Hello
I understood that RS-422 is a differential protocol with resistive termination between the lines and load capacitance is determined by cable.
But, we want to use capacitors near the board connector for circuit protection/compliances requirements. I need to determine the maximum differential capacitance value that the driver (ISO1432B) can handle it without to much stress.
Could you please clarify?
thank you
Hi Johnson,
There is no defined limit for the maximum capacitance value that the ISO1432B driver can handle. The maximum capacitance that your system can withstand is dependent on the data rate that you are operating at. In addition to this, the design of your system will also determine the differential capacitance value that you will be able to add onto the bus. A longer cable will add more capacitance to the bus, and impact the data rate that you will be able to operate at. The plot below shows the relationship between cable length and data rate. As previously mentioned, cable length is directly related to the capacitance present on the driver, so you can use this as a reference when determining the maximum differential capacitance value that you can add near the board connector.
In addition to this, I would also like to point out that in the device datasheet, a capacitance value of 50 pF was connected to the bus when measuring the provided switching characteristics of the driver. All of these measured parameters are guaranteed, so you can also use this as a reference. These parameters can be found in section 8.13 of the ISO1432B datasheet.
Regards,
Kenneth
