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information remains unchanged. If a Command pattern
is sent to the output latch or if Sync is received, CSTRB
is pulsed and Data outputs remain in their previous
state. Reception of a Sync pattern clears the Command
outputs to all 0’s, since Sync is a legal command.
FUNCTIONAL DESCRIPTION
System Configuration
The TAXIchip system provides a means of connecting
parallel data systems over a serial link (Figure 2). In
LOCAL Mode (normal operation mode) each TX/RX
pair is connected over a serial link which can be a Fiber
Optic or Copper Media (Figure 3).
Noise-induced bit errors can distort transmitted bit pat-
terns. The Am7969 Receiver logic detects most noise-
induced transmission errors. Invalid bit patterns are
recognized and indicated by the assertion of the viola-
tion (VLTN) output pin. This signal rises to a logic “1”
state at the same time that Data or Command outputs
change and remains HIGH until a valid pattern is
detected by the Data Decoder. The error detection
method used in the Receiver cannot identify bit
errors which transform one valid Command or Data pat-
tern to another. Fault-sensitive systems should use ad-
ditional error checking mechanisms to guarantee
message integrity.
The Am7968 Transmitter accepts inputs from a sending
host system using a simple STRB/ACK handshake.
Parallel bits are saved by the Am7968’s input latch on
the rising edge of a STRB input. The input latch can be
updated on every CLK cycle; if it still contains previously
stored data when a second STRB pulse arrives, Data is
stored in the input latch, and the second ACK response
is delayed until the next CLK cycle.
The inputs to an Am7968 Transmitter can be either Data
or Command and may originate from two different parts
of the host system. A byte cycle may contain Data or
Command, but not both. Data represents the normal
data channel message traffic between host systems.
Commands can come from a communication control
section of the host system. Commands occur at a rela-
tively infrequent rate but have priority over Data. Exam-
ples include communication specific commands such
as REQUEST-TO-SEND or CLEAR-TO-SEND; or
application specific commands such as MESSAGE-
ADDRESS-FOLLOWS, MESSAGE-TYPE-FOLLOWS,
INITIALIZE YOUR SYSTEM, ERROR, RETRANSMIT,
HALT, etc.
Am7968 Transmitter
The Transmitter accepts messages from its parallel in-
put pins (Command or Data). Once latched into an
Am7968, aparallelmessageisencoded, serialized, and
shifted out to the serial link. The idle time between trans-
mitted bytes (evident by lack of STRB) is filled with
Sync bytes.
Am7969 Receiver
Receivers accept differential signals on the SERIN+/
SERIN– input pins. This information, previously
encoded by an Am7968 Transmitter, is loaded into
a decoder.
The Am7968 Transmitter switches between Data and
Command by examining Command input patterns. All
0s on Command input pins cause information on the
Am7968’s Data input pins to be latched into the device
ontherisingedgeofSTRB. All other Command patterns
cause a Command symbol to be sent in response to an
input strobe. The pattern on the Data inputs is ignored
when a Command symbol is sent. In either case, if there
is no STRB before the next byte boundary, a Sync sym-
bol will be transmitted. The sync pattern maintains link
synchronizationandprovidesanadequatesignaltransi-
tion density to keep the Receiver Phase-Locked-Loop
(PLL) circuits in lock. It was chosen for its unique pattern
which never occurs in any Data or Command mes-
sages. This feature allows Sync to be used to establish
byte boundaries.
When serial patterns are received, they are decoded
and routed to the appropriate outputs. If the received
message is a Command, it is stored in the output latch,
appears at the Command output pins, and CSTRB is
pulsed; Data output pins continue holding the last Data
byte and DSTRB stays inactive. If a Data message fol-
lows the reception of a Command, Command output
pins continue holding the previous Command byte and
CSTRB stays inactive. The command outputs will retain
their states until another Command signal is received
(Sync is considered to be a valid command which, when
decoded, sets Command outputs to “0” and issues a re-
sulting CSTRB).
Byte Width
The Sync pattern utilized by TAXIchip set keeps the
automatic gain control (AGC) fiber-optic transceiver cir-
cuits in their normal range because the pattern has zero
DC offset.
The TAXIchip set has twelve parallel interface pins
which are designated to carry either Command or Data
bits. The Data Mode Select (DMS) pin on each chip can
be set to select one of three modes of operation: eight
Data and four Command bits, nine Data and three Com-
mand, or ten Data and two Command. This allows the
system designer to select the byte-width which best
suits system needs.
The Am7969 Receiver detects the difference between
Data and Command patterns and routes each to the
proper Output Latch. When a new Data pattern enters
the output latch, DSTRB is pulsed and Command
10
Am7968/Am7969
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