PLC driving digital circuit crazy!

backtoplc

Member
Join Date
May 2005
Posts
16
I have a panasonic FP-X series PLC and a high speed digital ckt in the same rack.
The PLC reads the output of the high speed ckt.
The switching of the relay is driving the high speed ckt nuts. Teh output goes unexpectedly goes high!
The PL
Any of you know the solution?
My first thought was to move the ckt away from PLC and also add caps. But how many caps can you add.
 
check if the grounds are good (low resistance), add supressors to indictive loads.
do proper shielding of the sensitive signals (ground at one point only), also do proper
shielding of the noise producers, separate as much as practical, before you start adding caps.

adding capacitors lowers impedance to AC or any changing signals (and specially
high speed) but also introduce signal delays and distortion. be careful when
and where you add this, lowering impedance can help fight the noise but using resistor
instead of capacitor is likely to produce better results in high speed applications.
note: this will work for DC as well, and increase load on signal source
so do the math before putting crowbar as shunt.
 
Last edited:
Panasonic FP-X series PLC has high speed inputs, which are designated to work with "high speed digital circuits". At the same time this PLC has relay outputs. Everything is built in one unit. Do you think that switching of these relay outputs is affecting your high speed electronics? Maybe then these outputs are swithching large current and you need to use interposing relay.
 
Thanks for the reply. Just switching of relays is not affecting the circuit.
Three PLC relays are connected to the circuit. Basically, they are used to Enable the logic gates. The voltage at the Enable pin is 5V normally. When the relay activates, the voltage is 0V.
Two of the relays are used to control direction of a DC motor. The 3rd relay is used to activate a analog switch.
The 2 motor relays are causing the problem.
 

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