Baud rate simulation

Question

I need to create a null modem simulation to test various pieces of software. I've been using socat to do this:

socat -d -d pty,raw,echo=0 pty,raw,echo=0 &

But I also need to simulate the basic baud rates of the serial port and the effects of UART buffers of various lengths. socat's baud rate (b####), ispeed, and ospeed parameters don't seem to have any effect when doing this type of pty loopback. For example:

socat -d -d pty,raw,echo=0,b50,ispeed=50,ospeed=50 pty,raw,echo=0,b50,ispeed=50,ospeed=50 &

Should result in a very slow data rate. Yet when passing data through this loopback, the data is transferred at the maximum machine speed. This doesn't surprise me since there's no real hardware in the loop.

So, is there a way to simulate UART timing short of writing custom code?

My ultimate aim is to simulate a large Ethernet-to-Serial device like a Moxa 5600 (16 ports) and how the effects of serial throughput affects the TCP socket streams etc... etc...

Thanks,

Answer 1

I don't regard this as a practical solution, but it is a fun one. Why simulate a null modem, when you can simulate a real modem. minimodem is available as a package in some distributions and converts data into modem audio tones, and vice versa. To try it out, just listen to this audio at baud rate 10 and 0.1 volume:

$ echo 'the quick brown fox' | minimodem  -v 0.1 --tx 10

This takes about 20 seconds to run. If you direct the output to a file, it is instantaneous, but you can then "play" the file with sox and have the tones interpreted back to data:

$ echo 'the quick brown fox' | minimodem  -v 0.1 --tx 10 -f out.wav 
$ file out.wav
  out.wav: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 48000 Hz
$ sox out.wav -t wav - | minimodem -f - --rx 10
  ### CARRIER 10.00 @ 1590.0 Hz ###
  the quick brown fox
  ### NOCARRIER ndata=20 confidence=33.775 ampl=0.064 bps=10.00 (rate perfect) ###

This is still instantaneous, but shows that it works.

To play the tones in the wav file at the right speed, it needs to be played through an audio device. But we can use software loopback devices to do that:

$ sudo modprobe snd-aloop
$ aplay -l
 **** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC662 rev1 Analog [ALC662 rev1 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM]
  Subdevices: 7/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7
card 1: Loopback [Loopback], device 1: Loopback PCM [Loopback PCM]
  Subdevices: 8/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7

We now have 8 loopback devices (the module has a parameter to ask for more). For example, we can now play sound on card 1, device 1, subdevice 2, and listen to it on card 1, device 0, subdevice 2. To play, taking 20 seconds:

$ sox out.wav -t alsa hw:1,1,2 
out.wav:
 File Size: 1.96M     Bit Rate: 768k
  Encoding: Signed PCM    
  Channels: 1 @ 16-bit   
Samplerate: 48000Hz      
Replaygain: off         
  Duration: 00:00:20.40  
In:47.7% 00:00:09.73 [00:00:10.67] Out:467k  [    -=|=-    ]        Clip:0    

To listen:

$ sox -q -t alsa hw:1,0,2 -t wav -c 1 - | minimodem  --rx 10 -f -
### CARRIER 10.00 @ 1590.0 Hz ###
the quick brown fox
### NOCARRIER ndata=21 confidence=10.265 ampl=0.060 bps=10.00 (rate perfect) ###

The characters take 20 seconds to appear, one by one.