ML4644
FUNCTIONAL DESCRIPTION
Figure 1 is a block diagram of a Four Port Multiplexer
using the ML4644. All AUI interfaces are transformer
coupled as required in an AUI connection.
RECEPTION
The receive function consists of detecting data on receive
differential data input pair of the MAU port (RxIN) and
transmitting this data out of the RxOUT ports of all the DTE
port. This data will only be passed onto the DTE port if it
meets the unsquelch criteria of the AUI receiver circuit. This
provision prevents any noise on the AUI cable from being
misinterpreted as data and transmitted to the RxOUT pins.
The receiver squelch circuit rejects signal typically with
pulse width less than 20ns or a voltage level more positive
than –250mV. Once the receiver is unsquelched, it
remains so until reception of the idle signal which is more
positive than –170mV for longer than 180ns.
TRANSMISSION
The transmit function consists of detecting data on any of
the four differential data input pairs (TxIN1, TxIN2, TxIN3,
and TxIN4) and transmitting this data out of the TxOUT
pair of the MAU port.
Only data that meets the unsquelch criteria of the AUI
receiver circuit will be passed onto the TxOUT port.
LOOPBACK
If the ML4644 is connected to a MAU, or connected to
other 4642/44s that are connected to a MAU, the MAU
should be used to provide loopback. The loopback function
of the MAU will loop data back to the RxIN port which will
pass is to all FxOUTs and the
REC
pin will go low. In some
configurations, there will be no MAU, and a ML4644 will
be the last chip in the Tx path. In this case, the RxIN port
will never be active, since it is not connected, and the
REC
pin has no function. The
REC
pin may be grounded and this
will put the ML4644 in loopback mode, and data on the
TxIN pair will be looped back to all RxOUT ports.
COLLISION
There are two possible collision scenarios.
1. Collision from the network connected to the MAU.
2. Collision between two or more DTEs attached to the
multiplexer.
In the case of a network collision, the MAU will send a
collision presence signal to the multiplexer. The ML4644
will propagate this signal to the CDOUT pins of each of the
DTE ports. The collision signal is a 10MHz ±0.01% signal.
When a collision event occurs between two or more DTE
ports, the ML4644 will send the collision presence signal to
each of the DTEs via the collision ports. At the same time a
5MHz JAM signal is sent to the network.
SQE TEST FUNCTION
The Signal Quality Error (SQE) test function allows the
DTE to determine whether or not the collision port is
functional. After each transmission, during the interpacket
gap time, the collision signal will be activated on the
CDOUT port of the same DTE port data has been
transmitted, for typically 1µs. The SQE function is not
activated at the DTE ports that are in jabber state. The SQE
function of each port can be disabled by tying the SQEN
pin high.
JABBER FUNCTION
The jabber function prevents a malfunction transmitter
from continuous transmission and thus loaded down the
network. Within the ML4644, there is a jabber timer. The
timer starts at the beginning of a received packet and
resets at the end of each packet. If the packet lasts longer
than 7 to 20ms the jabber circuit will disable the
offending TxIN receiver (the transmission of excessively
long packet is thus terminated) and generates a collision
signal to the collision port of the offending DTE port. The
DTE port will exit the jabber state when the transmission
goes idle.
LED DRIVERS
The ML4644 has ten LED drivers. 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 open collector driver.
All LED drivers except the jabber have pulse 1ms pulse
stretchers. The pulse stretchers provide adequate on time
for the LED to be visible.
CASCADING THE ML4644 FOR EIGHT PORT
MULTIPLEXER APPLICATION
An 8 port multiplexer can be realized by using two
ML4644s and one ML4642. In this configuration (see fig.
2), the SQE function of the ML4642 should be disabled to
prevent false collision signalling. The SQE function in this
configuration is performed by each DTE port of the
ML4644 independently. Only one crystal is required,
since X3 of the ML4644 with the crystal can drive X1 of
the other chip.
ML4644
ML4642
ML4644
Figure 2. Block Diagram of an Eight port Multiplexer.
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
