Synchronous Interfaces; Modems And Ntus - Allied Telesis AR Series Hardware Reference Manual

58 AR Router
Asynchronous interfaces follow the RS-232 standard. A range of speeds is
selectable using the SET PORT command from the Interfaces chapter of your
router's Software Reference.
Not all signals need to be connected. In particular, both DCD and CTS are not
required unless the interface has been specifically set up to use them. See the
Terminal Server chapter of your router's Software Reference for more details. A
typical terminal connection will only require RXD, TXD and GND, but refer to
the equipment manual for the connected device for more specific data.
If LPD-based printer ports are required, then the router may expect to 'see' the DTR
signal from the printer connected to the DCD pin of the router. This is used to detect
both the fact that the printer is 'online' and that it is 'powered-on'. This prevents print
jobs from being sent to a disconnected printer and lost. See the Printer Server chapter of
your router's Software Reference for more details.

Synchronous Interfaces

Synchronous interfaces use an AMPLIMITE 50-way connector wired as a
universal connector for RS-232/V.28, V.35 and X.21 interface standards in both
DTE and DCE modes. A transition cable is required to convert the
AMPLIMITE to the correct physical interface for the standard required (e.g.,
DB15 for X.21). The transition cable can be ordered from your supplier, or you
can construct your own cable.
Transition cables do more than simply change the physical connector. The
router hardware reads the cable type from the AMPLIMITE plug and enables
or disables the internal driver electronics as required. To function correctly, the
cables must be identical to the cables described in "Cables And Loopback Plugs"
on page 63.
DCE cable types reverse the direction of data lines as well as clock lines. This cable is
intended to be used when the router supplies the clock for the attached device. This is
commonly used when the router is connected to the synchronous interface of another
router or a host, without using an intermediate modem or NTU (Network Terminating
Unit). Therefore, the DCE cable must only be connected to a router interface which is
set to generate the clock. See the Interfaces chapter of your router's Software Reference
for more information.
Transition cables can be changed at any time. It is not necessary to power-down
the router.
When an RS-232 DCE transition cable is connected to a synchronous interface
configured to generate clocks, the maximum clock speed is 38400 bps. When an
X.21 DCE or V.35 DCE transition cable is connected to a synchronous interface
configured to generate clocks, the maximum clock speed is 2 Mbps.

Modems and NTUs

Modems and NTUs are used to transfer digital information over wide area
Telecom links. Strictly speaking, a modem is an analog device, while an NTU is
digital device.
Modems can be either asynchronous or synchronous. The distinction is that
synchronous modems provide a bit clock for both transmitted and received
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