RS485 resistors

Get rid of the 4.7K and series 10 keep the terminator. Make sure the terminator should be 220. Make sure there is a decent ground for the signals. RS-485 is really a 3 wire system 2 signal one ground.

Aren't most wires more like 100 ohms?

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John Devereux

Usually, for UTP connections, the termination resistors are more like

120R, which matches the typical characteristic impedance of a twisted pair.

Biasing to define the idle state (assuming the reciever does not pre-empt this by having a controlled built-in offset voltage) is typically done with two resistors, one from A (let's say) to Vdd, and one from B (let's say) to GND. In order to get a couple hundred mV you need a fairly large current given the termination resistors (60R for two in parallel, usually), so values like 680R or 620R are common.

Since the two bias resistors are effectively in series with each other and in parallel with the termination resistor from the pov of the UTP, ideally you'd like to increase the termination resistor a bit, maybe to 133R, but often people don't bother.

For short runs, and low speeds, termination is not very important; almost anything works. You do need to establish a common ground between transmitter and receiver or you'll have problems.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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From a transmission POV, 135 Ohms is usually used at and below 56 kbit/s. From there to 1.544 Mbit/s about 110 Ohms is good, so your 100 Ohms should be fine.

Yes, so the OP's 220+10+10 is likely too high.

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John Devereux

That's how I see it.

I write here, but my answer is for all the others, whom I thanks too of course.

I'm not sure the ground is actually *common*. The cable may have a length of more 450 m. Both transceivers have a DCDC converter to isolate the ground: digital lines are optocoupled.

This is the previous design, working now. I have to redesign the whole system so if I can improve the serial communication it will be appreciated.

Let's say the maximum speed I can reach with this configuration is only

19200 bps. I need more.

Furthermore, the cable is very complex it carries several coaxial cables, optical fibers, ecc... So I don't know if its impedance is about

100 ohm.

Marco / iw2nzm

The maximum speed and distance depend on:

- the losses of the cable, Belden has high quality low-loss cables

- the length of the cable or used speed

- the allowed losses

- termination (prevents reflections)

19200 is very low. At 400 meter you can go much faster with RS485. I know a place where there is a 2 km cable running at 62,5 kbit.

You can check for information about cables and terminations in networks like Bitbus (I worked lot with it in the past, see

Profibus etc. Also use good transceivers like the SN75LBC176.

Make sure you termination impedance is the same as the cable characteristic impedance.

Regards, Pieter

I'm looking at it. Does the SN75LBC176 give better performances than MAX485? What are the critical parameters I must check on the datasheet?

I'm afraid I don't know the impedance of the cable. Perhaps I might try to measure it directly using a VNA.

Thanks for your answer, Marco / iw2nzm