Microsoft KB Archive/43135
RS-232C 25-Pin and 9-Pin Connector Information PSS ID Number: Q43135 Article last modified on 04-13-1989 PSS database name: PSS
The following is a list of signals used by serial (COM) ports, also known as the RS-232C standard:
25-Pin Connector 9-Pin Connector
Pin Signal Pin Signal
1 Chassis Ground 1 Carrier Detect 2 Transmit Data 2 Receive Data 3 Receive Data 3 Transmit Data 4 Request to Send 4 Data Terminal Ready 5 Clear to Send 5 Signal Ground 6 Data Set Ready 6 Data Set Ready 7 Signal Ground 7 Request to Send 8 Carrier Detect 8 Clear to Send
20 Data Terminal Ready 9 Ring Indicator 22 Ring Indicator
Note: The signals from Pins 9-19, 21, and 23-25 were all associated with secondary signals that are no longer utilized in the computer environment; this is the reason for DB9 connector.
There are two general cable configurations associated with the RS-232C Communications Standard: Data Terminal Equipment (DTE) and Data Communication Equipment (DCE).
Data Terminal Equipment (DTE) is normally associated with serial devices such printers, plotters, and miscellaneous pointing devices including mice, data tablets, trackballs, etc. The following is a general cabling diagram:
1 - 1 4 - 5 2 - 2 6 - 8 - 20 3 - 3 7 - 7
Data Communication Equipment (DCE) is normally associated with serial devices such as modems or direct line communications. The following is a general cabling diagram:
1 - 1 4 - 5 2 - 3 6 - 8 - 20 3 - 2 7 - 7
Note: The difference between DTE and DCE is that serial devices, such as printers, plotters, and pointing devices, are generally one-way communication equipment (printers and plotters from computer, pointing devices to computer), while modems are two-way communication devices; this is the reason for the reversal of Transmit Data (Pin 2) and Receive Data (Pin 3).
Copyright Microsoft Corporation 1989.