Electronic Design

Circuit Provides Ground-Start Supervision For Phone Switching

Ground start is the primary analog signaling method used between telephone switches such as the central office (CO) and a private branch exchange (PBX). It offers a way to indicate on-hook and off-hook conditions in a voice network. There’s one major distinction between ground start and loop start (the typical home telephone service): ground start requires ground detection to occur in both ends of a connection before the tip and ring loop can be closed. This is done to eliminate a condition known as “glare.” Glare occurs when both ends of a telephone line or trunk are seized at the same time for different purposes or by different users.

Ground-start supervision resolves this problem with a simple handshake that ensures glare won’t occur. The circuit shown below uses a Clare IAA110 optically isolated dual relay and optocoupler to demonstrate how groundstart supervision can be implemented. The input control signals that are sent to the IAA110 can be driven by the user’s microprocessor.

This circuit’s CO side is a simplified arrangement that’s used to illustrate the ground-start signaling protocol. It may be used to depict a foreign-exchange station (FXS) as well. A foreignexchange office (FXO) can be represented by the customer premise equipment’s (CPE’s) data-access arrangement (DAA).

Before describing the circuit in detail, it may be helpful to review the operational states of the ground-start protocol:

  1. Idle state (on-hook)
  2. CPE seizes trunk
  3. CO seizes trunk

During the idle state (on-hook), the CPE monitors the TIP lead for application of ground by the CO. The battery from the CO appears on RING lead. Also, the CO monitors the RING lead for application of ground by the CPE.

When the CPE seizes the trunk line first, the CPE grounds the RING lead. The CO senses that the RING lead is grounded and, in response, grounds the TIP lead. Next, the CPE senses that TIP is grounded and closes the loop by activating hookswitch K3. The CPE then removes the ground on the RING lead. Communication can now begin.

If seizing the trunk first, the CO grounds the TIP lead and superimposes ringing voltage over the RING-lead battery. The CPE detects that TIP is grounded and ringing. To prevent a glare condition, the TIP ground must be sensed within 100 ms. Then, the CPE closes the loop by activating hookswitch K3. Consequently, the CO will sense the dc current from the CPE. Communication can now begin.

This circuit performs all the necessary ground-detection and assertion functions required by the ground-start signaling protocol.

In the idle state (on-hook), the −−TIP_EN signal should be pulled low to initiate TIP ground monitoring. The −−RING_EN line should also be pulled high or left open so that RING ground is deactivated. When −−TIP_EN is activated, relay K1 will turn on. This creates a path from the CPE’s 48-V power supply through LED D3, R4, and D5. In the idle state, the TIP lead is floating to make certain that no current will flow through this path.

For RING ground assertion, setting −−RING_EN low turns on relay K2. This relay creates a low-resistance path from the RING lead through K2 and R5. The current flow due to the grounded RING lead is detected by the CO.

During TIP ground detection, the CO grounds the TIP lead by closing the SW1 relay. Current then flows through the K1 relay path. In doing so, it activates D3, which is optically coupled to Q1. As Q1 saturates, it causes −−TIP_DET to go low. This indicates that the CO has placed ground on the TIP lead. Diode D6 protects D3 from high reverse voltages and prevents LED D4 from turning on. Resistor R4 limits current to about mA with a battery voltage of 48 V. Diode D5 ensures operation with the correct polarity applied to TIP and RING leads. Resistor R3 sets the turnon threshold of D3 to about 1.9 mA. Note that a 48-V battery voltage isn’t mandatory to perform the TIP ground detection. If changing voltage, adjust R4 for approximately 3 mA of current in the path. The TIP connection on the CPE side to the loop resistance on the CO side must be taken into account when selecting R4.

After TIP ground is detected and before the loop is closed, −−TIP_EN should be deactivated. Doing so will disable the TIP ground-detect circuit, preserving the longitudinal balance in the communication state.

Additional information about ground-start signaling and associated regulatory requirements can be found in the Bell System technical reference (PUB 47001 1982) and the Code of Federal Regulations (FCC Part 68.3).

See associated figure

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