The IEEE 802.3 standard is designed for 500 meters of Ethernet cable and 185 meters of Cheapernet (RG58A/U) cable. To extend such segments to 1000 meters of Ethernet cable and 300 meters of Cheapernet cable requires utilization of Transmit mode collision detection. This method isdescribed below.

COLLISION DETECTION SCHEMES

The collision circuitry monitors the coaxial DC level. If the level is more negative than the collision threshold, the collision output is enabled.

There are two different collision detection schemes that can be implemented with the CTI; Receive mode, and Transmit mode. The IEEE 802.3 standard allows the use of receive and transmit modes for non-repeater node applications. Repeaters are required to have to receive mode implementation. These different modes are defined as follows: Receive Mode: Detects a collision between any two stations on the network with certainty at all times. Transmit Mode: Detects a collision with certainty only when the station is transmitting.

Table I summarizes the receive and transmit mode definiReceive Mode: The Receive mode scheme has a very simple truth table. However, the tight threshold limits make the design of it difficult. The threshold in this case has to be between the maximum DC level of one station (-1300 mV) and the minimum DC level of two far stations (-1581 mV). Several factors such as the termination resistor variation, signal skew, and input bias current of non-transmitting nodes contribute to this tight margin. On top of the -1300 mV minimum level, the impulse response of the internal low pass filter has to be added. The CTI incorporates a 4-pole Bessel filter in combination with a trimmed on board bandgap reference to provide this mode of collisiondetection.

Transmit Mode: In this case, collision has to be detected only when the station is transmitting. Thus, collision caused by two other nodes may or may not be detected. This feature relaxes the upper limit of the threshold. As a result of this, longer cable segments can be used. With the CTI, a resistor divider can be used at the Collision Detection Sense (CDS) pin to lower the threshold from receive to transmitmode.