Events to be transferred by the optical link must be formatted as 32-bit
wide words. In addition to the user data, an additional bit (known as
"D39") is set on the last word only. The user presents data, provides a
strobe and then must wait for an acknowledgment strobe back from the OLS \
before changing the data. A timing diagram is shown in Figure 
.
Figure: Timing diagram for data into the OLS .
When not transmitting user data, the OLS continuously sends idle
characters to the OLD . As user data arrives it is stored in a register.
Before the first data character is sent, the OLS sends a special "comma"
character, which marks the user data. Each 8-bit user byte is then taken
in turn and its parity is calculated. The byte is then encoded as a 10-bit
character using the FC-1 scheme. The character is then clocked into the
Source OLC -266 card synchronously with a 26.5625 MHz clock. When four
characters have been transferred, a control character is then sent. This
consists of the four parity bits, the parity bit of the control byte, a
reserved bit, a sequence bit and D39 (0 for all words except the last).
The structure of the control byte is shown in Figure .
Figure: Structure of the control byte.
The six characters comprising a comma, 4 data characters and a control
character are known as a "transmission word". Each transmission word is
followed by 2 idle characters. This structure is shown in
Figure .
Figure: Structure of transmitted data.
The Source OLC -266 runs a VCO at 265.625 MHz which is phase-locked to the user clock. Incoming 10-bit characters are serialised and the bit-stream is transmitted over the optical fibre at this rate. Note that this is a purely serial line with no clock pulse. When the link is first established, the transmitter sends synchronisation characters which lock the phase of the receiver clock to that of the transmitter. One of the reasons for the 8-10 bit encoding scheme is to ensure that all characters transmitted consist of an even mix of 1s and 0s. This avoids transmitting "DC" codes (such as 11111111) and ensures that there are frequent changes of state which keep the clocks synchronised. The Destination OLC -266 deserialises the bit-stream and reforms 10-bit characters which it delivers to the OLD .
The OLD receives the 10-bit characters and decodes them to recover the original bytes of data. These are then grouped in a 40-bit wide register (the original 4 data bytes plus the control byte) and then strobed into the IB .
