AMD
TAXI Technical Information Publication #89-06
S u b je c t : TAXl fo r FDDI Ap p lic a t io n s ?
Qu e s t io n :
Can the TAXIchip set be used for FDDI physical layer applications?
An s w e r:
The TAXIchip set is code compatible with the FDDI physical layer but there are restric-
tions in the design which would cause difficulty in using the TAXIchip set for the physical
layer of an FDDI node. The TAXIchip set by itself cannot be used to build a fully
compliant FDDI node, although it provides several of the functions required.
The TAXI Transmitter is compatible with FDDI at the physical layer electrical interface
and can send all codes specified by FDDI. An exception to the encoding is that Quiet-
Line-State (QLS) is defined as fiber-dark for FDDI, requiring a static SEROUT=LOW,
and the Transmitter defines the equivalent of QLS, as Command F, as no-transitions,
with no control of the static logical state.
The TAXI Receiver is also compatible with FDDI at the physical layer electrical interface
and can recognize the codes specified by FDDI, with restrictions. The restrictions
concern Master-Line-State (MLS), Halt-Line-State (HLS), and the carrier detect function.
MLS and HLS are terms describing a data stream composed of a consecutive string of
HQ and HH symbols respectively, representing a line-state condition. The Receiver will
decode these symbols, but it does not count them to signal line-states as required
by FDDI.
MLS and HLS are relatively long run-length signals with 10 and 5 bit-times between
transitions respectively, as compared to a maximum limit of 3 bit-times for data. The
Receiver PLL was designed for wide operating frequency range, with tradeoffs in the
ability and time required to capture long run-length data sequences. The FDDI specifica-
tion allows 100 µs for the Receiver to lock upon and detect MLS following a long period
of QLS. A typical TAXI Receiver will meet these criteria but the production parts are
neither tested nor guaranteed for this condition. There are no problems associated with
tracking the MLS signal once the PLL has acquired lock.
HQ and HH, within the TAXI Receiver, require proper byte framing for detection. MLS and
HLS as specified by FDDI are not framed, therefore the transition may be located at any of
the ten bit locations. The result, as decoded within the TAXI Receiver, will be as follows:
MLS:
00100 00000
00000 00100
all other
=
=
=
=
=
HQ
QH
CMD-A
CMD-D
Violation
CMD-8
10% probability
10% probability
80% probability
20% probability
80% probability
HLS:
00100 00100
all other
HH
Violation
The FDDI line state definition does not preclude the insertion of an occasional sync
into the MLS or HLS data stream for proper framing, solving the recognition problem.
If full FDDI compliance is required, MLS and HLS must be detected external to the
TAXIchip set.
The carrier detect function, as specified by FDDI, requires the flagging of a QLS to the
MAC layer as long as the fiber is dark. The TAXI SERIN inputs must be static for this
condition to be met by the TAXI Receiver. This problem must be addressed directly by
the Optical receiver or gating of its outputs.
Functions of the FDDI MAC layer interface are not directly addressed in the TAXI
designs.
100
TAXIchip Integrated Circuits Technical Manual
AMD [ AMD ]
