RS232 information

 Function  OFF ON
 Output voltage   –15V to –5V   +5V to +15V 
 Input voltage   –15V to –3V   +3V to +15V 
 Logic level  1 0
 Condition  Mark Space

25-way D-type

  1. (DTE === DCE) Protective Ground
  2. (DTE --> DCE) Transmitted Data
  3. (DTE <-- DCE) Received Data
  4. (DTE --> DCE) Request To Send
  5. (DTE <-- DCE) Clear To Send
  6. (DTE <-- DCE) Data Set Ready
  7. (DTE === DCE) Signal Ground
  8. (DTE <-- DCE) Data Carrier Detect
  9. reserved
  10. reserved
  11. reserved
  12. Secondary Data Carrier Detect
  13. Secondary Clear To Send
  14. Secondary Transmitted Data
  15. (DTE <-- DCE) Transmitter Signal Element Timing
  16. Secondary Received Data
  17. (DTE <-- DCE) Receiver Signal Element Timing
  18. reserved
  19. Secondary Request To Send
  20. (DTE --> DCE) Data Terminal Ready
  21. (DTE <-- DCE) Signal Quality Detector
  22. (DTE <-- DCE) Ring Indicator
  23. (DTE <-> DCE) Data Signal Rate Select
  24. (DTE --> DCE) Transmitter Signal Element Timing
  25. reserved

Serial format

The data line is normally OFF=low=1. Each character has the following format:

start bit  ON=high=0
data bit 0  ON=high=0 or OFF=low=1
data bit 1  ON=high=0 or OFF=low=1
data bit 2  ON=high=0 or OFF=low=1
data bit 3  ON=high=0 or OFF=low=1
data bit 4  ON=high=0 or OFF=low=1
data bit 5  ON=high=0 or OFF=low=1
data bit 6  ON=high=0 or OFF=low=1
data bit 7  ON=high=0 or OFF=low=1
stop bit(s)  OFF=low=1


DTE: Data Terminal Equipment (e.g. CRT or keyboard); normally uses male socket.
DCE: Data Communication Equipment, or Data Circuit-terminating Equipment, (e.g. modem); normally uses female socket.

DTR, DSR: The DTE turns DTR to ON to instruct the DCE to establish the communication link, and turns DTR to OFF to instruct it to disconnect the link. The DCE echoes the relevent state on DSR as soon as it has completed these functions.

RTS, CTS: The DCE turns CTS to ON if DTR, DSR, and RTS are all ON; before the DTE may transmit data to the DCE via TxD it must ensure that DTR, DSR, RTS, and CTS are all ON or not implemented. If the DTE turns RTS to OFF, then the DCE completes transmission of the data that the DTE has sent to it before turning CTS to OFF; the DTE may not turn RTS back ON before this has happened, and hence this handshaking may be performed after each block has been sent from the DTE to the DCE to prevent overrun. RxD is OFF whenever no data is being transmitted from the DCE to the DTE, and in half-duplex it must remain OFF whenever RTS is ON and also for a brief interval after it turns OFF to allow for transmission to be completed.

RI: The DCE turns this ON when it detect a ringing signal.

DCD: The DCE turns this ON when it detects a valid carrier signal, and transmission cannot occur while it is OFF.

Simplex: only one-way communication possible.
Half-Duplex: two-way communication possible, but not simultaneously.
Full-Duplex: simultaneous two-way communication possible.

Frequency Shift Keying: one constant frequency is used to represent logic state '1', and another is used to represent logic state '0'; in full-duplex using telephone wires, two distinct pairs of frequencies (four frequencies in total) must be used so that data being transmitted in each direction can be distinguished.
Phase Shift Keying: a constant frequency sine-wave is periodically shifted in phase; if, for example, four different phase shifts of 45, 135, 225, and 315 degrees are possible then two bits can be encoded (00, 01, 10, and 11 respectively) in each shift.
Phase Amplitude Modulation: in addition to phase-shifts as described above, a change in amplitude is also possible at each shift, hence increasing the number of logic states that can be represented by one phase and/or amplitude change.