83C795 Datasheet, PDF (88/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

LAN CONTROLLER OVERVIEW
83C795
7.5.6 Link Integrity Test Function
E ach T P driver transmits a s hort pos itive pulse
periodically when it is not s ending data. T hese
puls es are received at the other end of the twisted
pair cable, s ignaling that the link is operating
correctly. T he time between link tes t puls es is
compared to the expected range at the receiver to
help ignore nois e pulses . If the link tes t fails (no
puls es or data received within a fixed time period)
then the LLE D pin is set high and AUI interface is
s el ected. I f the li nk i s r es tor ed, the chip
automatically s elects T P interface.
T he Link Integrity Tes t is als o us ed to correct the
receiver connection’s polarity. T he Link Integrity
Tes t s ignal and S tart-of-Idle both have pos itive
polarity. T hepolarity correction s tatemachinelooks
at both of thes etodeterminewhether toflipreceiver
polarity. Polarity correction can be dis abled by a bit
in the MANCH R egister (MANCH.E NAPOL).
T he Link Integrity Tes t can be disabled by a bit in
the General Control R egis ter (GCR .LIT ). Dis abling
L ink T es t forces the 83C795 to s elect the
twisted-pair interface.
7.5.7 Jabber Protection
If the internal trans mit enable s ignal is active for
more than 46 ms ec, the twis ted-pair trans mitter will
be dis abled and a collis ion indication is s ent to the
MAC trans mitter circuit. When the internal transmit
enable s ignal has been inactive for more than 368
ms ec, the internal collis ion indicator will become
inactive and the twis ted-pair trans mitter will be
re-enabled. Jabber protection for the AUI port is
provided by an external MAU.
7.5.8 SQE Test (Heartbeat Test)
In twis ted-pair operation, a brief internal collision
indication will be sent to the MAC trans mitter after
each packet is trans mitted. When an AUI port is in
us e, an external MAU will provide this s ignal.
7.5.9 Status Indications
To as s is t in ins tallation and management of the
network, indicator LE Ds can be driven directly by
four outputs from the 83C795. T hes e s how the
res ult of Link Test, polarity check, trans mit and
receive activity. T he LE D outputs can be readback
through the MANCH regis ter to s upport network
management functions .
83C795
TPX2+
TPX1+
TPX1-
TPX2-
20 MHz
Low Pass
Elliptic
Filter
100 Ohms
R31 || R32 = 50 Ohm = R33 || R34
Ratio of R31 to R32 determines how much smaller the
second half of the pulse is than the first half
Phone
Jack
Polarity
TPX1
TPX2
+
-
+ +-
+
--
++ -
-
+
+
-
+ --
+
-+
+- -
+
+
1
1
0
0
1
0
1
FIGURE 7-3. SIMPLIFIED TRANSMIT CIRCUITRY
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