RS485 support in pxa255

Question

I want to use rs485 placed on my card. I'm working on arm-linux and with pxa255 processor. I have already checked "serial.h" located in arm-linux tool chain but unfortunately i couldn't find the appropriate rs485 settings struct although it is supported in some other environments like cris. So now, do i have to write a low-level driver to enable rs485 or is there any other easier way to do this?

Answer 1

I'd recommend reviewing this page about Linux RS-485 support.

Answer 2

There seems to be quite a lot of confusion about serial ports with regards to RS485 mode support on Linux drivers. I think I might be able to shed some light answering this question.

First off, you should ask yourself: why do I want to activate RS485 support on my serial port? I don't know what development board or hardware you're using but be aware that most UARTs work with single-ended TTL levels only (5V or 3.3V levels). That means you won't be able to talk to RS485 devices directly because they only handle differential signals. If you want more details you can see this link: https://www2.htw-dresden.de/~huhle/ArtScienceRS485.pdf for a good introduction.

So you can write as many lines of code and drivers as you want but that won't help your TTL UART talk to RS485. Even if you have a device that supports RS485 on its driver, the hardware itself needs to have a level translator IC (like a MAX485) for you to be able to use it. Fortunately, our beloved Linux Kernel developers know a lot about hardware and you don't have to worry about this (for most devices anyway). Then, answering your question: if you are not able to find the RS485 settings on your driver it will most likely mean your hardware does not offer support directly.

Luckily for you, the solution is quite easy: just get a cheap USB to RS485 dongle. You can find them from 5-10$ and they should work fine out of the box for most scenarios. In my experience, FTDI chips work well.

If you really have to go with one particular UART for any reason whatsoever, for instance, imagine you want to use the UART on your Raspberry Pi or any other where you only have TX and RX signals accessible. Then you have a slightly more difficult challenge ahead of you and you need to study a bit more about how RS485 devices work. The key aspect here is to understand that RS485 on a half-duplex two-wire link (see note below for more details on this). To be able to share a two-wire bus where one or multiple devices are listening and only one is talking at any time, you need an additional control signal (Drive Enable/~Read Enable) on top of your TX and RX (note that when you go on single-ended TTL you have two signals referred to GND using three wires compared to one differential bus with two wires for the RS485 side). Your RS485 transceiver (say MAX485) will need this RE/~DE signal to arrange who is talking and who is listening on the bus, namely: all devices listening on the bus will have this signal low (read enable set) while the talker will be high (drive enable).

Here we get to the essence of your question: where can you get this additional signal? Well, the answer is: it depends on your UART chip. Some chips don't offer anything to work with, then you're left with the option of manually driving in your software application a GPIO line or flow controls signals (RTS or DTR) for UARTS where you have them available. You can read more details on this topic for the particular case of the FTDI chips here: RS485: Inappropriate ioctl for device. There is also a reference to a nice hardware solution to this problem using the mythical 555 timer IC.

Note: more confusing areas: one, RS485 is half-duplex and sometimes mixed up with RS422, which is full duplex and needs four wires; and two: where I say two-wire and four wires I should better say three-wire and five wires because the differential voltage signals need a GND to serve as a return path; sadly, most practical implementations of RS485 are two-wire and they seem to work reliably for most people, but that's a huge topic itself and we can talk about it somewhere else.