What exactly is "serial?"

[erikbgithub]

Problem

In monk we use the pyserial framework for serial connections. Yet serial itself is not a standard like USB.

Solution

Investigate what a serial connection is, how the standard works, which options exist and which errors might be possible.

[erikbgithub]

General Investigation

Serial Port on Wikipedia (link ff)

• Standards are RS-232, and maybe UART
• USB is a new replacement for RS-232
• unidirectional communication
• Macs sometimes used the RS-422 standard with Mini-DIN connectors
• other connectors:
  • DE-9
  • 8P8C
• there exist Virtual Serialports
• without flow control typical errors:
  • wrong characters are received
  • characters are lost, because not read fast enough

Settings

• notation: Data/Parity/Stop (typical 8/N/1, also default in DFE products)
• Speed:
  • bits / seconds and bauds the same for serial?
  • port speed and device speed must match
  • not all ports support all bitrates
  • framing data reduces actual data transfered, so real bitrates are lower
  • commonly supported: 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bit/s
• Data bits:
  • 5 (for Baudot code), 6 (rarely used), 7 (for true ASCII), 8 (for any kind of data, as this matches the size of a byte), or 9 (rarely used)
  • also little endian vs big endian (oftren not configurable)
• Parity:
  • sends additional bit for each byte, so that the sum is either always even or always odd
  • can detect an uneven number of byte errors
  • settings: none (N), odd (O), even (E), mark (M), or space (S)
  • odd is most useful
  • none is most used, with a communication protocol on top ensuring data safety
• Stop Bits:
  • additional bits that signal the end of a byte
  • usually 1, but for slower machines also 1.5 or 2 possible
• Flow Control:
  • common handshakes: RS-232 RTS/CTS or DTR/DSR
  • also sometimes use XON/XOFF

PySerial Docs (link)

• allows all researched configuration, while object creation and afterwards
• to debug the python code it's possible to connect to loop://logging=debug
• command for fast debugging is in a terminal:
  $ python -m serial.tools.miniterm loop://logging=debug

UART to Serial Terminal (link)

• MIDI protocol might implement some data safety features on UART
• Problems with the first characters might occur when speed is >9600 baud
• explains how to use screen as serial terminal (e.g. closing with CTRL+A k y)

Serial Programming Guide for POSIX Compliant Operating Systems (link)

• empty line is always a never stopping 1
• start bit is always a 0
• stop bits always value 1 (like the "no communication" part it's basically a waiting time until the next frame is sent)
• a break is a long 1 (about .5 or .25 sec) and is used to reset communication or change modes

Bit für Bit (link)

• original TTY (=teletype) interface was working with 20 mA vs 0 mA signals instead of the common 0V vs 5V or -12V to -3V vs +3V to +12V interpretation of today's serial connection types
• RS-232 was developed for computer to modem communication
• sender should have the same amount or more stop bits then the receiver (note by @binschek: but maybe not good in bidirectional communication)
• decreasing baud rates increases reliability

[erikbgithub]

Typical Problems

• sync time in the beginning (probably needs active I/O)
• one side might lose characters on the way
• one side might misinterprete characters
• one side might have different baudrates
• one side might have differently working clocks (which leads to misinterpretation of baudrates)
• one side might stop sending because it waits for a control signal (hardware or software flowcontrol activated on one side only, or control characters lost)
• one side might drop received data because it waits for a control signal (hardware or software flowcontrol activated on one side only, or control characters lost)
• if the connection is lost for some time mid-sending it's quite hard to find the beginning of the next character (might be solvable by small periods of at-least-one-frame-long waits)

[erikbgithub]

Interesting reads:

• Wikibook about Serial Programming
• good summary of everything tty

[erikbgithub]

answered