RS-485 bias & termination resistors with 3.3v transceiver and 100 ohm cable

Question

I am trying to connect two boards together with a few feet of Cat 5e cable. Both boards will be using 3.3v RS-485 transceivers (ISL83483 or similar).

Using this calculator, I entered 3.3v for supply voltage and 100 ohms for Rt1.

It gives a negative number for max. unit loads (-5.85). It says 1.31 if I reduce the noise margin to zero (which isn't good).

I know about the newer failsafe transceivers; however, I want to maintain compatibility with devices using older generation/non-failsafe transceivers. I also know it is commonplace to run short and/or low-speed networks without termination, but I have read multiple good answers on Stack Exchange that conclude termination really should be used.

Am I missing something or is it borderline impossible to have bias and termination resistors using 100 ohm cable and 3.3v transceivers?

Answer 1

It is not impossible in practice, but you need to understand how everything affects the bus and how to make it work.

The first problem why the calculator fails is that it uses values from RS-485 specs how a compliant driver must be able to deliver at least 1.5V into a 54 ohm load, which approximately equals terminations and 32 unit loads. This comes from the assumption that terminations are 120 ohms, and that the specs are not defined what voltage should be driven in a 100 ohm environment.

Because you have already chosen 100 ohm terminations which is a 50 ohm load even without any tranceivers connected to the bus, it already exceeds the rated load so that is why the number of devices is negative.

So a standard driver does not need to drive 1.5V into a 100 ohm bus with loads. It can still work even if it drives less, as long as it exceeds the input thresholds with the margin you want.

Like the standard says, 100 ohm bus may be used as long as you make tradeoffs to the values.

In practice, since you have only two devices, short bus, low speed, and slew limited driver, you might just consider the standard 120 ohm terminations even if the cable has 100 ohm impedance.