Let’s Get Technical
01 0010 11
Start D0 D1 D2 D3 D4 D5 Stop
8 bit times
based on TxCLK
Figure 3. A block diagram of the fiber receiver. Serial
data from the fiber is converted back into parallel.
Figure 4. The format of a transmission frame.
of on or off pulses of light representing the bits, but with a specific order
to the bits. In fact, two additional bits
are added for framing, since this is an
asynchronous communication link
(there is no clock transmitted along
with the data). These are the Start and
Stop bits. The Start bit is always low
and the Stop bit is always high. Figure
4 shows the format of the entire eight-bit frame of data that is transmitted.
Note that after the Start bit, the
six data bits are clocked out in order
from D0 (LSB) to D5 (MSB). The last
bit in the frame is the
Stop bit. This frame
format is the digital waveform that is
input to the fiber transmitter (Tx).
When no transmission is taking
place, the normal state of the idle signal is high. The fiber transmitter
keeps the fiber dark during this time.
Why have the fiber sitting there filled
with light when nothing is being transmitted? This is also the waveform that
comes out of the fiber receiver (Rx).
So, you can see the importance of
the Start bit being a zero. Whenever
the signal goes low, we have the
beginning of a new transmission
frame. The Stop bit guarantees we
return to the high, idle state of the signal when transmission is complete.
Now it is time for a little math.
Suppose the transmitter clocks bits out
at the rate of 1.25 Mbps (1,250,000
bits/second). This would correspond to
a TxCLK frequency of 1.25 MHz. The
following questions come to mind:
1. What is the time for a single bit?
2. What is the time for a transmission frame?
3. How many frames can be transmitted in one second?
4. How many bits of data can be
transmitted in one second?