Termination resistors using bipolar transistors for RS-485?

Question

I am planning to use RS-485 on a network and I do not have too much experience on the standard but I understand that termination resistance must be provided for considerable lengths and transmission speeds. For CAT5e the impedance matching value is typically 120 ohms. If the impedance varies depending on the cable length, then:

1. Does the impedance matching resistor's value needs to be adjusted depending on the length of wire/speed of transmission, or can this be "activated" as a fixed value (120 ohms) whenever the length of cable increases or the user desires to increase the transmission speed?

2. Is it possible to provide electronic activation of the IM resistor using bipolar transistors? I have in mind something like this:

3. As the activation of the transistor may depend on how the polarity is present at the A/B lines of the RS-485, I figure it could be possible to activate a set of resistors via transistors managed by a microcontroller GPIO. Is this correct? if not, can you share a specific circuit arrangement for this?

If the resistance needs to be controlled dynamically (i.e. depending on length, wire type CAT5e = 120 ohms and CAT6 typically 100 ohms) then depending on a command, I figure I can program the 100 ohm IM resistor set or the 120 ohm set if I can use a controllable termination.

I am planning on using MAX3081 with low bias for communications. The recommendation on their diagrams is to use terminators across A/B lines, not from signal to ground (in case someone asks).

Let me know your thoughts/recommendations on this.

Answer 1

The impedance should not vary with the length of the cable. It is a combination of the cable inductance with its capacitance:

From this article.

See also excellent answers here.

There are ICs with "Activatable" termination (such as LTM2881) but that is for connections that are not point to point.

Your proposed "solution" would just try to short the RS-485 lines to ground directly, not provide a controlled termination. And the 120R resistors are always connected. Also note that "A" is not Tx and "B" is not Rx. "B" is just "A" inverted.

Answer 2

the tricky part with your updated schematic would be getting the voltage on GPIO1 and GPIO2 to a voltage somewhere between A+0.6V and B+0.6V the absolute bus voltages in RS485 are poorly constrained. (could be anywhere +/- 12V)

maybe something like this instead:

^{simulate this circuit – Schematic created using CircuitLab}

this uses a current source based on the GPIO voltage, Q5 and R4 to feed a current mirror Q3 Q4 which sends the current to bias the termination transistor on.

Only one transistor is used exploiting inverted beta.

If you don't have a 12V supply handy, perhaps use an optocoupler instead.