3 0 Functional Description
(Continued)
3 3 COLLISION FUNCTIONS
The collision circuitry consists of two buffers two 4-pole
Bessel low pass filters (section 3 1) 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 1 1
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-
nal to the CD
g
pair in the event of collision jabber or
heartbeat conditions This line driver also features zero dif-
ferential idle state
3 4 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
TL F 11085 – 3
FIGURE 2 Receiver Timing
TL F 11085 – 4
FIGURE 3 Transmitter Timing
3
NSC [ NATIONAL SEMICONDUCTOR ]
