ML4642
FUNCTIONAL DESCRIPTION
Before data will be transmitted to the RXOUT pins of the
DTE ports it must meet the unsquelch requirements for the
RXIN receiver circuitry. The squelch circuitry at the RXIN
differential receiver input performs the same function as
that of the TXIN squelch circuitry using the same noise
rejection criteria.
inter-packet gap time, the collision signal will be
activated on the CDOUT pair of the same port as the
TXIN pair which received the packet, for typically 1 µs.
The SQE function will not be activated on DTE ports of
the ML4642 which are in the Jabber state. The SQE
function is enabled on both DTE ports when the RXLED/
LPBK/SQE pin is grounded.
COLLISION
JABBER
There are two conditions that constitute a collision from
the point of view of the ML4642:
The jabber function prevents a babbling transmitter from
loading down the network. Within the ML4642 is a jabber
timer on each TXIN receiver. Each timer starts at the
beginning of a received packet and resets at the end of
each packet. If a packet lasts longer than 7 to 20ms the
jabber logic disables its corresponding TXIN receiver (thus
preventing its data from being retransmitted) and
generates a collision signal on the babbling port’s CDOUT
pair. When the TXIN pair finally goes idle, a second timer
measures 0.5 seconds of idle on TXIN prior to re-enabling
the receiver and turning off the collision signal. If the
TXIN pair becomes active again before the 0.5 seconds
has expired, the timer is reset and measures another 0.5
seconds of idle time.
a) If data is received at the TXIN inputs of both DTE ports
simultaneously a local collision occurs within the
ML4642.
b) If the CDIN input is active at any time other than the
inter-packet gap window allowed for the SQE Test
function described below.
In either of the above circumstances it is necessary for the
ML4642 to drive the CDOUT pairs on both DTE ports with
the collision signal. The collision signal consists of a 10
MHz +/– 15% square wave matching the AUI
specifications and capable of driving a 78ý load. The
collision signal shall turn on within 2 bit times of the
origination of the collision condition and shall turn off
within 2–5 bit times after the collision condition subsides.
The jabber function can be disabled on both ports by tying
the JAB1/JABD pin to ground.
During a collision condition there are two sources for data
to be transmitted to TXOUT, TXIN1 and TXIN2. The highest
priority source for data to be transmitted to TXOUT is the
TXIN1 receiver.
LED DRIVERS
The ML4642 has six LED driver pins. Each DTE port has a
transmit LED and a jabber LED and the MAU port has a
receive LED. Additionally, there is a collision LED which
indicates the presence of a collision condition. All LED
drivers are active low 10mA current sources.
For example if TXIN2 begins transmission then TXIN1 turns
on, the collision oscillator will turn on and TXOUT will
switch from TXIN2 to TXIN1. If the collision ends by
TXIN1 turning off first, TXOUT will switch from TXIN1 to
TXIN2, and 2–5 bit times later the collision oscillator will
turn off.
The TXLED, RXLED, and CDLED outputs have 50ms pulse
stretchers on them to enable the LEDs to be visible. The
JLED outputs do not have pulse stretchers on them
because their conditions occur long enough for the LEDs
to be visible.
The MAU port’s CDIN receiver contains squelch circuitry
to prevent noise from causing the erroneous detection of a
collision signal. A signal on the CDIN pair will not be
considered active until it exceeds the same squelch
requirements as those of the TXIN receivers.
Two of the ML4642 LED outputs serve as configuration
pins as well. RXLED/LPBK/SQE and JAB1/JDIS may be
tied through a resistor to V , tied through a resistor and a
CC
LED to V or grounded. Additionally RXLED/LPBK/SQE
CC
may be tied to a specific voltage. When these pins are
grounded or tied to a 0.6 Volts they become configuration
inputs. Otherwise when tied high they become status
outputs.
LOOPBACK
The loopback function allows the ML4642 to emulate a
coaxial transceiver by propagating the TXIN data back
out the RXOUT pair of the same DTE port that is sourcing
the data as well as the RXOUT pair of the idle DTE port.
This allows the Ethernet controller sending the data to
monitor its transmit packets and detect network faults.
CASCADING THE ML4642 FOR 4 AND 8 PORT DESIGNS
The configurability of such functions as loopback, jabber,
and SQE allows ease of cascading multiple ML4642 chips
for larger fan-out designs. Figure 10 shows a four port AUI
Multiplexer design. For a type 0 configuration both jabber
and transmit LEDs are available on a per port basis for
status. The RXLED/LPBK/SQE pins are tied through a
resistor to 5 volts, and CDLED is wire OR’ED with the
other chip for one collision detect status LED per system.
There is also only one receive LED status output which is
displayed in a type 2 configuration. This particular pin in
The loopback function is enabled at both DTE ports when
the RXLED pin is tied to ground, or 0.6 volts.
SQE TEST FUNCTION
The Signal Quality Error (SQE) Test function allows the
DTE to determine whether or not the collision detection
circuitry is functional. After each transmission, during the
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