Beware: when reading such sites, it is always important to refer to the original source document as the authoritative work.



That's a good question, and the answer requires a bit of background understanding:

The RS232 Standard specifically applies to the connection between a DTE and a DCE; in fact, its full title is


A DCE is a modem, or any other similar device for connecting to a communication network (eg, the PSTN - Public Switched Telephone Network).

A DTE is the termination of the data circuit; ie, the source and/or the sink of the data being transferred over the network.
Thus a DTE may be a computer, a terminal, a printer, etc, etc,..
"DTE" is most certainly not synonymous with "terminal" (as some sites erroneously state).

+-----+                                             +-----+
|     |         +-----+           +-----+           |     |
| DTE +==<===>==+ DCE +---<--->---+ DCE +===<===>===+ DTE |
|     |         +-----+           +-----+           |     |
+-----+                                             +-----+

Thus the DCEs (modems) and network just "pass through" the data between the two DTEs - in fact (apart from call setup and clearing) the DCEs should be completely transparent to the DTEs



No. The primary purpose of the DCE is just to transparently transfer data - the commands are merely incidental and, in some cases, you don't use any commands to the DCE at all!

Thus the Tx pin on the DTE is an output that sends data to beTransmitted;
The Tx pin on the DCE is also used for data to beTransmitted - hence it is an input to the modem!

In fact, the RS232 standard actually refers to Interchange Circuits - it defines Circuit BA as "Transmitted Data" which connects the DTE's output to the DCE's input
(this corresponds to V.24 circuit 103)