Comtech EF Data CRS-170A - Manual (Page 26)

CRS-170A L-Band 1:1 Redundancy Switch MN/CRS170A.IOM
Introduction Revision 13
1–6
• Fourth, +12V is routed from each modem to the CRS-170A for operational power. This
power from both modems is diode OR’d so that the switch remains active if power is lost
from one of the modems.
Mode Switches – The Mode DIP switches, located on the antenna side of the CRS-170A, allow
the switch to operate in two different modes: Normal (Intelligent Controller) Mode and
SDM-300L/CDM-600L (Dumb Controller) Mode.
• Use Normal (Intelligent Controller) Mode for CDM-625/A, CDM-850,
CDM-840, CDM-760, CDM-750, CDM-710GL, CDM-710, CDM-700, and
CDM570L/AL modems.
• Use SDM-300L/CDM-600L (Dumb Controller) Mode for SDM-300L and
CDM-600L (CLM-9600L) modems.
IF Switching – The CRS-170A provides L-Band redundancy, but when switching occurs, it
accommodates more than just the L-Band signals:
• On the Tx side, it switches the Tx L-Band signal, the 10 MHz reference to the BUC, DC power
to the BUC and, on CDM-570L modems, FSK signaling to the BUC.
• On the Rx side, it switches the 10 MHz reference to the LNB, and DC power to the LNB. The
Rx L-Band signal from the LNB is split in a power divider to continuously drive both the
Online and Offline demodulator. The Offline demodulator can then maintain lock so that it is
ready to quickly assume Online status for receive traffic when a switchover is required.
Data Switching is accomplished inside each modem, in the data interface itself. A ‘Y’ cable
connects both the Online unit and the Offline unit to the terrestrial data source. Transmit data
drives both modems in parallel so that the Backup modulator is always ready to come Online.
Termination impedance for the modem line receivers is active in the Online unit. On the Rx side
of the interface, TRI-STATE® control of the Offline unit line drivers keeps the Offline unit from
interfering with the Rx data traffic. Send timing output from the Offline unit is also TRI-STATE® 1 .
The Offline Modem holds its Rx terrestrial data in TRI-STATE ® as commanded by the
CRS-170A Switch. Because of this, it is very important to power down the Offline
Modem before disconnecting the modem from the Switch.
Manual Switching is user-enabled locally via the Online unit; locally using the modem’s front
panel, or remotely using serial remote control or the modem’s Web Server (HTTP) Interface.
You must control Automatic Switching Selection via the Online unit. Use
the CRS-170A Switch Conditions Unit+Tx and Unit+Rx DIP switches,
located on the antenna side of the CRS-170A, to configure the conditions
that cause an automatic switchover. These settings provide a great deal
of flexibility in the operation of the switch: you can select between
1 TRI-STATE is a registered trademark of National Semiconductor.