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Next: Flow Control Up: IEEE 1355 HS-Link Technology Previous: HS-Link Signals

Character Encoding

HS-Links use an 8B/12B DC balanced encoding scheme, where 8 bits of data are encoded into 12 code bits, i.e. the encoding overhead is 50%. Table 1 shows the encoding of the HS-Link characters. The clock recovery using the delay-locked loop scheme requires a positive going synchronisation pulse at the beginning of every character. Therefore a start and a stop bit need to be added to every byte in addition to the parity bit and the inversion bit, which is required to maintain DC balance. The data can be sent inverted to ensure that the disparity, i.e. the difference between the number of ones and zeroes, tends towards zero. The inversion bit indicates the polarity of the data bits in the character. The odd parity check will detect all single bit errors.

 
Table 1: HS-Link Character Encoding  

The encoding scheme allows the transmission of the 256 data values plus 126 control characters. 8 of these control characters are reserved for the low level link protocol. In order to ensure a continuous stream of characters, which is required to keep the receiver calibrated, IDLE characters are sent when no data is available. The flow control character (FCC) is used for the flow control mechanism. The end-of-packet (EP) character is used to terminate packets and can be replaced by the exceptional end-of-packet (EEP) character to indicate that an error has occurred. The other four control characters are used during the link startup and shutdown procedure.



Stefan Haas
Tue Mar 31 11:54:08 MET DST 1998