4384363 Transceiver for Local Network Using Carrier-Sense Multiple Access Collision Detection May81

This document describes a transceiver for Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) local area networks. The primary innovation addresses a challenge when using industry-standard emitter-coupled logic (ECL) circuits for both the receiver and transmitter.

The core problem is that an ECL current source, used for the transmitter output, has a "compliance limit." In the event of a collision, the voltage swing on the coaxial cable could exceed this limit, causing the transmitter output to saturate. This saturation would prevent the collision detection mechanism from functioning correctly, rendering collisions undetectable.

The invention solves this by employing a dual, contiguous power supply scheme:

1. Transmitter Output Stage: Powered by a voltage range of approximately -5 to -10 volts.
2. Receiver Input and Collision Detection Circuits: Powered by a voltage range of approximately 0 to -5 volts.

This power supply arrangement allows the transmitter output to swing about 6-7 volts without exceeding its compliance limit, thus ensuring that collisions remain detectable.

Additional key features and protections include:

• Isolation Circuitry: Prevents transceiver short circuits from shorting out the coaxial cable.
• Guard Circuitry: Automatically disconnects the transmitter from the channel if it becomes "stuck on" and attempts to monopolize the network, typically after a predetermined transmission packet length.
• Collision Simulation: An integral mechanism allows for testing the collision detection circuitry by simulating a collision at the end of every transmission.
• Staircase Waveform Generation: The transmitter generates a staircase waveform (rather than an abrupt transition) to reduce high-frequency components and phase distortion, minimizing the size of necessary filter capacitors.