ITU X-Series Serial Interfaces

X.20 (Interface between DTE and DCE for start-stop transmission services on PDN

X.20 defines an asynchronous serial interface consisting of a Transmit and Receive circuit (plus their associated ground/common return).

The real content of X.20 is how call control is signalled over the serial interface. This is achieved without additional signals by "playing" with continuous-zero and continuous-1 signals on the T and R cirrcuits.

  • electrical characteristics can be X.26, X.27 or V.28
  • connector: 15-pole ISO 4903
  • characters for call control: 7-bit IA5 alphabet with even parity

Call control

Call control is used to
  • request outbound calls (and provide status from the network)
  • indicate inbound calls (and accept or refuse them by the user)

Call control state machine

Call control time chart

X.20bis (Use of DTE designed for async duplex V-series modems on PDN)

This interface has significantly more circuits than X.20:

Number Designation Comment
102 Signal ground / common return
103 Transmitted data
104 Received data
106 Ready for sending
107 Data set ready ON: ready for data; OFF: DCE clear indication/confirmation
108/1 Connect data set to line ON: Call request for direct call/call accepted; OFF: DTE clear request/confirmation
108/2 Data terminal ready see above
109 Data channel received line signal indicator
125 Calling indicator ON indicates incoming call
141 Local loop back
142 Test indicator
  • electrical charactreristics acccording to V.28
  • connector: 25-pole ISO 2110


See X21

X.21bis (Use of DTE designed for sync duplex V-series modems on PDN)

  • electrical charactreristics acccording to (either)
    • V.28 / 25-pole ISO 2110
    • X.26 / 37-pole ISO 4902
    • V.35 / 34-pin ISO 2593 (for 48 kbps)

Updated by laforge 10 months ago · 4 revisions

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