Appendix E - Synchronous and Asynchronous Communications
Sealevel Systems
ACB-II
Page
19
This pre-defined character is called a sync character or sync flag. Once the
sync flag is received, the communications device will start character
assembly. Sync characters are typically transmitted while the
communications line is idle or immediately before a block of information is
transmitted. To illustrate with an example, let's assume that we are
communicating using eight bits per character. Point A is receiving a clock
from point B and sampling the receive data pin on every upward clock
transition. Once point A receives the pre-defined bit pattern (sync flag), the
next eight bits are assembled into a valid character. The following eight bits
are also assembled into a character. This will repeat until another pre-
defined sequence of bits is received (either another sync flag or a bit
combination that signals the end of the text, e.g., EOT). The actual sync
flag and protocol varies depending on the sync format (SDLC, BISYNC,
etc.).
For a detailed explanation of serial communications, please refer to the
book
Technical Aspects of Data Communications
by John E. McNamara,
published by Digital Press (DEC) 1982.
