AT&T AT&T manual 190

trunks. If Ground Start trunks are used, a 55A1 Ground Start Button must be provided at each PFT set.

Only FCC registered single-line voice terminals may be used for PFT stations. Rotary sets must be used for dial pulse PFT trunks; touch-tone sets must be used for touch-tone PFT trunks.

In the event of a Power Failure Transfer (switch has lost power or a major fault has occurred) a contact closure is provided to the Central Office (CO) over a dedicated pair of wires. The CO then makes busy all DID trunks. When power is restored, the closure is removed and the CO restores DID service. External alarm contacts are provided on the front of the ETU for use as required.

Note: It is recommended that customers with Disservice make provisions with their CO to provide this arrangement.

The ETUs are mounted on the cross-connect backboard. Connections are via 25-pair receptacle-ended (CO and SIP) and plug-ended (switch line and trunk) connectors. Modular jacks are provided for the -48V control signal from the CPU (Call Processor Unit) and for additional ETUs. Screw terminals are provided for the connection of external alarms.

When calculating Unit Loading (see Section 5, “Unit Loads”), all ETU loading counts against Cabinet 1.

The 10B ETU is mounted on the cross-connect field as shown in Section 6, “Environmental Requirements.”

ETU Power Failure Transfer connections are shown in Figure 2-36.Part (a) on the figure shows a single-line voice terminal that has been connected as a Power Failure Transfer station. In normal operation, the Call Processor CP supplies -48V dc to the ETU. The voice terminal is connected through the ETU to the station port CP and can support all calling activities. The trunk connection through the ETU to the trunk port supports normal trunk calls.

Part (b) on Figure 2-36shows the ETU connections when a Power Failure Transfer has occurred. The transfer is initiated by the removal of the -48V dc to the ETU. All connections through the system are dropped, and direct connections between PFT stations and CO trunks are established. A contact closure toward the CO makes all DID trunks busy.

When the system is again able to process calls, normal service is automatically restored.

A multiple ETU arrangement is shown in Figure 2-37.As discussed earlier, separate -48V dc

control signals from the Call Processor are provided via			legs	7 and 8 on Octopus cable C2.
The 25-pair	cable from the Analog “Line” CP	provides	connectivity for eight voice terminals
at the Line	input to the ETU. Since the ETU	supports	only	five PFT stations, three of the

voice terminals are wired straight through the ETU and are not switched during service interruptions. A similar condition exists for the 25-pair cable (D) from the CO Trunk CP to the Trunk input of the ETU. Three of the eight trunk port appearances are wired straight through the ETU to the CO and are not switched. Trunk ports connected by legs 2 and 3 of the splitter cable are wired directly to the TAE Block.

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