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maximum load capacitance for RS-422 protocol

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genadyt

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maximum load capacitance for rs-422 protocol

Hi guys,

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?
 

DenisMark

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rs 422 protocol

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).

Press "help" button if my help is usefull.
 

genadyt

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rs422 protocol

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.
 

IanP

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rs422 load

The RS422 protocol does not specify "load capacitance" ..
The exact value has to be determined in an experimental way ..

For distance ≈100m and speed ≤115.2kbps (over CAT5) start with, say, 1nF ..

Regards,
IanP
 

DenisMark

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capacitance on rs422

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
 

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