Anyone knows what is the maximum load capacitance for RS-422 drivers?
Does protocol specifies this value or I should determine it from the datasheets of vendors? If I should determine it, what is the right way?
RS-422 works on differential line with line termination resistors at the both ends of line. Line doesn't have capacitive behavioural, so drivers see only equvalent resistance of line and capacitance of PCB (or stab capacitance). Usually dinamic characterictics are measured with load capacitance 100pF (in datasheets).
Thank you very much for response.
I probably didn't put my question right. I certainly know that RS-422 is differential protocol with resistive termination between the lines, but I want to use EMI Pi filter near the board connector, so I need to determine the max. capacitance of that filter in such a way that the driver can handle it without to much stress.
Thanks.
As designer of transceiver ICs, i think that the additional current for capacitance charging must be less than 10mA. So driver can handle it without to much stress.
Cmax<=Imax/(du/dt)
=>
du=Vdiff=3.0V - typical
dt=10%*Tbit=0.1/(fdatarate) - typical (30% max according standart)
=>
Cmax=0.1*Imax/(fdatarate*3.0V)=3.3e-4/fdatarate
fdatarate=115,2k => Cmax<3nF
fdatarate=500k => Cmax<660pF
Try this simple estimation