RS485 minimum node spacing - 3 x RS485 device on PCB

Question

I would like to ask something about RS485 minimum node spacing which is described in (Page 5)

TI Design guide

I would like to create device which consists from 3 existing RS485 devices. My plan was that I put these 3 devices together on single 2 layer PCB and connect them via single RS485 line (A, B). According to material above this shouldn´t be a good idea because there will be very small length traces between these devices.

Do you have any experience with this approach of placing more RS485 devices on same board? Is there any technique which could help the signal integrity?

Thanks

Answer 1

termination is only required on long lines with high data rates. The purpose of the termination is to make the end of line impedance same as wire impedance to avoid reflections of the wavefront. But this is totally unnecessary on a short bus.

My company manufacturers 100,000+ devices a year (for 10+ years) with a 2 meter RS-485 bus and we don't use terminators.

There are many app. notes on this.

Answer 2

RS-485 is a differential bus. This means it is very noise immune. There are two lines A and B. You need to bias the bus so the (A-B) is around -200mV when the bus is idle. Try a potential divider like: 5V->1k2 B 120 A 1k2 0V. That's the biasing for the RS-485 bus its self. That will mean when no RS-485 chip transmit is enabled the bus will be in idle state.

A small bus length does not matter. What does matter is the termination. You have to make it so that when idling both A and B are near the middle voltage with ([(A-B) < -200mV] see above) but when operating does not have open ends to the bus. Put 120 Ohms at either end of the bus as well.

This is pretty good https://www.maximintegrated.com/en/app-notes/index.mvp/id/763

Answer 3

It depends on your data rate. Lower frequencies can be closer together, what you are trying to avoid is crosstalk. Use termination resistors on the lines and you should not have a problem. Most commercial solutions are internally terminated to some degree. That said, if the bus length is long, especially if its longer that the 'official' maximum length, you get into a grey area where terminations and line conditioning play a larger role. Always terminate at the receiver, never at the transmitter. The purpose is to change an LC circuit into an RLC circuit, which generally has better frequency characteristics. Terminating at the transmitter makes the problem worse though.

Answer 4

There is no problem with short distances and bus termination is not required. If a device is always a master there is no need to bias the bus. If you have a multi-master system, where there are times that no device is driving the bus, a bus bias scheme might be required, but it would depend on the RS-485 drivers being used. Most modern drivers don't require a bias. A bias circuit can be found in many RS-485 driver app notes. Its essentially a bus termination with a resistor connected to one of your power supplies. It doesn't hurt to terminate a bus if it isn't required.