Technical data

Managing Peripheral Devices
8.7 Managing Modems
Table 8–4 Common Pinouts on the EIA-232 DB23 Connection
Pinout Description
Data Terminal Equipment
(DTE)
Transmit information through pin 2, and receive information
through pin 3, among other standardized pin assignments.
Data Communications
Equipment (DCE)
Transmit information through pin 3, and receive information
through pin 2, among other EIA-232 pin assignments.
Straight-Through and Cross-Over Wiring
Descriptions of straight-through and cross-over wiring follow:
• DCE devices communicate straight-through with DTE devices: the
transmit pin on each end of the cable is wired to the corresponding
receive pin on the other end. Pin 2 on one cable is connected to pin 2 on
the other cable, and pin 3 on one cable is connected to pin 3 on the other
cable.
• Equipment wired with a DCE pinout requires a cross-over to
communicate with another connector wired DCE; pins 2 and 3 on one
cable are connected to pins 3 and 2 on the other cable, respectively. A
cross-over is required in certain situations, because two transmit pins or
two receive pins cannot be wired together. As a specific example, you need
a cross-over to wire two DTE devices together, or to wire two DCE devices
together.
A cable with cross-over wiring is sometimes referred to as a null modem
cable, because a null modem cable of an appropriate length could logically
replace all components of a modem-based communications connection;
that is, it could replace the local serial cable, the local modem, the
intervening telephone circuit, the remote modem, and the remote serial
cable.
Table 8–5 describes the most common connectors used to wire a modem.
Table 8–5 Connectors
Connector
1
Description
DB9 A 9-pin connector, containing a row of four pins, and a row of five pins.
The DB9 can have the EIA-574 commonly used on PC systems or an older
standard connection used on MicroVAX consoles.
DB25 A 25-pin connector, with a row of twelve pins and a row of thirteen pins.
The DB25 typically uses the EIA-232 pinout and can be wired as Data
Terminal Equipment (DTE) or as Data Communications Equipment (DCE).
MMJ A 6-pin modular jack, which uses DEC-423 signaling, commonly referred
to as DECconnect wiring. DECconnect wiring greatly simplifies wiring
devices, as one need consider only the appropriate adapter for the device
connection; the associated BC16E cabling is wired consistently.
1
All connectors in this table are available in both male and female genders.
The pinouts and applications for the common connectors are shown in
Table 8–6.
8–28 Managing Peripheral Devices