Serial ports

Serial port pinouts

Fig's 1&2 show the more common 9 pin serial pinouts, on both the computer side and also the device side respectively. Fig.3 gives a more visual representation of the male variant, most commonly found non the PC side. I think this was historically due to a male being more prone to damage, hence the female end being more commonly used on cables, but I may be wrong.

Fig.5 is particularly useful when testing cables, although I have only really connected pints 2&3 for this purpose. The idea here is that you make a plug that connects the pins together and then you can open the port and whatever you type should be echoed back exactly. 

Fig's 6-10 provide details about Null Modem connectivity.

Fig.11 provides more pinout details for the DB25 connectors

Fig.1 [source]

Fig.2 [source]

Fig.3 [source]

Fig.4 [source]

Fig.5 [source]

Fig.6 [source]

Fig.7 [source]

Fig.8 [source]

Fig.9 [source]

Fig.10 [source]

Fig.11 [source]

What follows is some more information on terminal settings.

stty -F /dev/ttyS0 -a can be used to view terminal settings, while stty is used to change settings.

The output of stty -F /dev/ttyS0 -a could be;

 

speed 2400 baud; rows 0; columns 0; line = 0;
intr = <undef>; quit = <undef>; erase = <undef>; kill = <undef>; eof = <undef>; eol = <undef>; eol2 = <undef>;
swtch = <undef>; start = <undef>; stop = <undef>; susp = <undef>; rprnt = <undef>; werase = <undef>; lnext = <undef>;
flush = <undef>; min = 0; time = 1;
parenb -parodd cs7 -hupcl -cstopb cread clocal crtscts
-ignbrk -brkint ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt -echoctl -echoke

 

 with some explanations;

• crtscts: Hardware flow control (RTS/CTS) is enabled.
• -ixon: Software flow control (IXON) is disabled.
• -ixoff: Software flow control (IXOFF) is disabled.
• parenb: If present, it indicates that parity is enabled. If absent, parity is disabled.
• parodd: If present, it indicates that odd parity is used. If absent, even parity is used.
• rows: This typically indicates the number of rows or lines that are available on the terminal screen.
• columns: This usually indicates the number of columns that are available on the terminal screen.
• line: This refers to the number of lines per page.
  

The setting of a terminal with stty, for example;

stty crtscts -ixon -echo cs7 parenb -icrnl -inlcr -ocrnl onlcr speed 2400 -F /dev/ttyS0

• crtscts: Enables hardware flow control using RTS/CTS (Request to Send/Clear to Send).
• -ixon: Disables software flow control (IXON).
• -echo: Disables echoing of input characters.
• cs7: Sets the character size to 7 bits per byte.
• parenb: Enables parity generation on output and parity checking for input.
• -icrnl: Disables mapping of CR (Carriage Return) to NL (Newline) on input.
• -inlcr: Disables mapping of NL to CR on input.
• -ocrnl: Disables mapping of CR to NL on output.
• onlcr: Enables mapping of NL to CR-NL on output.
• speed 2400: Sets the baud rate to 2400 bps (bits per second).
• -F /dev/ttyS0: Specifies the serial port device file, in this case, /dev/ttyS0


• Hardware Flow Control (crtscts): Enables hardware-based flow control, which uses the RTS and CTS signals to control data transmission between devices.

• Software Flow Control (-ixon): Disables software-based flow control. This is important when hardware flow control is used.

• Echo (-echo): Disables the echoing of input characters back to the terminal.

• Character Size (cs7): Sets the character size to 7 bits.

• Parity (parenb): Enables parity generation on output and parity checking for input.

• Input Carriage Return (-icrnl): Disables mapping of CR to NL on input.

• Input Newline (-inlcr): Disables mapping of NL to CR on input.

• Output Carriage Return (-ocrnl): Disables mapping of CR to NL on output.

• Output Newline (onlcr): Enables mapping of NL to CR-NL on output.

• Baud Rate (speed 2400): Sets the baud rate to 2400 bps.

• Serial Port (-F /dev/ttyS0): Specifies the serial port device file, in this case, /dev/ttyS0.