Specifications

Chapter 5 - 030 Processor Module
66 Adept MV Controller User’s Guide, Rev C
DTE, DCE, or AT-style?
There are two ways to make an RS-232 connection: detailed research and full
understanding; or a try-it-and-see method. In general, you will do no harm if you make an
incorrect connection between two genuine RS-232 ports. Try either of the connection types
described above (DTE or DCE) and see which works. If neither works, either the other
device has a non-standard pinout, or the devices are incorrectly configured (for example,
different speeds or different handshake methods).
The following information may assist if you want to attempt the research method.
Otherwise, we recommend you skip the rest of this sub-section.
The RS-232C standard (and its successors, EIA-232-D and ANSI/EIA/TIA-232-E) define
two types of apparatus:
• DTE: Data Terminal Equipment
• DCE: Data Circuit-terminating Equipment (used to be known as "Data
Communications Equipment.")
Modems are supposed to be DCE devices. Terminals are supposed to be DTE. Both are
supposed to use 25-pin D-Sub connectors. The standard only defines the connector and
pinout at the “point of demarcation”, which is to be no more than 3 meters from the DCE
device. Computers and printers are not explicitly addressed by the standard. In practice,
they often have a DTE-type pinout. Some have a DCE pinout. Increasingly, many
computer devices use an IBM PC-AT style 9-pin connector, which is strictly neither DTE
nor DCE.
The standard assumes that you have two devices, one DTE, the other DCE. If you need to
connect two DTE devices, the standard assumed that you would use a pair of modems or
other DCE devices. To directly connect two DTE devices, you need to use a null modem to
make the necessary connections. A null modem is an adaptor connector (or cable) that has
DCE pinouts at each end. Null modems are readily available at most computer stores. If
you need to build your own null modem, the table below shows the typical pinouts.
Table 5-7. Typical 25-pin Null-Modem Adaptor Pinout
25-Pin D (male) 25-Pin D (female) 25-Pin D (male) 25-Pin D (female)
1 Shield 1 Shield 5 CTS 4 RTS
2TXD 3RXD 6 & 8 DSR & CD 20 DTR
3RXD 2TXD 20DTR 6 and 8 DSR & CD
4 RTS 5 CTS 7 Ground 7 Ground
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