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© Sealevel Systems, Inc.
4021 Manual | SL9154 11/2021
Appendix E - Asynchronous and Synchronous Communications, Continued
Synchronous Communications
Synchronous Communications is used for applications that require higher data rates and greater error
checking procedures. Character synchronization and bit duration are handled differently than
asynchronous communications. Bit duration in synchronous communications is not necessarily pre-
defined at both the transmitting and receiving ends. Typically, in addition to the data signal, a clock signal
is provided. This clock signal will mark the beginning of a bit cell on a pre-defined transmission. The source
of the clock is predetermined, and sometimes multiple clock signals are available. For example, if two
nodes want to establish synchronous communications, point A could supply a clock to point B that would
define all bit boundaries that A transmitted to B. Point B could also supply a clock to point A that would
correspond to the data that A received from B. This example demonstrates how communications could
take place between two nodes at completely different data rates. Character synchronization with
synchronous communications is also very different than the asynchronous method of using start and stop
bits to define the beginning and end of a character. When using synchronous communications, a pre-
defined character or sequence of characters is used to let the receiving end know when to start character
assembly.
Figure 11 - Synchronous Communications Bit Diagram
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 predefined
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.).
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