IBM 5251 Series Maintenance Information Manual page 170

When information is to be sent on the cable, a group of
five 1 bits (1010101010 half bits) is sent to establish bit
synchronization. Immediately after the 1 bits have been
sent, a group of three half bit l' s and three half bit 0' s
is sent to establish frame synchronization. After the
frame synchronization half bits have been sent, the
frame is sent; the sync bit is sent first.
1 Bit
Cell
c c c c c c c c c c c c c c c c c c c
Il
=
logical 1
=
10
n
=
logical 0
=
01
n
=
11
U
=
00 11 and 00 are valid only during frame synchronization.
L
I 1'0
Bit Synchronization-k--Frame-l
(5 logical' bits) Synchronization Sync Bit (bit 15 of the first frame)
- T o line
4·10