AMD
Fig u re 2 -1
Ba s ic TAXIc h ip Op e ra t io n
6. The Data
comes out the
RX and Data
Strobe is
1. Parallel Data
is Entered by
the User
Raised
4. The Serial
Data is Sent
Out
Data
Strobe
Data In
Am7968
Transmitter
Am7969
Receiver
Data Out
Strobe
Acknowledge
3. The Data is
Encoded and
then Converted
into a Serial
Stream
5. The Re-
2. The Data is
Strobed in by
the User
ceiver takes the
Serial Data and
Converts it
Back to Parallel
Data and then
Decodes it
12330E-3
Three different widths are possible: 8 Data and 4 Command bits, 9 Data and 3 Com-
mand bits, and 10 Data and 2 Command bits. This choice of data and control bus widths
allows flexibility to meet different system bus width requirements, while providing the
capability of merging control and data into a common data stream.
2 .2 Op e ra t io n a l Mo d e s : Lo c a l, Ca s c a d e a n d Te s t
A TAXIchip set point-to-point link can be operated in one of three modes: Local,
Cascade, or Test. Local mode consists of a single Transmitter communicating with a
single Receiver over the serial medium. Cascade mode for Am7968/7969-125 consists
of a single Transmitter driving two or more daisy chained (cascaded) Receivers over a
single serial medium. Cascade Operation for Am7968/Am7969-175 consists of a single
Transmitter driving a single Receiver as shown in Appendix C, TAXI TIPs #13 and #14.
Cascade mode permits direct interface with 16-bit, 32-bit and wider busses. The link
may be operated in any of the above modes using the TAXl’s internal PLL for bit rate
generation and tracking, or the link may be run in Test Mode with external frequency
multiplying and data tracking PLLs.
3 .0 DATA ENCODING, VIOLATION AND S YNCS
3 .1 Da t a En c o d in g
Any form of serial data transmission requires some form of encoding before the data are
output to the transmission medium. Encoding is the process of converting a set of m
data bits to a set of n code bits.
The purpose of the encoding operation is to include clock information in the data stream.
Without this timing information, the Receiver would not be able to distinguish adjacent
bits of the same value. For example, if we transmit a thousand ONEs followed by a
ZERO, the Receiver might detect only 999 ONEs, or perhaps 1001 ONEs, followed by
one or two ZEROs. An accurate clock is needed to tell the Receiver when to sample the
incoming bit stream to determine if the bit is a ONE or a ZERO. Since the Transmitter
and Receiver have only one data path between them, the clock (timing) information
must be included in the serial data stream.
53
TAXIchip Integrated Circuits Technical Manual
AMD [ AMD ]
