AN00014.TXT

Advanced Communication Board Developer's Toolkit 1995
    

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Subject: Differences between DTE and DCE

Data Circuit-Terminating Equipment (DCE)

Data Circuit-Terminating Equipment (DCE) is defined as: The interface 
equipment that couples Data Terminal Equipment (DTE) to a transmission 
circuit or channel and a transmission circuit or channel to a DTE.

DCE equipment is typically installed at a users premises and provides all 
the functions that are required to establish, maintain, and terminate 
a connection (i.e. a modem, channel bank, CSU/DSU etc.).  It provides the 
signal conversion and coding between the DTE and the line which is defined 
as the medium of transmission (i.e. telephone line, satellite up-link, 
microwave transmitter, etc.).  The typical DCE connector pin-out is 
different than the pin-out of the DTE to provide for a direct connection 
between the DCE (modem) and DTE (computer serial port). 
EIA-232, EIA-530, EIA-449, CCITT V.35, V.36, V.37 and MIL-188-C/114 provide 
for DCE pin-outs.  

Please refer to the example below for DCE to DTE connection.

Data Terminal Equipment (DTE)

Data Terminal Equipment (DTE) is defined as: 1.) The equipment comprising the 
data source, the data sink, or both.  2.)  Equipment, consisting of digital 
end-instruments, that converts user information into data signals for 
transmission, or reconverts the received data signals into user information. 
3.)  The functional unit of a data station that serves as a data source or a 
data sink and provides for data communication control to be performed in 
accordance with link protocol.

DTE equipment can be thought of as the instrument or device originating 
or receiving the data. The serial port on the computer is the perfect example.  
The computer has no control over the medium of transmission, i.e. 
telephone line, satellite up-link, microwave transmitter etc., it merely sends 
the data through the serial port as it is told and receives the data back when 
expected.  The software layer operates the same if it is connected to a modem, 
channel bank, CSU/DSU or to another DTE through a "null modem" cable.
        
        DTE to DCE example using RS-232 pin-out:

               DTE SIGNAL              DCE SIGNAL
                 TD (2)                  RD (2)
                 RD (3)                  TD (3)
                 RTS(4)                  CTS(4)
                 CTS(5)                  RTS(5)
                 RXC(17)                 TXC(17)
                 TXC(15)                 RXC(15)

This is a simplified way to view the DCE-DTE interconnect. The actual RS-232 
pin out shows a signal either from or to the DCE. Thus TD (BA) would actually 
be transmitted from the DTE to the DCE and RD (bb) would be transmitted from 
the DCE to the DTE.