Datasheet

DS21Q50
57 of 87
16.2 Termination
The DS21Q50 is designed to be fully software-selectable for 75W and 120W termination without the need
to change any external resistors. The user can configure the DS21Q50 for 75W or 120W receive
termination by setting the IRTSEL (CCR5.4) bit. When using the internal termination feature, the
external termination resistance should be 120W (typically two 60W resistors). Setting IRTSEL = 1 causes
the DS21Q50 to internally apply parallel resistance to the external resistors in order to adjust the
termination to 75W. See Figure 16-3 for details.
16.3 Receive Monitor Mode
When connecting to a monitor port, a large resistive loss is incurred due to the voltage divider between
the E1 line termination resistors (Rt) and the monitor port isolation resistors (Rm) as shown in
Figure 16-1. The four receivers of the DS21Q50 can provide gain to overcome the resistive loss of a
monitor connection. This is typically a purely resistive loss/gain and should not be confused with the
cable loss characteristics of an E1 transmission line. By setting the receive monitor mode register as
shown in Table 16-1, the receiver can be programmed to provide 30dB of gain.
Figure 16-1. Typical Monitor Port Application
PRIMARY
E1 TERMINATING
DEVICE
Rm
Rm
MONITOR
PORT JACK
E1 LINE
X
F
M
R
Rt
DS21Q50
SECONDARY
TERMINATING
DEVICE
Table 16-1. Receive Monitor Mode Gain
MONITOR MODE
(ADDRESS = 1Fh) REGISTER
VALUE
GAIN (dB)
70h 30
00h 0