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SX-200 General Information Guide
62 Revision A 50003510
Power Supply
All power for the SFT unit is provided from the -48 Vbat source on the system. A source of -12
V powers the electronic circuitry on the card. This supply is derived from the -48 V input and
powers all the SFT circuitry except the transfer relays. The relays are powered by a
transistor-regulated -41V source, also derived from the -48 Vbat input. Thus, in the event of
Vbat varying between the standard -42.5 V to -56.5 V, the current drain remains constant.
Transfer Relays
Each circuit in the SFT uses a four form C relay to transfer between normal and SFT modes
of operation.
Loop Detector
When a transfer relay enters SFT mode, the loop detector connects in series with the loop
between the extension and CO trunk facility. This circuit prevents the extension from returning
to normal operating mode before an SFT mode call is completed. When the SFT mode call is
completed, the extension is returned to normal operating mode.
SFT Control Leads
The transfer control sensor on the SFT senses a loop closure across the SFT and SFT return
(SFTR) leads. When a loop closure is sensed, the power to the relays is removed, the relays
are released, and all circuits enter the transferred state.
Power Consumption
The total current drain for the SFT is typically 80 mA.
Power Dissipation (watts)
Power Supply TYP. (Watts) TYP+20%
-48Vbat 3.18 3.81
@Vbat=-56 V 3.71 4.45