PLCTIE 0]

At 984 inputs the signal comes along with two status bits within the register. MSB Bit#1 (984LD) Bit#15 (IEC) = value out of range Bit#4 (984LD) Bit#12 (IEC) = 0 underflow 1 = overflow What to do: Copy Input Register 3x to holding register 4x Reset [MSB Bit#1 (984LD) Bit#15 (IEC)] to zero and if [bit#4 (984LD) Bit#12 (IEC)] = 1 limit value to 4095 You could also perform a fault dection with the two bits, but as you have seen, if the measurement is around 4mA you could get an underflow. I hope this information helps. Best regards Eric (sorry for my English, my mother tongue is German)

[Modiconbob 2]

This is an early model card. The card it was modeled after would "lock-in" at zero if the value went negative and "lock-in" at 4095 if the value went above 20 mA. However this card does what it is told.......this caused many issues when folks were doing upgrades from the older systems to this card. You must program in logic some limits to prevent this, or use a "newer" model card such as the B875-111. I would still sugest the limits be programmed as someone may later substitute an older card in an emergency and really screw things up.