MYSON

TECHNOLOGY

MTD492

MTD492 Revision 3.5 01/07/1997

3/8

RX+,RX-

O

3,6

4,12

Reception Data Output. A balanced differential output drives the

data

recovered from the network to the MCC. These outputs are also open

sources, and pull-down resistors of 510

Ω to VEE are required. These

pins are biased at -2V during idle time. When an extended period of

collision condition happens, the receiving path is disabled. Any

interruption of collision will re-enable the receiving path.

TXO

O

15

28

Transmission Output. Should be connected to the coaxial cable via

either one (Cheapernet) or 2 serial isolation diodes (Ethernet).

GND

10

17

Positive Supply Pin(Ground). Should be connected to the COAX

shield.

VEE

4,5,

13

5, 6, 7,

8, 9, 10,

11, 20,

21, 22,

23, 24,

25

Negative Power Supply. -9 Volts. A 0.1

µF decoupling capacitor

must be connected across GND and VEE as close to the device as

possible.

3.0 FUNCTIONAL DESCRIPTIONS

3.1. Transmission Path

The transmission data is input from TX+/- pins differentially. In general, this differential signal is coupled

through an AUI isolation transformer. In the MAU design, it is preferable that an equivalent 78 Ohm load be

placed across these 2 pins for proper loading for the signal source. Improper load termination may cause

excess undershoot at the end of the packet, which causes the dribble bit to be transmitted erroneously.

The transmission signal is first checked against the on-chip DC/AC squelch condition. If the signal is

greater than the squelch threshold (-175 to -300 mV) and the width is wider than 17 nsec, the squelch is

turned off. The squelch remains off until the DC squelch condition is not met or an end-of-packet IDL is

detected (at the end of the packet, data remains at 1 for longer than 175 nsec). Once IDL is detected,

MTD492 provides a 0.8 usec rejection window that prevents dribble bit transmission at the AUI interface.

When the squelch is off, the transmission path is enabled and data is fed into a waveform shaping circuit

followed by the transmission output buffer. The waveform shaping function controls the output rise/fall time

between 20nsec and 30 nsec, and minimizes the mismatch between the rise/fall time. MTD492 provides a

current source output that should be connected to the coaxial cable through at least one isolation diode.

When data is high, the output current is virtually zero (there is approximately less than 1mA output in this

state to maintain the linearity of the output buffer); when data is low, the output current peaks at around 80

mA. This provides an approximate 2V peak to peak swing on a 25 Ohm load. Due to the nature of

Manchester code, the average output DC current is half of the peak - 40 mA, i.e. 1V DC average on the

cable 25 Ohm load.

3.2. Reception Path

The signal on the coaxial cable is first buffered. The DC average of the signal is extracted by a low pass

filter. When the DC average exceeds the carrier sense threshold (Vcs), the reception data path is turned

on. The signal goes through a high pass filter for equalization of high frequency loss on the cable and then

is compared with its center value. The comparator output is amplified by the AUI driver to provide adequate

driving for the RX+/- output.

The end of the packet is determined by 2 conditions. If the received data is high longer than 175nsec or the

DC average does not meet the carrier sense threshold, the reception data path is turned off. MTD492 then

appends the IDL pulse to the end of the packet. A rejection window of 1 usec that blocks the data reception

path is also turned on.