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Differential Pair for MAX485E

Didan , 06-04-2019, 10:13 AM
Dear Forum, I have doubt how to length mathing (if necessary, idk if it is really necessary). Iam checking a layout board in my company and looking on it I realized theres a mismatch for RS-485 communication using chip MAX485E. So I have a connector which I will read MODBUS using RS-485 protocol and looking on MAX485E datasheet it says that differential driver output R= 27ohm (RS-485) I think it means I need to do length matching for this impedance, but how to calculate it using Saturn calculator, for instance or any other calculator like Kicad calculator?. I would like to know how to calculate trace width and gap to route differential pair in this situation. I have a 2 layers board, using FR-4 (Er=4.6) ; TanD = 0.02 ; Height of substrate (thickness): 1.6mm ; Rhou = 1.72e-08. Im using Kicad with diff pair width = 0.2mm and width gap = 0.25mm set as default in Software. If someone have any document to help me about theory too gonna be good. THanks
chitransh92 , 06-05-2019, 04:10 PM
Hello @Didan
The bus impedance of the RS485 is approximately 100-120 Ohms differential and control trace impedance as ~50 Ohms
Make sure to follow below for efficient RS485...
- Since you have 2 layer board, use a ground polygon beneath the RS485 communication (Both Control and Bus circuitry)
- Several points in the layout as shown in the attached image can be improved, however you can start off by placing the component as shown in attached image as this will reduce stub length.

For the Saturn PCB calculator:

- Differential Pairs tab -> Play around with the values and use conductor height as you dielectric distance
- A quick example gives a impedance of ~125 Ohms with
-> Conductor width = 18 mils
-> Spacing = 6 mils
-> Height of conductor (this is distance from plane) = 60 mils

For reference document..

Search the web for "RS485 layout guidelines" and you shall find what you are looking for.
A good reference at https://www.renesas.com/in/en/www/do...data-links.pdf

Thank you.

Didan, 06-07-2019, 12:54 PM
Hi chitransh92. I dont have sure about what u suggested to do about your red marked. Theres a problem about signal AB+ and AB- ?
robertferanec , 06-09-2019, 02:13 AM
When you try to calculate track geometry (track width and spacing) for specific impedance on 2 layer PCB, usually you end up with very weird numbers - means you may give up on trying to use that width and space. If you really have to use 2 layer PCB for differential pairs, in most cases you may want to just use standard width and clearance (same as you use for other tracks), BUT keep the tracks on PCB as short as possible.
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