83C694 Datasheet, PDF (11/33 Pages) List of Unclassifed Manufacturers – TWISTED PAIR INTERFACE AND MACHESTER ENCODER/DECODER

83C694D
ARCHITECTURE
The 83C694D supports the IEEE 802.3 loopback
design (section 14.2.1.3) which provides for con-
tinuous loopback from transmit to receive in normal
operation. This means that transmitted data is al-
ways looped back during TP operation, simulating
the physical broadcast characteristics of 802.3 co-
axial cable networks.
2.8 LINK TEST FUNCTION
Each TP driver transmits a short positive pulse
periodically when it is not sending data as shown in
Figure 5-4. These pulses are received at the other
end of the TP cable, signalling that the link is
operating correctly. The time between link test
pulses is compared to the expected range at the
receiver, to avoid false detection of noise pulses as
link test pulses.
If the link test fails (no pulses or data received in a
fixed time period), then the LNK pin is set high and
data transmit and receive on the TP interface is
disabled.
2.9 AUI/TP AUTOSELECT
The 83C694D can automatically select which me-
dia to transmit and receive on, based on the link test
state. If the link test fails, the AUI transmitter and
receiver are enabled while the TP transmitter, re-
ceiver, and loopback are disabled. If link test
passes, the AUI operation is disabled and TP op-
eration is re-enabled. The only exception to this is
when MODE1 is set low and TP operation is en-
abled continuously.
2.11 STATUS INDICATIONS
To assist in installation and management of the
network, indicator LEDs can be driven by four out-
puts from the 83C694D. These show the result of
Link Test, polarity check, and transmit or receive
activity.
An LED test feature is built into the 83C694D. All
LEDs turn on for 2/3 second after a reset to the
device.
2.12 TEST MODE
Three test modes can be selected when the SEL
pin is set to intermediate voltages. These modes
and their corresponding voltages are:
• Internal counter speedup (1.75 V)
• RXC and RXD enable (2.5 V)
• Output tristate (3.5 V)
Internal counter speedup is used for fast board-
level testing of timed functions such as LED power-
up blink and link test pulse period.
RXC and RXD enable is used to test internal VCO
functions without using a full data packet receive.
Output tristate is used during board-level testing to
enable short/open testing. It is also used to test
other devices resident on the board. This function
does not tristate transmit (pins 22-27) or X2 outputs.
2.10 JABBER AND SQE TEST FUNCTIONS
If TXE is high for greater than 46 ms, the TP
transmitter will be disabled and COL will go active
high. If TXE then goes low for more than 368 ms,
the TP transmitter will be re-enabled and COL will
go low.
In TP operation, a short pulse will be output on COL
after each packet is transmitted. This is required as
a test of the TP transmit/receive path, and is called
SQE Test or CD Heartbeat.
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