GB2063025A - Active termination of a transmission line - Google Patents

Abstract

A transmission line 30 is terminated by logic gate 31 e.g. a 74126 three state buffer, whose output state is the same as its input state, and which is provided with a positive feedback impedance R of value equal to the characteristic impedance of the line 30. <IMAGE>

Description

SPECIFICATION Actively terminated transmission line The present invention relates generally to the active termination of a transmission line and in particular to a circuit in which a transmission line is terminated.

Previous proposals have been made for terminating transmission iines to avoid losses and reflections. Conventionally, a transmission line may be terminated with its characteristic impedance. In British Patent 1292614, a transmission line is terminated by a circuit including an inverting amplifier to the input of which the line is connected and provided with a resistor connecting its input to its output, producing negative feedback. The resistor is chosen in conjunction with the input and output impedances of the amplifier so that the circuit presents to the line the characteristic impedance of the line.

When logic is used at signalling speeds of 10 M bauds and above, wires and printed-circuit board conductive tracks assume some of the characteristics of transmission lines. Cross talk and fast edges may cause wires or tracks to ring to such an extent that a logic threshold is crossed, causing false triggering. This is illustrated in Figure 1, where a signal peak P during a logic '0' portion of a signal exceeds a logic threshold. If each line were terminated with its characteristic impedance, this would overload the outputs. For example, a TTL logic '1' of 3V across 75Q gives a current of 40mA.

An object of the present invention is to provide an active termination for a transmission line in a logic system.

According to the present invention, there is provided a logic system including: a transmission line of characteristic impedance ZO; a logic gate having an input connected to the output of the line, and an output, the gate being such that the logic state of its output is the same as the state of the input; and an impedance of value Z0 providing a positive feedback path from the output of the gate to the input of the gate.

For a better understanding of the invention, reference will now be made by way of example, to the accompanying drawings, in which: Figure I is a waveform diagram of a logic signal on an unterminated transmission line, Figure 2 is a waveform diagram of a logic signal on a line terminated in accordance with the invention, and Figure 3 is a diagram showing a circuit in accordance with the invention.

The exemplary circuit shown in Figure 3 enables a transmission line 30 to be terminated with its characteristic impedance without significantly loading the line. A 74126 buffer is used with a positive feedback resistor R of value approximating and preferably equal to the characteristic impedance of the line.

A 74126 buffer is a TTL logic gate in which the logical state at the output Y is the same as the state at the input A when the control input C receives logic '1'.

The output Y is disabled when the control input receives logic '0'. As the output Y is always at the same state as the input A, the resistor R conducts very little. The input, however, always sees a low impedance, correctly terminating the transmission line, and reducing ringing, as illustrated in Figure 2.

In an experimental circuit including a transmission line of 75Q characteristic impedance a 74126 buffer was used for its high current driving capability in the logic '1' state. A 75Q feedback resistor was used successfully. The optimum value of the feedback resistor will depend upon the characteristic impedance of the line being terminated.

1. Alogicsystem including: atransmission line of characteristic impedance Zo; a logic gate having an input connected to the output of the line, and an output, the gate being such that the logic state of its output is the same as the logic state of its input; and an impedance of value Zo providing a positive feedback path from the output of the gate to the input of the gate.

2. A system according to Claim 1, wherein the line comprises a conductive track of a printed circuit board.

3. A system according to Claim 1 or 2 wherein the impedance is a resistance.

4. A system according to Claim 1,2 or 3, wherein the gate is a buffer gate.

5. A circuit substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Actively terminated transmission line The present invention relates generally to the active termination of a transmission line and in particular to a circuit in which a transmission line is terminated. Previous proposals have been made for terminating transmission iines to avoid losses and reflections. Conventionally, a transmission line may be terminated with its characteristic impedance. In British Patent 1292614, a transmission line is terminated by a circuit including an inverting amplifier to the input of which the line is connected and provided with a resistor connecting its input to its output, producing negative feedback. The resistor is chosen in conjunction with the input and output impedances of the amplifier so that the circuit presents to the line the characteristic impedance of the line. When logic is used at signalling speeds of 10 M bauds and above, wires and printed-circuit board conductive tracks assume some of the characteristics of transmission lines. Cross talk and fast edges may cause wires or tracks to ring to such an extent that a logic threshold is crossed, causing false triggering. This is illustrated in Figure 1, where a signal peak P during a logic '0' portion of a signal exceeds a logic threshold. If each line were terminated with its characteristic impedance, this would overload the outputs. For example, a TTL logic '1' of 3V across 75Q gives a current of 40mA. An object of the present invention is to provide an active termination for a transmission line in a logic system. According to the present invention, there is provided a logic system including: a transmission line of characteristic impedance ZO; a logic gate having an input connected to the output of the line, and an output, the gate being such that the logic state of its output is the same as the state of the input; and an impedance of value Z0 providing a positive feedback path from the output of the gate to the input of the gate. For a better understanding of the invention, reference will now be made by way of example, to the accompanying drawings, in which: Figure I is a waveform diagram of a logic signal on an unterminated transmission line, Figure 2 is a waveform diagram of a logic signal on a line terminated in accordance with the invention, and Figure 3 is a diagram showing a circuit in accordance with the invention. The exemplary circuit shown in Figure 3 enables a transmission line 30 to be terminated with its characteristic impedance without significantly loading the line. A 74126 buffer is used with a positive feedback resistor R of value approximating and preferably equal to the characteristic impedance of the line. A 74126 buffer is a TTL logic gate in which the logical state at the output Y is the same as the state at the input A when the control input C receives logic '1'. The output Y is disabled when the control input receives logic '0'. As the output Y is always at the same state as the input A, the resistor R conducts very little. The input, however, always sees a low impedance, correctly terminating the transmission line, and reducing ringing, as illustrated in Figure 2. In an experimental circuit including a transmission line of 75Q characteristic impedance a 74126 buffer was used for its high current driving capability in the logic '1' state. A 75Q feedback resistor was used successfully. The optimum value of the feedback resistor will depend upon the characteristic impedance of the line being terminated. CLAIMS

1. Alogicsystem including: atransmission line of characteristic impedance Zo; a logic gate having an input connected to the output of the line, and an output, the gate being such that the logic state of its output is the same as the logic state of its input; and an impedance of value Zo providing a positive feedback path from the output of the gate to the input of the gate.

2. A system according to Claim 1, wherein the line comprises a conductive track of a printed circuit board.

3. A system according to Claim 1 or 2 wherein the impedance is a resistance.

4. A system according to Claim 1,2 or 3, wherein the gate is a buffer gate.

5. A circuit substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawing.