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Hello all,
in order to protect the RS485 transceiver against ESD, EFT and surges we add a TVS device parallel to the bus lines.
Unfortunately, these devices add some capacitance on the bus lines, which, some how, affect the communication.
I feel that the bus speed will be restricted / decreased, as the total bus length too.
The number of nodes will also be decreased, as this number will actually be a multiplier of the effect.
My question is, how we can quantitate the effect of TVS capacitance on bus speed, bus length and nodes number,
in order to select the proper / best TVS device for our application.
An idea I have is to express the TVS capacitance to bus length equivalent capacitance
and then use the various graphs (length vs speed) available on many RS485 guides.
Assuming a cable capacitance for RS485 as 50pF/m, a TVS with 300pF would be equivalent to 6 meters of cable,
so each node equipped with such TVS would decrease the total bus length by 6 meters.
Dear Max,
thanks for the reply.
Your approach seems to me as a complete electrical network analysis,
but not easy to perform on a bus which is not known in detail.
A lot of assumptions needed and in every installation all the elements of the network will be different.
We are designing a series of "slave" RS485 modules, the transceiver IC (THVD1500, THVD1510) has only 1/8 UL resistive load
and although we could claim a capability of 256 nodes on the bus, the added TVS device/devices will introduce a capacitance on the lines which may lower the number of nodes we could claim.
The higher bus speed we are interesting to is 115.2Kbs or 250Kbs (max).
But the bus installation our modules are going to be used is every customer's decision, so it is not possible to know the actual bus speed (lower than the above upper limits),
neither the nodes number / bus length / cable type.
We would like to offer a guidance to our customers for the bus nodes number in relation to the bus speed and cable length, as recommended/safe operation conditions of our modules.
This is a point where we would need some help.
Regarding the driver equivalent output resistance (THVD1500), what you mean is to calculate the two slopes (of the high side and low side drivers) and add them?
What Is a typical RS485 driver equivalent output resistance?
Regards,
Nikolas