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MH88634BV-4

Central Office Interface Circuit

Mitel Networks 

MH88634B

Advance Information

Network Balance Impedance

The MH88634BV-2/BT-2’s Network Balance

Impedance is fixed at 600Ω.

The MH88634BV-4 Network Balance Impedance is

fixed at 900R.

2-4 Wire Conversion

The device converts the balanced 2-Wire input,

presented by the line at Tip and Ring, to a ground

referenced signal at TX. This circuit operates with or

without loop current; signal reception with no loop

current is required for on-hook reception enabling the

detection of Caller Line Identification (CLI) signals.

Conversely the device converts the ground

referenced signal input at RX, to a balanced 2-Wire

signal across Tip and Ring.

The 4-Wire side (TX and RX) can be interfaced to a

filter/codec, such as the Mitel MT896X, for use in

digital voice switched systems

During full duplex transmission, the signal at Tip and

Ring consists of both the signal from the device to

the line and the signal from the line to the device.

The signal input at RX, being sent to the line, must

not appear at the output TX. In order to prevent this,

the device has an internal cancellation circuit. The

measure of attenuation is Transhybrid Loss (THL).

Transmit and Receive Gain

The Transmit Gain of the device is the gain from the

balanced signal across Tip and Ring to the ground

referenced signal at TX. It is set at 0dB.

The Receive Gain of the device is the gain from the

ground referenced signal at RX to the balanced

signal across Tip and Ring. It is set at -2dB.

Supervision Features

Line Status Detection Outputs

The MH88634 supervisory circuitry provides the

signalling status outputs which are monitored by the

system controller. The supervisory circuitry is

capable of detecting: Ringing Voltage; Forward and

Reverse loop battery; Forward and Reverse loop

current; and Switch Hook.

Ringing Voltage Detect Output (RV)

The RV output provides a logic 0 when ringing

voltage is detected across Tip and Ring. This

detector includes a filter which ensures that the

output toggles at the ringing cadence and not at the

ringing frequency. Typically this output switches to a

logic 0 after 50ms of applied ringing voltage and

remains at a logic 0 for 50ms after ringing voltage is

removed.

The threshold is different on the two variants. The

900R part has been designed to meet the ring

sensitivity requirements of TR57, 30 not detecting

ringin below 35Vrms.

Forward Loop and Reverse Loop Detect Outputs

(FL & RL)

The FL output provides a logic 0 when either forward

loop battery or forward loop current is detected, that

is the Ring pin voltage is more negative than the Tip

pin voltage.

The RL output provides a logic 0 when either reverse

loop battery or reverse loop current is detected, that

is the Tip pin voltage is more negative than the Ring

pin voltage.

Switch Hook (SHK)

The SHK output is active if either forward loop or

reverse loop current is detected, or if forward or

reverse battery voltage is detected.

Control Input

The MH88634 accepts a control signal from the

system controller at the Loop Relay Control input

(LRC). This energises the relay drive output Loop

Relay Drive (LRD). The output is active low and has

an internal clamp diode to VRLY.

The intended use of this relay driver is to add and

remove the Line Termination from across Tip and

Ring, as shown in Figure 3.

If this Control input and the Supervisory Features are

used as indicated in Figure 3, Loop-Start Signalling

can be implemented.

Mechanical Data

See Figure 9 for details of the mechanical

specification.

2-268

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