ML4658
SYSTEM DESCRIPTION (Continued)
The AUI drivers are capable of driving the full 50 meters
of cable length and have a rise and fall time of typically
3ns. The rise and fall times match to within 1ns. In the
idle state, the outputs go to the same voltage to prevent
DC standing current in the isolation transformers.
At the start of a packet transmission, no more than 2 bits
are received from the DO circuit and not transmitted onto
the twisted pair. The difference between start-up delays
(bit loss plus steady-state propagation delay) for any two
packets that are separated by 9.6µs or less will not
exceed 200ns.
TRANSMISSION
The output stage of the transmitter is a current mode
switch which develops the output voltage by driving
current through the terminating resistor and the output
filter. The transmitter employs a center tap 2:1 transformer
The transmit function consists of detecting the presence
of data from the AUI DO input (Tx+, Tx–) and driving
that data onto the transmit twisted pair (TxTP+, TxTP–).
A positive signal on the Tx+ lead relative to the Tx– lead
of the DO circuit will result in a positive signal on the
TxTP+ lead of the chip with respect to the TxTP– lead.
where the center tap is tied to V (5V). While one pin of
CC
the transmit pair (TxTP+, TxTP–) is pulled low, the other
pin floats. The output pins to the twisted pair wires, TxTP+
and TxTP–, can drive a 100Ω, 150Ωload, or a variety of
impedances that are characteristic of the twisted pair
wire. RTSET selects the current into the TxTP+, TxTP–
pins. This current along with the characteristic impedance
of the cable determines the output voltage.
Before data will be transmitted onto the twisted pair from
the AU interface, it must exceed the squelch
requirements for the DO pair. The Tx squelch circuit
serves the function of preventing any noise from being
transmitted onto the twisted pair. This circuit rejects
signals with pulse widths less than typically 20ns and
voltage levels more positive than –175mV. Once the Tx
squelch circuit has unsquelched, it looks for the start of idle
signal to turn on the squelch circuit again. The transmitter
turns on the squelch again when it receives an input
signal at Tx± that is more positive than –175mV for more
than approximately 180ns.
Once the characteristic impedance of the twisted pair is
determined, one must select the appropriate RTSET
resistor as well as match the terminating impedances of
the transmit and receive filter. The RTSET resistor can be
selected as follows:
RTSET = (RL /100)× 220Ω
where R is the characteristic impedance of the twisted
L
pair cable.
BINARY
0
1
1
1
0
0
1
1
0
TxTP+
TxTP–
OUTPUT AFTER
TRANSMIT FILTER
INPUT INTO
RECEIVER
Figure 11. Transmit Pre-Equalization Waveform
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
