When operating in the 531.25
Mbaud mode, data byte 0,
Tx[00:09], is active and is
clocked into the input latch a
single byte (10 bits) on each
rising edge of TBC. In the 1062.5
Mbaud mode both data byte 0,
Tx[00:09], and data byte 1,
Tx[10:19], are active. In 1062.5
Mbaud mode, data byte 0 and
data byte 1 are clocked into the
transmitter on the rising edge of
every clock cycle, (TBC). There is
one minor variation possible in
the 1062.5 Mbaud mode, referred
to as “ping-pong” mode. Ping-
pong mode is selected by setting
the PPSEL pin (#34) high. In this
mode the transmitter clocks data
into the input latch one byte per
half clock cycle. Data byte 0 is
transmitted on the rising edge of
TBC and data byte 1 is trans-
mitted 1/2 clock cycle later. See
Figure 16 for timing information.
The input latch will stop sending
the data applied to the Tx[00:09]
data pins when a low is applied to
the -COMGEN pin (#32) and will
send the pre-set special Fibre
Channel character, K28.5 instead.
The 8B/10B coding scheme,
adopted by Fibre Channel, con-
verts 8 bit data words into 10 bit
representations of the actual
data. Of all the possible combina-
tions of 10 bit binary words, the
8B/10B code reserves 256 of
them to represent the valid
combinations of 8 bit data. Some
of the remaining combinations
are reserved for special functions.
The character reserved for
defining the transmitted word
boundary has been defined as the
K28.5 character, also known as a
comma character. The receiver
will automatically reset registers
and clock when it receives a
comma character (this will be
discussed in more detail in the
receiver operation section). Every
valid 8 bit data word is actually
represented by one of two 10 bit
codes, indicating either positive
or negative running disparity.
The input latch only generates
the K28.5 character with positive
disparity (0011111010).
In Figure 2, the Frame
Multiplexer utilizes shift registers
and a multi-stage multiplexing
scheme to convert the 10 or 20
parallel data bits to a serial data
stream. This serial data stream is
then fed directly into the Input/
Output Select portion of the
transmitter.
The I/O Select function allows use
of both the internally serialized
Fibre Channel data stream and an
externally supplied Fibre Channel
data stream denoted as
±
SI (pins
11 and 12). By using the proper
settings of TS1, TS2, and EWRAP
(pins 76, 75, and 71
respectively), the internal data
stream and the external data
stream can be directed to various
combinations of the cable driver
output, the laser driver output,
and the electrical loopback
output. The possible I/O
combinations are listed in the
Input Output Select Table and the
functionality is described in more
detail in the Transmitter Laser
Driver Operation section below.
The cable driver function
provides a 50
Ω
differential cable
driver output at pins 5 and 6
(± SO). The simplified circuit is
the O-BLL section shown in
Figure 10. A similar output is
provided to allow electrical
loopback, or wrap of the local
data back to the local receiver for
diagnostics. This is denoted as
±
LOUT on pin 8 and pin 9.
The final function on the
transmitter chip is the Laser
Driver block which provides a
high speed differential output,
±
LZOUT, at pins 19 and 20.
There are several other laser
control I/Os which will be
HDMP-1512 Input Output Select Table
Mode
0
1
2
3
4
5
6
7
658
TS1
0
0
0
0
1
1
1
1
TS2
0
0
1
1
0
0
1
1
EWRAP
0
1
0
1
0
1
0
1
Data Source For:
±
SO
±
LZOUT
±
LOUT
NA
Internal
NA
NA
NA
Internal
Internal
Internal
NA
Internal
NA
Internal
Internal
NA
NA
NA
Internal
Internal
Internal
±
SI
NA
Internal
NA
±
SI
Active Outputs
±
SO
±
LZOUT
±
LOUT
no
yes
no
no
no
yes
yes
yes
no
yes
no
yes
yes
no
no
no
yes
yes
yes
yes
no
yes
no
yes
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