22
FALLBACK SWITCH AND LOOPBACK SWITCH
• The command BMyzz<CR> applies only to Loopback Switch units. It sets the
default loopback timer with the yzz operand, where y is D, H, or M for days,
hours, or minutes and zz is 1-99 days, 1-24 hours, or 1-60 minutes.
L
OOPBACK
S
WITCH
C
ONSIDERATIONS
If you use a “B” command to place a Loopback Switch in loop mode, the standard
loop configurations (illustrated below) assume that the Loopback Switch is
connected to a customer DCE interface. The incoming Receive Data is connected
directly to the outgoing Send Data with no delay or buffering of any nature. If the
data is asychronous (unclocked), this technique will function under all
circumstances. However, if data is synchronous, the DCE-supplied Transmit Clock
speed and phase must be synchronized to the Receive Clock. Depending on the
nature of the attached DCE, you can do this in one of several ways:
• If the attached DCE is a DSU or other device connected to a digital network,
both the Transmit Clock and the Receive Clock are typically slaved to a master
network clock and are therefore speed-synchronized and usually phase-
synchronized to each other.
• If you can’t confirm that the Transmit Clock and Receive Clock are both speed
and phase synchronized, the attached DCE must be able to accept an External
Transmit Clock and, when the loop is activated, must be configured in that
mode. The Loopback Switch will loop the Receive Clock output to the
External Transmit Clock input, thereby forcing the two clocks to be speed- and
phase-synchronized to each other.
Table 5-1. Standard Loopback Switch loop for RS-232 DCE
From
To
Signal
Pin(s)*
Signal
Pin(s)*
RD
3
SD
2
DSR
6
DTR
20
DCD
8
RTS
4
SCR
15
SCT
24
*Referenced to RS-232 DB25 connector.
