Allied Telesis AR Series Hardware Reference Manual page 64

64 AR Router
The transition cable does 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 this section.
The DCE cable type reverses the direction of data lines as well as clock lines. This cable
is intended to be used when the router supplies the clock for an attached device. This is
commonly used when the router is connected to the synchronous port 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 port that is set to generate
the clock. See the Interfaces chapter of your router's software Reference for more
information. The AMPLIMITE connector supports both DTE and DCE modes for all
three physical standards.
Please pay particular attention to V.35 connections. The router interface is fully
compliant to V.35 but many DCE (DSU/CSU) devices are not. Because of the low signal
levels used for V.35 (500mV point-to-point), issues such as ground loops and excess
cable length become more important than with X.21. In particular, some DCE devices
connect the cable ground to chassis or safety ground. This can create an earth loop
condition through the shield wire of the V.35 interface. The symptom of this condition is
unexpectedly high received packet errors (CRC, Abort, etc.) as seen in the
SHOW SYN=n
command's output In these cases the suggested remedy is to break the earth
COUNT .
loop by disconnecting (and isolating) pin A inside the M34 connector. This action does
not compromise the safety of the product in any way.
Transition cables can be changed at any time. It is not necessary to power-down
the router.
Good quality data grade cables should be used for all cables. All transition cables must
be made from paired cable with an overall foil shield. Metalised backshells must also be
used. Poor quality cables will limit the maximum speed or the maximum length of the
cable, or both.
It is very important that the cables do not degrade EMC emission standards of the router
(e.g., FCC, CISPR). The cable foil shield must be connected to the AMPLIMITE
metalised backshell with a 360 degree ferrule. In addition, on specified cables, a clip-on
cable ferrite must be attached to the cable at the AMPLIMITE end, no more than 25 mm
from the rear of the backshell of the AMPLIMITE connector. Contact your authorised
Allied Telesyn distributor or reseller for a list of approved ferrites.
Table 31 on page 65 lists the wiring diagrams to follow when constructing
transition cables for each physical interface standard.
C613-03031-01 REV C