Texas Instruments | AN-679 Point to Point Fiber Optic Links | Application notes | Texas Instruments AN-679 Point to Point Fiber Optic Links Application notes

Texas Instruments AN-679 Point to Point Fiber Optic Links Application notes
AN-679 Point to Point Fiber Optic Links
Literature Number: SNOA155
National Semiconductor
Application Note 679
Filipe Sanna
Louise Yeung
April 1990
3.1 Synchronization Timing Examples
4.1 System Block Diagram
4.2 Channel Block Diagram
5.1 Data Interface
5.2 Control Bus Interface
6.1 Pull-Up Scheme
6.2 GAL Scheme
A station using the ANSI X3T9.5 (FDDI) physical layer standard can transmit and receive data at 100 Mbits/sec
through a fiber optic cable. However, with several physical
layers connected together in parallel, each with its own fiber
optic cables for transmission and reception, the station can
transmit and receive data at much higher speeds. National
Semiconductor’s physical (PHY) layer devices can be connected together in parallel to achieve such high bandwidth
point-to-point links. The National devices required to implement a PHY layer are the DP83231 Clock Recovery Device
(CRDTM device), the DP83241 Clock Distribution Device
(CDDTM device), and the DP83251/55 Physical Layer Controller Device (PLAYERTM device). The bandwidth of the
system depends on the number of PHY layers usedÐeach
set of PHY layer devices contributes 100 Mbits/sec to the
FDDI PHY layers in parallel require only one pair of fiber
optic cables, one pair of transceivers, and one set of PHY
layer chips per channel, while yielding a typical data
throughput of 800 Mbits/sec (for a system with eight channels).
Any application where data throughput is the limiting factor
to system performance is a candidate for a high-speed
point-to-point link. For example, a point-to-point link can be
installed between a CPU and a disk controller to speed up
information storage and retrieval times. Another application
area could be in the display capabilities of a graphics workstation which can be combined with the data processing
power of a supercomputer to achieve visualization for data
intensive simulations (Figure 1). As a third example, a highspeed networking backbone using a point-to-point link is depicted in Figure 2. Here, separate FDDI rings are connected
together with a high speed link which provides bridging between rings without loss of performance.
Fiber optics afford a greater physical separation between
stations than electrical signals. Hence, a fiber point-to-point
link can be used to extend SCSI or IPI transmissions up to
one kilometer.
The use of parallel FDDI PHY layers is a cost effective
method to increasing the data throughput in multiples of 100
Mb/s. This task can be accomplished utilizing an existing
FDDI fiber plant.
TL/F/10797 – 1
FIGURE 1. Point-To-Point Link between a
Supercomputer and a Graphics Workstation
TL/F/10797 – 2
FIGURE 2. High Speed Networking Backbone
Using a Fiber Optic Point-To-Point Link
BMACTM , PLAYERTM , CDDTM , and CRDTM are trademarks of National Semiconductor Corporation.
C1995 National Semiconductor Corporation
Point-to-Point Fiber Optic Links
Fiber Optic Links
RRD-B30M75/Printed in U. S. A.
After travelling through the fibers, the data arrives at Station
II. Each PHY layer on the receiving end reads data from the
fiber and presents its byte to the corresponding PHYnÐIND
port (at 12.5 MHz) in Station II. SYSÐIND then rejoins the
bytes back into the 32-bit word sent by Station I and can
present the data to the host (at 12.5 MHz). This demonstrates how Station I can send 32 bits to Station II in 80 ns,
giving an effective data throughput of 400 Mbits/sec.
Even though this is a non-FDDI application, the general
rules for FDDI framing must be followed. In particular, each
frame must start with a JK symbol and end with valid FDDI
ending delimiter (Figure 4). Furthermore, the frame size
must be between three and 4500 bytes long (see PLAYER
device datasheet for more detail). At least four pairs of idle
symbols should be inserted between the frames to allow for
readjustment of the PLAYER device’s elasticity buffer. However, to guarantee at least one opportunity to recenter the
elasticity buffer between frames in the event of clock drift or
a single line hit in the interframe gap, the user is advised to
insert eight idle symbol pairs.
A system using four PHY layers in parallel is shown in Figure
3. The diagram demonstrates conceptually how data is
passed from Station I to Station II at 400 Mbits/sec. Each of
the four PHY layers in Station I is connected to a PHY layer
in Station II with fiber optic cables. Since National Semiconductor PHY layers are full duplex, each pair of PHY layers is
linked by two fibers, one for transmission in each direction.
The system interface shown contains two parts.
SYSÐREQ, which handles data transmission, and
SYSÐIND, which handles data reception.
Suppose that Station I wants to transmit a 32-bit word of
data to Station II. SYSÐREQ in Station I takes the data and
splits it into four bytes, one for each PHY channel. Each
PHY layer reads its byte from the PHYnÐREQ ports of
SYSÐREQ (at 12.5 MHz) and sends the data out across
the fiber as an 8-bit serial stream at 100 Mbits/sec. (Note
that due to the 4B/5B encoding scheme used by the PLAYER devices the data actually passes through the fiber as a
10-bit serial stream at 125 MHz.)
TL/F/10797 – 3
FIGURE 3. System Using Four PHY Layers in Parallel
The synchronization process is repeated with each new
frame received, and may not always proceed exactly as described above. Depending on the skew between the fastest
and slowest channels, the PLAYER devices will either synchronize the bit streams or generate an error. Figure 5
shows four different scenarios for the synchronization of
several PHY channels. Each is described in the following
TL/F/10797 – 4
FIGURE 4. Valid Frame Format
3.1 Synchronization Timing Examples
Figure 5a presents the timing waveforms for a single PLAYER device which was the first device to receive a JK at the
Elasticity Buffer (EB) in a situation where the last JK received by another PLAYER device is 60 ns behind it.
PMDÐIND shows a simplified version of the data coming
into the PLAYER device from the CRD device. The Local
Byte Clock (LBC) is a 12.5 MHz TTL signal from the CDD
device which is used by the PLAYER device. After a propagation delay, data appears at the A Indicate Port of the
PLAYER device on each rising edge of LBC. The CR line is
pulled high on the first falling edge of LBC after the PLAYER
device completely receives the JK symbol. The delay in this
signal (t1) is due primarily to the external propagation delay
of the CR line pullup structure and some delay time within
the PLAYER device itself.
The important point to note in this scenario is that the CS
signal does not go high for some time (t2) after the CR signal goes high. This indicates that this PLAYER device was
one of the first PLAYER devices in the system to receive its
JK symbol, since CS e 1 only when all of the PLAYER
devices have received a JK symbol. Since the CS signal
failed to go high before the first falling edge of LBC after the
CR line is released, this PLAYER device outputs a second
JK symbol to the host through the A Indicate Port. By the
next falling edge of LBC, the CS signal has gone high and
all of the PLAYER devices outputting the first byte of data
after the JK symbol to the host.
Figure 5b depicts the timing waveforms for a single PLAYER
device which was the first device to receive a JK symbol in a
situation where the last JK symbol received from another
PLAYER device is less than 20 ns after the first JK symbol.
Again, PMDÐIND shows the beginning of a frame coming
from the CRD device. The PLAYER device releases the CR
line upon reception of the JK symbol and after a short delay
(t3), the CR signal goes high. In this scenario, however, the
CS signal goes high within one LBC period, so that the
PLAYER device shown only has to report one JK symbol to
the host before outputting data. This indicates that all of the
symbols are coming in with a skew of less than 40 ns between the slowest and fastest channels. The delay between
the release of CR and the assertion of CS (t4) depends on
skews in LBC between the channels, the reaction time of
the wired-AND structure used to create the CS signal, and
the skew between the data coming in on the different channels.
Figure 5c demonstrates the timing waveforms for a channel
which receives the JK symbol pair last. Here, the CS signal
goes high immediately after the PLAYER device releases
the CR line. The only delay (t6) is due to skews in LBC
between the channels and the reaction time of the external
wired-AND structure. Since this PLAYER device senses the
CS signal within the first falling edge of LBC, it only needs to
output one JK symbol to the host before outputting the data
Figure 5d shows an error situation where one or more of
the channels never receives a JK symbol. In this case, the
The FDDI standard specifies the maximum clock drift between two stations to be 100 parts per million (ppm) peakto-peak and the maximum frame size to be 4500 bytes.
However, it is possible to transmit more than 4500 bytes per
packet if tighter clock tolerances are observed. The equation to determine the maximum allowable frame size is given
Clock Drift per Bit
The clock drift per bit is calculated by taking the maximum
difference in the CDD device frequencies between the
transmitting station and the receiving station multiplied by
8 ns. For example, if the 12.5 MHz crystal used by the CDD
device in the transmitting station had a a 25 ppm accuracy
error and the 12.5 MHz crystal used by the CDD device in
the receiving station had a b30 ppm accuracy error, then
the clock drift per bit would be 55 ppm (peak-to-peak) x 8 ns
or 4.4 ps. The maximum frame size that could be transmitted using these crystals is thus 40 ns/4.4 ps or 9090 bytes,
significantly more than the 4500 specified.
FrameMAX e 40 ns
Since the data traveling from Station I to Station II passes
through different devices and different fibers, it may not arrive at each of the PHY channels on the receiving end at
exactly the same time. Hence there must be some method
of aligning the incoming bit streams so that data is passed
to the PHYnÐIND ports in the correct sequence. The JK
symbol serves as the reference point for synchronizing the
bit streams among channels. National Semiconductor
PLAYER devices have an open drain output called Cascade
Ready (CR-pin 48) which is released when a JK is received
on that channel. ANDing the Cascade Ready (CR) pins of all
the PLAYER devices creates a signal (called Cascade Start)
which indicates that all of the channels have received a JK
symbol. This signal is tied to the Cascade Start (CS-pin 47)
input of each PLAYER device and indicates when all of the
devices achieve synchronization.
The first PLAYER device that receives a JK symbol pair will
present that pair to the host (through the A Indicate Port).
Meanwhile, it will activate the open drain CR output. If needed, it will output a second JK to the host as it waits for
synchronization from the other PLAYER devices. During this
time, the incoming data can be temporarily stored in the
elasticity buffer. The ability of the PLAYER devices to output
two consecutive JK symbols yields an 80 ns synchronization
window. Each PLAYER device that receives a JK symbol
will present a JK symbol to the host and release its CR line.
Once all of the PLAYER devices have released their CR
lines, the CS signal feeding each PLAYER device will go
high. At this point the read pointers of all the PLAYER device’s elasticity buffers will be aligned and all of them will
output JK symbols to the host. Simultaneous reception of
JK symbols on every channel informs the host that synchronization has occurred, and that the subsequent data bytes
will be properly aligned.
JK symbols to the host, but when the CS signal fails to rise
the device is forced to output the incoming data and report a
Cascade Sync Error (CSE) in the Receive Condition Register B (RCRB).
PLAYER device shown recognizes its JK symbol and releases the CR line. Since one of the PLAYER devices never
gets a JK symbol and hence never releases the CR pin, the
CS signal fails to go high. The PLAYER device shown in this
diagram attempts to compensate for skew by outputting two
TL/F/10797 – 5
FIGURE 5a. First PLAYER device to receive a JK symbol in a large skew scenario. The CS
signal goes high about 60 ns after the CR line is asserted. This indicates that some of the
channels received JK symbols more than one LBC period after the channel shown.
TL/F/10797 – 6
FIGURE 5b. First PLAYER device to receive a JK symbol in a small skew scenario. The CS signal goes
high shortly after assertion of the CR signal, indicating that all of the channels received JK symbols within t4.
TL/F/10797 – 7
FIGURE 5c. Last PLAYER device to receive JK symbol (in any scenario). Since this device
was the last to assert its CR line, t6 is due only to the delay of the wired-AND structure.
TL/F/10797 – 8
FIGURE 5d. PLAYER device which receives JK symbol in scenario
where one or more channels never receive JK symbols.
Figure 7 illustrates the source of timing deviations between
the channels and demonstrates the need to minimize timing
skews between the channels wherever possible. In Station
I, we are concerned with TXC and TBC while in Station II we
examine LBC, since Station I is transmitting to Station II.
The time parameters shown in the figure represent the maximum deviations in propagation delay between channels.
For example, if t1 were 10 ns, this would mean that TXC/
TBC could arrive at PHY1 up to 10 ns before arriving at
t1 represents the skews in TXC/TBC between the channels,
t2 encompasses the skews in the PHY layer’s transmitting
path, t3 represents the differential skews amongst the fibers, t4 includes all of the skews inherent in the PHY layer’s
receiving path, and t5 represents the skews in LBC between
the channels in the receiving station. All of the skews together must not exceed 80 ns in order to prevent synchronization errors, and smaller total skews will provide greater
stability across temperature and power fluctuations.
In a worst case scenario where all devices were badly
skewed, t2 together with t4 yields a base 30 ns of skew
between the channels. This leaves 50 ns available for differential skews in the clock signals and the fiber. It is recommended that t1 be held to 4 ns and t5 kept under 8 ns to
prevent misclocking of the data. Hence, the maximum skew
among fibers should be less than 38 ns.
4.1 System Block Diagram
The number of PHY layers connected together in parallel is
limited only by the timing budget of the CS line (explained in
Section 5.2) and the timing skews between channels. As an
example, a system level block diagram using four PHY layers connected together in parallel is presented in Figure 6.
All of the PHY layers within a given station are driven with a
single set of clock signals, and all are controlled and monitored by the host system through the Control Bus interface.
Each channel has two dedicated fibers, one for transmission and one for reception. The full duplex architecture eliminates the need for complex handshaking between the two
stations. The four channels communicate through the CR
and CS signals. For simplicity, the CR lines are shown connected to a pullup resistorÐa more detailed look at the connection of these pins is given in Section 6.
The global clock scheme should be arranged to minimize
the skews in the clock signals between PHY layers. Smaller
clock skews between channels will leave more tolerance for
device skews and fiber optic variations. For further recommendations concerning the CDD device in a multiple PLAYER device environment, see the CDD device datasheet
(CDD Device Driving Multiple PLAYER Devices).
TL/F/10797 – 9
FIGURE 6. PHY Layer Block Diagram for an Example System Using Four PHY Layers in Parallel
For example, for a typical FDDI fiber optic cable, s e 1.9 c
108 m/s, v e 0.005, and w e 0.001. Solving for l with
these parameters results in a length of 667 meters.
The following equation summarizes the tradeoff between
cable length and variance:
I (1 a v)
I (1 b v)
k 38 ns
s (1 b w)
s (1 a w)
where: s is the speed of the signal in the cable
I is the average length of the fiber in meters
v is the variance in the length of the cable
w is the variance in the speed of the signal through
the fiber
TL/F/10797 – 10
t1 e Worst case clock skew between two PHYs
t2 e Worst case skew between PLAYER device propagation delays
t3 e Worst case skew between Fibers
t4 e Worst case skew between PLAYER device propagation delays
t5 e Worst case clock skew between two PHYs
Total skew e t1 a t2 a t3 a t4 a t5
Note: Total skew must not exceed 80 ns in order to prevent synchronization errors.
FIGURE 7. Origin of skew between channels. Adding all of the skews
(t1 through t5) gives the total possible skew for the system.
PLAYER device, then the Cascade Sync Error (CSE) bit will
be set in the Control Bus register RCRB by the first PLAYER
device to have recognized a JK symbol.
4.2 Channel Block Diagram
Figure 8 shows the components which constitute a single
PHY channel (the CDD device is common to all channels so
it is not shown here). The fiber optic transceivers are standard FDDI devices which translate electric signals to light
pulses and vice versa. The fiber optic receiver accepts data
from the fiber and sends two pairs of differential ECL signals
to the CRD device, namely signal detect and data. The CRD
device extracts a clock signal from the incoming data and
passes a resynchronized equivalent of this data and a recovered clock signal to the PLAYER device, as well as signal detect and clock detect signals.
Within the PLAYER device, the incoming data stream is decoded (from 5B to 4B) and placed in the elasticity buffer.
When in cascade mode, the elasticity buffer is used not only
to absorb variations between the received clock and the
local clock, but also to smooth out skews between incoming
data presented to the different PHY channels. If all of the
PLAYER devices receive JK symbols within 80 ns of each
other and release their CR pins, then the CS pin will go high
and all of the PLAYER devices will read from the first data
location of the elasticity buffer. This cell contains the first
byte of data received after the JK symbol. Hence, the elasticity buffers facilitate the coordination of data output between the different PHY channels. If the last PLAYER device receiving a JK does so more than 80 ns after the first
5.1 Data Interface
The system interface should consist of a transmit holding
register and buffer for transmission and a buffer and receive
register for incoming data. A state machine is required to
decode the symbols coming from the PLAYER device so
that only data is stored. Furthermore, a controller will be
required to monitor and manage the PLAYER device
through the Control Bus interface. This controller must handle the initialization of the PLAYER device and report error
conditions to the host.
Each PLAYER device takes ten bits of data at the A Request Port, a pair of 4-bit data symbols plus a parity and
control bit. (See the PLAYER device datasheet for the
PHYÐMAC byte wide interface table.) The system interface
can thus generate parity and control for each PLAYER device separately and check control and parity coming from
each channel. To simplify the system interface, however,
the parity pins can be tied to ground and parity checking can
be disabled in the Current Transmit State Register (CTSR).
Parity information coming from the PLAYER device can similarly be ignored.
FIGURE 8. Block diagram of one PHY channel. The components shown here are repeated for each separate channel in the system.
TL/F/10797 – 11
Thus if m is 20 pF and n is 2, the maximum pullup resistor is
500X, which meets the specification for the minimum resistor size. However, it is apparent that for many PLAYER devices a passive pullup resistor is too slow.
In error situations, one or more PLAYER devices may report
a Cascade Sync Error, but they may not do so simultaneously depending on when they receive JK symbols. The
Cascade Sync Error (CSE) bit of the Receive Condition
Register B (RCRB) will be set by each PLAYER device
which receives a JK but does not sense the CS pin go high
before the second falling edge of LBC from when CR was
released. CS has to be set approximately within 80 ns of CR
release. If a JK symbol is completely corrupted from a line
hit or bad connection, the PLAYER device on that channel
will not report a CSE. Only the data on the channel(s) which
did not report a CSE will be corrupt, however, these are the
channels which were unable to synchronize with the rest of
the group. All of the PLAYER devices which receive a JK
symbol (and release the CR pin) will read data from the first
cell of the Elasticity Buffer. Therefore, a line hit on a single
fiber will not wipe out the entire frame. The rest of the channels may still output synchronized data. This is particularly
important in applications where partial data reception is still
useful. For example, during screen updates in high resolution graphics systems, only one line of pixels would be lost
instead of an entire block of the screen blanking out.
6.2 GAL Scheme
As a result, we recommend using external logic devices for
any system with three or more cascaded PLAYER devices.
The choice of devices is limited by the propagation delay as
well as fan in or fan out. Each CR pin should be pulled up
with a 400X resistor and fed into the AND gate. A recommended device to perform the AND function is the
GAL16V8A chip, which offers 8 inputs and supplies 8 outputs with a propagation delay of 10 ns. This chip will allow
up to eight PLAYER devices to be cascaded together while
still maintaining the necessary delay, fan in and fan out
characteristics. The device should be place in the electrical
center of the cascaded devices to prevent excessive timing
skews among the chips. Figure 9 depicts an eight channel
system using the GAL16V8A to AND the CR signals together.
5.2 Control Bus Interface
If no JK symbols are corrupted, but they arrive with more
than 80 ns of skew, all of the PLAYER devices will eventually report a CSE error. Hence the control microprocessor has
the ability to pin point the corrupted channel or determine if
the problem is due to excessive skew between the channels. Note that the Control Bus registers can be programmed to assert the interrupt (INT) pin upon detection of
the CSE flag.
To place the PLAYER devices in Cascade Mode, the Mode
Register (MR) must have the Cascade Mode (CM) bit set to
one. The Cascade Synchronization Error (CSE) of the Receive Condition Register B (RCRB) is set to one if the CS
signal fails to go high within 80 ns of recognizing the JK
symbol. The RCRB also reports Elasticity Buffer errors
through the EBOU bit, signaling a loss of data from the fiber.
These bits must be cleared by the Control Bus controller.
When the number of PLAYER devices and the total capacitance is small, it may be possible to tie all CR pins and CS
pins together and use a single pullup resistor. The lower
limit of the pullup resistor is calculated as follows. The CR
pins typically sink a 13 mA maximum, so the equation for
the smallest resistor which should be used is:
RMIN e VCC/0.013X
Hence for a voltage supply of 5V, the resistor value is
5/0.013 e 385X. The upper limit of the pullup resistor depends on the capacitance of the system and the number of
PLAYER devices used. Restricting the timing budget (tb) to
20 ns (worst case) for the AND function, we arrive at the
following equation:
RMAX e tb/(m x n) X
TL/F/10797 – 12
FIGURE 9. Example of an eight channel system
using a GAL16V8A chip to perform the AND
function on the CR lines. The resulting signal (CS)
is fed back into each of the PLAYER devices.
where: m is the capacitance associated with each PLAYER
device’s CR line (including the IC capacitance (4
pF), the socket capacitance, and the trace capacitance) measured in pF
n is the number of PHY channels (number of cascaded PLAYER devices).
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