83C795 Datasheet, PDF (82/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

LAN CONTROLLER OVERVIEW
83C795
7.3.1 AUI Differential Receiver
With the standard 78Ω transceiver AUI cable, the
differential input must be externally terminated. T his
requirement may be satisfied by connecting two 39Ω
resistors in series with one optional common mode
bypass capacitor.
To prevent nois eat the AUI R X+/- input fromfalsely
triggering the decoder, a s quelch circuit rejects
s ignals with puls e widths les s than 25 ns ec
(negative going), or with levels les s than 175 mV.
When the input exceeds the s quelch levels , the
analog phas e-locked loop locks into the incoming
s ignal and Manches ter decoding takes place. T he
internal carrier sens e s ignal is activated and the
receive data (R XD) and the receive clock (R XC)
become available within s even bit times . At the end
of a frame when normal mid-bit trans ition on the
di fferential input ceas es , carr ier s ens e is
deactivated. T he receive clock remains active for
an additional 5 bit times.
7.3.2 Twisted-Pair Differential Receiver
T he receiveds ignal fromthe unshieldedcable may
benois y, s ominimumvoltageandtiminglimits mus t
be met before the receiver logic is enabled. A
’S mart S quelch’digital nois efilter is us edin addition
to the analog squelch circuit in the receiver.
I f the input polarity is r ever s ed i t wi ll be
automatically detected and corrected. When
polarity is normal, the PLE D (MANCH.3) bit will be
s et.
T he phas e-locked loop and Manches ter decoder
are the s ame circuits us ed by the AUI receiver.
7.3.2.1 Extended Length For Twisted-Pair
S etting the XLE NGT H bit (GCR .7) increases the
s quelch levels us ed by the T P receiver. T his
enables the adapter to be connectedto acablethat
is longer than s pecified by the 802.3 s tandard.
7.3.3 Manchester Decoder
Decodi ng i s accompl i s hed by an anal og
phas e-l ock ed l oop that s epar ates the
Manches ter-encoded data s tream into clock
s ignals and NR Z data. T his loop can tolerate up to
20 ns ecof jitter on each s ignal edge, exceedingthe
802.3 requirements .
7.3.4 Carrier Sense
T he AUI interface determines carrier presence by
requiring the differential received s ignal to exceed
the negative s quelch level for a minimal period,
nominally 25 ns ec. T he twis ted-pair interface
requires that the pos itive and negative s quelch
levels be exceeded s everal times for a period of at
leas t 20 nsec.
When operatingin loopback mode, internal transmit
enable s ignal is returned to the MAC as the carrier
indication.
7.3.5 Collision Detect
Collision detection for the AUI interface is indicated
when the differential s ignal on the CD input pair
exceeds the negative squelch thres hold. Collision
detection for the twis ted-pair interface is indicated
when there is carrier s ensed while the trans mitter
is active.
7.3.6 Loopback Mode
When theLAN controller is programmedto operate
in loopback mode 2, it provides a s ignal to the line
receiver s ection which caus es the Manches ter
decoder to derive its input from the encoded
trans mit data ins tead of the differential receivers of
AUI and T P interfaces. T he AUI and T P interfaces
are ignored while the loopback indication is active.
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