80-Bus News


November–December 1983 · Volume 2 · Issue 6

Page 55 of 67

transitions, just a continual high or low to the end of the data block. This is an invitation to lost sync, particularly if the clock is an integral part of the signal, how can this be overcome? The answer is what is called bit stuffing, which gives rise to a highly efficient data transmission system known as NRZI.

First a look at NRZI, it stands for Non Return to Zero Inverted (it is actually used on our disk systems but no-one shouts about it). It’s not a case of high means a 1 and low means a 0. It is a case of change of input state means a change of output data state. Lets assume that the input signal is low, and the output data state is currently 0. The next input signal transition from low to high will cause a change of output state from 0 to 1. No different from what we already know, but if the output data state were already high when the input low to high transition occurred then the data state would change from 1 to 0. It is the input transition edges associated with the transition which causes the output to change from one data state to another. Thats where the Non Return to Zero bit comes in, an input transition to zero does not necessarily mean that the data state returns to 0, it could go to 1. None of this matters so long as the decoder knows what is to happen, and knows its starting states.

It should be obvious that to be a synchronous data transmission system, NRZI must have some sort of sychronising mechanism built in to allow clock recovery. It must know how many bits have passed between one input transition and the next. A continual steam of 1’s or 0’s is an invite to lost sync. If we sent a stream of 010101010.... then it would be no problem to recover the clock from the data itself, a simple phase lock would do it. Given the proven ‘sample and hold’ characteristics of phase locks, we could send quite a lot of 0’s before we needed to send a 1 to regain the sync, likewise we could send a lot of 1’s before we needed to send a 0 to regain sync. This is where the ‘bit stuffing’ technique comes in. The HDLC logic counts the number of similar bits sent. If five consecutive 1’s are sent, then the logic will ‘stuff in’ an extra 0 in the sixth position to regain the sync. Likewise if five consecutive 0’s are sent, then the logic ‘stuffs in’ an extra 1 in the sixth position. The receiving end knows all about this, and if it sees a transition in the sixth position it knows to throw that bit away. Very simple.

The only time this ‘bit stuffing’ does not take place is with the flags. So, if there are six consecutive 1’s received without the necessary ‘stuffed pit’, then one of two things has happened. It must be that start or end of a packet. If it’s the start, then no preceding data is of consequence, already having been dealt with. If it’s the end of a packet then either a data error occurred, in which case the preceding two bytes won’t add up to the CRC, making the packet invalid. Or the extra bit did indicate the end of the packet, in which case the CRC does add up and the packet is valid.

Of course with a high speed interface all the HDLC logic must be in hardware, no way can you do the ‘bit stuffing’, phase lock clock recovery and CRC checking in software when the data rate is approaching the cycle time of the processors involved. Special chips have been developed to handle this and although expensive, they can really shift. Another criteria with very high speed links is the clock recovery itself. Phase shifts in the connecting circuits can cause delays in the arrival of the transitions. These delays can be a substantial part of a ‘bit time’ when high speeds are considered. This one of the advantages of ‘bit stuffing, the clock signal is an integral part

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