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To clarify how the C152 does the NRZI decoding in SDLC mode.
The way NRZI decoding is described in the 8XC152 Hardware Description is as follows:
"pulses less than four sample periods are ignored, and assumed to be noise."
While this is true, it is also misleading. In SDLC mode, the C152 receive pin is sampled at 8 times the bit rate. One could assume that with the above statement, pulses less than 1/2 a bit time would be ignored. This is wrong. It turns out that a pulse that is between 3/8 and 1/2 a bit time in duration may or may not be sampled 4 times. This is because it takes only 3/8 of a bit time for 4 samples to occur. In the C152, it is indeterminate how a pulse that is 3/8 to 1/2 a bit time will be decoded.
This problem is particularly apparent when interfacing to an 8044 in a network where line drivers are being used. Pictures obtained by CMAD applications have shown waveforms produced by the 8044 that fall within the range of 3/8 to 1/2 bit time when an 8044 is responding to a master SDLC controller. When this occurs, the C152 may decode the incoming bit stream as random addresses which might match the C152s assigned address.
Currently, there are no specific work-arounds available that can be given out to customers. Not all 8044 users will have a problem. The customer must look at the waveforms produced by the 8044 in his network to determine if a problem exists. The flag recognition bug (See FaxBack document 2035) should also be examined for possible consequences.
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