Allen-Bradley 1779-KFMR User Manual page 77

Chapter 6
HDLC Data Link Layer
6 8
When the transmitting node sends five contiguous ones anywhere
between the opening flag and the closing flag of a frame, it automatically
inserts an extra zero bit. The insertion of the zero bit thus applies to the
contents of the address, control, data, and FCS fields. For example,
consider the following bit stream passed down from the preceding layer to
the HDLC data-link layer:
Time
0101 1111 1001
Before transmitting this bit stream across the link, the HDLC link-layer
must insert zeros as follows:
Zeros Inserted
0101 11110 1001
If the first zero were not inserted, the receiving node would interpret the
six ones bracketed by zeros as a flag sequence. This would be a flag
sequence even though it is not in a single byte because HDLC is bit
oriented. If the second and third zeros were not inserted, the receiving
node would interpret the 11 ones as an abort sequence. Although there is
no inherent reason to insert the fourth zero by itself, the algorithm must be
consistent. If the fourth zero were not inserted, the receiving node would
misinterpret the data because it would disregard the last zero of the
original bit stream.
The receiving node continuously monitors the bit stream it receives from
across the link.
If it receives a bit stream of a zero bit followed by five 1 bits followed
by a zero bit (0111110), it deletes the last zero bit and passes through
the rest (011111) as data.
If it receives a bit stream of a zero bit followed by six 1 bits followed
by a zero bit (01111110), it accepts this as a flag sequence.
If it receives a bit stream of seven or more 1 bits (1111111....), it accepts
this as an abort.
1011 1111 1111
1011 11101 11110
1011 1110
1011 11100