MYSON
TECHNOLOGY
MTD492
Coaxial Transceiver Interface
This datasheet contains new product information. Myson Technology reserves the rights to modify the product specification without notice. No
liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product.
MTD492 Revision 3.5 01/07/1997
1/8
FEATURES
Compatible with IEEE 802.3 10Base5 (Ethernet) and 10Base2 (Cheapernet).
Internal AUI squelch circuitry for noise rejection.
Transmission IDL detection at end of packet and dribble bit rejection window.
Reception dribble bit rejection window.
Reception and transmission mode collision detection.
Extended collision detection to turn off receiving path.
CD heartbeat externally controllable.
Advanced low-power, high-performance CMOS technology.
ESD protection greater than 2000 Volts.
16-pin PDIP and 28-pin PLCC packages.
GENERAL DESCRIPTION
The MTD492 transceiver integrates the coaxial cable interface functions of the Medium Attachment Unit
(MAU) in Ethernet or Cheapernet LAN applications. In an Ethernet 10Base5 network, MTD492 is mounted
on the thick Ethernet coaxial cable and connects to a station through an AUI cable. For Cheapernet
applications, MTD492 is connected to the Cheapernet coaxial cable through a BNC connector and is
usually mounted on the LAN adapter in a station.
BLOCK DIAGRAM
HIGH PASS
E Q U A L I Z A T I O N
L O W P A S S
FILTER
L O W P A S S
FILTER
C O M
-
+
L O W P A S S
FILTER
1 2 5
1 0 K
R E F E R E N C E
CIRCUIT
1 K
AUI
D R I V E R
AUI
D R I V E R
C A R R I E R
S E N S E
-
+
W A V E F O R M
S H A P I N G
+
-
D C / A C
S Q U E L C H
W A T C H D O G
TIMER 26ms
J A B B E R R E S E T
TIMER 0.4sec
10MHz
O S C
T R A N S M I T
S T A T E
MACHINE
R E C E I V E
S T A T E
M A C H I N E
X M T S Q
C L K
V E E
COLLISION
H B E
FXI
G N D
C O A X
V E E
X M T
C D S
R E C V
T X O
R R +
R R -
RX+
RX-
C D +
C D -
TX+
TX-
MYSON
TECHNOLOGY
MTD492
MTD492 Revision 3.5 01/07/1997
2/8
1.0 CONNECTION DIAGRAM
M T D 4 9 2 N
1 6 P I N P D I P
M T D 4 9 2 V
28 PIN PLCC
C D +
C D -
RX+
V E E
V E E
RX-
TX+
TX-
C D S
TX0
RXI
V E E
R R -
R R +
G N D
H B E
V E E
V E E
V E E
V E E
VEE(NC)
VEE(NC)
VEE(NC)
RX+
CD-
CD+
CDS
TXD
NC
RXI
VEE(NC)
V E E
V E E
V E E
V E E
VEE(NC)
R R -
RX-
TX+
TX-
HBE
GND
GND
RR+
1
2
3
4
5
6
7
8
2 4
2 3
2 2
2 1
2 0
1 9
1 8
1 7
2.0 PIN DESCRIPTIONS
Name
I/O
PDIP
Pin#
PLCC
Pin#
Description
TX+,TX-
I
7,8
13,14 Transmission Data Input. A balanced differential line receiver which
receives inputs from the off-chip Manchester Code Converter(MCC) to
the Transmitter. The common mode voltage on TX+ is set internally .
HBE
I
9
15
Heartbeat Enabler. The CD heartbeat test is enabled when HBE is
connected to Ground and disabled when HBE is connected to VEE.
This pin is internally biased at 0.5 VEE. Test mode is enabled if this
pin is left floating or biased in the range of -3.5V to -5.5V. Jabber is
disabled in test mode. Exiting the jabber disable mode requires at
least jabber reset time.
RR+,RR-
I
11,12
18,19 External Resistor. A 1k
/1% resistor should be connected across
these pins to correctly set internal operating currents. RR+ is
internally shorted to GND.
RXI
I
14
26
Network Receiving Input. Should be connected to the COAX center
conductor. Signals meeting receiver squelch limits are recovered and
output on RX+. RXI also detects the collision voltage level.
CDS
I
16
1
Collision Detection Sense. Connects directly to the COAX
shield, providing a reference for the collision detection voltage
level for reception-mode detection. An external bias network can be
used to shift the detection threshold for transmission-detection mode.
CD+,CD-
O
1,2
2,3
Collision Output. A balanced differential line driver drives this
output pair from the collision detection circuitry. A 10MHz signal from
the internal oscillator is transferred to these outputs in the event of
collision, excessive transmission (jabber), or during heartbeat
condition. These outputs are open sources, and pull-down resistors of
510
to VEE are required. To minimize power dissipation, both open
source outputs are disabled during idle condition, allowing the
common mode on the pull-down resistors to be pulled to VEE.
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
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
F 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.
MYSON
TECHNOLOGY
MTD492
MTD492 Revision 3.5 01/07/1997
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The RX+/- output pin is driven by a source follower. Therefore, an external pull-down load is required for
these 2 pins. During idle state, RX+/- is biased at around -1.5V.
MYSON
TECHNOLOGY
MTD492
MTD492 Revision 3.5 01/07/1997
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3.3. Collision Detection Path
In 10Base2 and 10Base5, a collision condition is determined by the DC level on the cable. The DC average
is extracted by a low pass filter. The output of the filter is compared with an internally set threshold (Vcd) to
determine the collision condition. The internal threshold is set for the reception-collision mode. By
definition, MTD492 will detect collision conditions caused by more than 2 stations simultaneously
transmitting.
The collision detection threshold can be shifted by applying a voltage to the CDS pin. This is typically done
by a resistor divider network between GND and VEE to implement the transmission-collision mode
detection. Transmission-collision differs from reception-collision in that transmission-collision will only
detect a 2-station collision if it is transmitting.
Once a collision is detected, MTD492 turns on the CD+/- pins. These 2 pins are also driven by source
followers, thus requiring external pull-down resistors. However, the followers are disabled during idle time,
and CD+/- will be pulled down to VEE and no current will flow through the external resistors.
Because Vcd is always larger than Vcs, when a collision occurs the reception path will also be turned on.
MTD492 implements an internal timer of 250 msec to detect the extended collision period. When a collision
condition period extends beyond this timer, the reception path is turned off. Any interruption of the collision
will reset the timer.
3.4. Reference and Control
The internal reference of MTD492 is generated by an on-chip bandgap circuit. An external resistor
connected between RR+ and RR- pins is used to set the reference current level. Typically, a precision 1K
Ohm resistor should be used. RR+ is internally shorted to GND, while RR- is biased at around -1.25V.
The heartbeat and jabber functions of MTD492 are implemented in the transmission state machine. The
heartbeat function is controlled by the HBE pin. MTD492 also provides a test mode that disables the jabber
function also controlled by the HBE pin. HBE has a 3-level input. It is biased internally at around -4.5V
(0.5Vee). When HBE is pulled high by a low value resistor (<4.7K), the heartbeat function is enabled; when
it is pulled low, the heartbeat is disabled. If it is driven to the mid-level or left floating, the jabber is disabled.
The exit of the jabber disable state takes about 0.5 sec (jabber reset time).
4.0 ABSOLUTE MAXIMUM RATINGS
DC Supply Voltage(VEE)
-12V
Input Voltage GND+0.3 to VEE-0.3V
Storage Temperature
-65
o
to 150
o
C
Ambient Operating Temperature 0
o
to 70
o
C
ESD Protection except for Pin14
2000V
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