Electronics, Networking, Software

RS-485 practice and theory

Just built two RS-485 adapters (version 2) and decided to write a little note about the RS-485 bus (and differential line) theory plus perform some testing of the devices. This material mostly interesting for newbies.

At the end of this article, you can find a funny way to test these devices 🙂

Present adapters are based on the USB to serial converter which means that for computer’s OS these devices are just a serial interface. Every byte written to the interface is transferred as is and should be received on the other end of this “pipe”.

Of course, RS-485 is nothing to deal with transmission protocols.
This standard defines only an electrical differential line with a transmitter, receivers, line termination, etc. What is actually on the line is defined by the top-level application.

Let’s do some experiments.
Connect the adapter to the Linux PC and run ‘dmesg‘ command to find the file name of the device. If everything is ok there must something like /dev/ttyUSBX, where X is a number of the device.
Now writing and reading can be performed on the device.
Connect the oscilloscope probe to the FT232 pin 1. This is TX. Now everything is ready to send some data and capture some bits. Of course, at this point, it’s not RS-485 but a simple serial line.

Execute the following command:

echo -n "h" > /dev/ttyUSBX

Where X is your device number.
With this command, we just sent to the device one symbol ‘h’ which is byte 0x68, according to ascii table.
Argument ‘-n’ prevents echo from using ‘\n’ (newline) symbol after ‘h’.

0x68 is 00010110 in binary so such a pattern should be captured.

Here can be seen in data bits. Also, there are some additional bits in the beginning and in the end of the sequence.
This is start and stop bits which can help to separate the actual data frame. There can be additional bits called ‘parity bits‘ and can be used 2 stop bits.  All this depends on the serial port configuration in the operating system.

You may mention that values are inverted, an idle line always represents ‘bit’ 1. This happens due to the resistor between the data line and the power line inside the FT232. It’s the default behavior for such data lines (but can be changed in FT232 EEPROM).

Also can be measured the length of the one bit – 104 microseconds and measure total frequency which is 9.6 kHz (1/104 * 1000).
Hmm, 9.6 kHz sounds familiar, of course, it’s the speed of the line! Baud rate: 9600. This is a way of how an unknown baud rate can be found.

Let’s check serial port settings to ensure that correct data was captured. Run the following command:

tty -F /dev/ttyUSBX -a

(don’t forget to replace ‘X’ with your device number)

In the output should be the following values:

speed 9600 baud

This means speed 9600, 8 data bits, 1 stop bit, no parity.

Now we can do some measures on the RS-485 line. It’s a differential line that uses two wires. Both wires are transferring the same signal but one signal is positive and the second is negative. Both signals are corresponding to each other, common-ground wire is not required (typically).
The receiver’s side device measures the voltage difference between the two wires. High enough (±200mV) difference interpreted as a single bit of the data. This allows excluding interference on the line because both wires are affected by interference in the same way and this interference is not-differential signal so it just subtracting from itself, giving almost zero value.
Additional shielding allows using the very long and stable line (but low speed) up to 1.2 kilometers which is great for industrial applications.
Please note, that it’s highly recommended to use common-ground wire in case long line, especially between different buildings. This can help to equalize ground voltages between devices and avoid problems that can ruin your day.

Endpoints of the RS-485 line should be terminated with a resistor which is equal to the resistance of the cable. For typical CAT5 twisted-pair, this is 120 Ω.
Also, you can see in my (and other) adapter additional resistors that pull up positive and pulls down negative wires of the RS-485 line.
These resistors form a voltage divider which can help to set initial bias voltage on the receiver inputs. This can help to stabilize the line so the receiver can easily detect voltage changes during data transmission.

The differential nature of the RS-485 means that we can’t directly connect our oscilloscope probes to the line. Actually we can, but all measures will be not precise and sometimes incorrect.

To capture differential line signals should be used differential probes, like this one:









As a bonus differential probe can be used to perform measures on the circuits connected to the generic AC voltage (like switching power supply) without destroying the oscilloscope and probe wires.

I don’t have such a probe so let’s do it in an incorrect way 🙂

Connect the first channel of the oscilloscope to the line A+ (pin next to the common wire) and the second channel to the line B-.
In this situation, ground terminals of the probes can be left unconnected but this increases the noise level. To reduce noise level connect these clips to the common ground wire. Please note that this ground connection will not help to correctly measure the actual signal because it’s a differential line and it’s not referenced to the common wire.

Let’s repeat sending the ‘h’ symbol using echo. We should capture something like this:

As expected there are two signals, one of the signals is inverted. But the overall picture looks familiar to the previous measures on the FT232 TX pin. Here even can be measured baud rate. Of course, both signals are quite noisy.

Most of the oscilloscopes contain simple Math functions. Using this functionality the second channel can be subtracted from the first channel.
The subtraction result must be inverted. After this result should look more correct. The noise level was decreased.

Data can be easily decoded.
Actually I’m not sure about this ‘bit’ between idle state and transmission of the start bit. It looks like transmitter switching from the high-z state to the tx mode. The area was marked on the picture. If I’m wrong please correct me down in the comments.

You can get more information about RS-485 and wiring here.

Now it’s time to have some fun. How about ‘movie’ through the RS-485 line?

Of course not a real movie but a very cool ASCII-Art version of the Star Wars.

Let’s test the ‘movie’. Connect to the special server using telnet:

telnet towel.blinkenlights.nl

If everything is correct you should see the ‘movie’ in ASCII. To exit type Ctrl-] and then Ctrl-d

Due to the nature of this ‘movie’ we can transfer this stream using any kind of serial line, in our case it’s RS-485.

Connect two adapters to each other and connect USB to the same (or even different) computers. One terminal will be server and the other is the client.

For the client, we need some kind of terminal program which can work with a serial line. The most popular utility for the console mode is minicom. Install minicom if you don’t have it.

Now on the server side run telnet again but this time redirect all output to the connected adapter:

telnet towel.blinkenlights.nl > /dev/ttyUSBX

Replace X with your device number.

On the client-side run minicom for the adapter device:

minicom -D /dev/ttyUSBX

Enjoy the ASCII movie on speed 9600 via RS-485 line 🙂

How it looks like on video:

Thanks for reading!

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