33 COLLISION FUNCTIONS

The collision circuitry consists of two buffers two 4-pole

Bessel low pass filters (section 31) a comparator a heart-

beat generator a 10 MHz oscillator and a differential line

driver

Two identical buffers and 4-pole Bessel low pass filters ex-

tract the DC level on the center conductor (data) and the

shield (sense) of the coax These levels are monitored by

the comparator If the data level is more negative than the

sense level by at least the collision threshold (Vth) the colli-

sion output is enabled

At the end of every transmission the heartbeat generator

creates a pseudo collision for a short time to ensure that the

collision circuitry is properly functioning This burst on colli-

sion output occurs typically 11 ms after the transmission

and has a duration of about 1 ms This function can be dis-

abled externally with the HBE (Heartbeat Enable) pin to al-

low operation with repeaters

The 10 MHz oscillator generates the signal for the collision

and heartbeat functions It is also used as the timebase for

all the jabber functions It does not require any external

components

The collision differential line driver transfers the 10 MHz sig-

heartbeat conditions This line driver also features zero dif-

ferential idle state

34 JABBER FUNCTIONS

The Jabber Timer monitors the Transmitter and inhibits

transmission if the Transmitter is active for longer than

20 ms (fault) It also enables the collision output for the fault

duration After the fault is removed The Jabber Timer waits

for about 500 ms (unjab time) before re-enabling the Trans-

mitter The transmit input must stay inactive during the unjab

time

TLF7405 – 3

FIGURE 2 Receiver Timing

TLF7405 – 4

FIGURE 3 Transmitter Timing

3