NetBurner Community Forum

For RS-485 on a CB34EX
the manual states:
JP23 / JP26 Connect 1-2 (optional), Half-Duplex Biasing and Termination
Connect 2-3 (optional), Full-Duplex Biasing and Termination

Can anyone provide more details on this description?
Does Termination, mean a 120ohm Termination?
What is Biasing?

Thanks,
Tim

Hello Tim,

Before going into those details, can you tell me what you are connecting to, and if that device has options for termination or biasing? It will be easier to explain if you can describe what it is and how many devices are on your 485 bus, as these are bus terms and selections dependent on the bus structure.

Take a look at page 4 here, where Rb is bias, and Rt is termination:
https://www.ti.com/lit/an/slla272d/slla ... 2553535528

This is not the part used on the CB34EX, but it has good graphics. The termination and bias in the CB34EX are all 120 ohms

Hi TomNB,
We connect to many different devices, each application is different. For the most part they are Modbus RTU RS-485 2-wire Half Duplex, and we are typically only connecting between a CB34EX and just one other device on either port. If we have two devices, we would try to use the 2 different ports.
Our cable runs are rarely over 6 ft., so we typically don't use 120ohm termination resistors. But when we do, we can either enable it on the other end, or we can install a physical resistor.
I reviewed your link, thanks, but I guess I still don't know if your JP23/JP26 jumpers are indeed adding the 120ohm resistors on a RS-485 2-wire setup?
Is there a situation where we may only want to use one of the 2 jumpers, or do we always use both?
Thanks,
Tim

Thanks Tim. The goal is bus impedance matching. A RS-485 bus is a multi-drop bus, meaning it can have more than one device on it. Whether just two devices or many more, only each end of the bus should have 120 ohm termination, and optionally bias resistors. The impedance matching reduces reflections, noise, etc.. The correct settings depend on the specifics of the bus and devices. If you find that with your cabling at 6' or less you don't need termination that is fine. If one device does have termination, you typically want it enabled on the other end as well.

To answer the question, yes, if you install JP23 and JP26 it does add the 120ohm termination and bias resistors. The different pins are used depending on if your using half or full duplex. JP23 and JP26 are each 3-pin headers.

If half duplex, install two jumpers, each in pin positions 1-2 of JP23 and JP26.
If full duplex, install two jumpers, each in pin positions 2-3 of JP23 and JP26.
You will never use just one jumper, otherwise the bus would be biased on one direction only.

That is what the manual is attempting to communicate on page 14.

Here is that section of the schematic:

Good info Tom, thank you very much!

Tom, another question,
If I'm doing RS-485 on both ports, the JP23/JP26 termination resistors only apply to the TSTRIP5 correct?
Are there no termination jumpers for RS-485 on the DB9?
Thanks,
Tim

Hello Tim,

That is correct. If you want termination on the DB9, they need to be added to the connector or cable.

Hi Tom, I appreciate the confirmation.
Thanks,
Tim