Quatech MPAP-100 User Manual - page 10
Quatech Syncdrive Plus User’s Manual
Synchronous Communication with Syncdrive Plus
Rev 01 (2/6/2003)
Page 6
Synchronous communication with Syncdrive Plus
Most data communications in personal computers is handled
asynchronously using standard communication ports. Asynchronous
communication transfers data one character at a time with
significant overhead due to the addition of the start and stop bits
required for each character. These additions can decrease the rate of
data transfer by 20% or more.
In contrast, synchronous communication transfers data in a format
referred to as a frame. Each frame consists of a block of data plus a
fixed amount of overhead from the insertion of control, synchronization,
and error detection characters. Since the amount of overhead is
independent of the data block size, the percentage of the total
transfer time devoted to the frame overhead diminishes as the size of
the data block increases.
Synchronous communication is further divided into bit-synchronous
and byte-synchronous transfers. Bit-synchronous transfers treat the
data block as a series of data bits with no specific character
boundaries. Byte-synchronous transfers treat the data block as a
series of fixed-length characters.
The first release of Syncdrive Plus transfers data in bit-synchronous
mode only. The bit-synchronous mode can be used to implement such
protocols as SDLC or HDLC. Syncdrive Plus does not implement any
specific protocol itself, but supports most protocols implemented by
the application software. Future releases of Syncdrive Plus will add
support for byte-synchronous modes such as BISYNC.
Understanding BIT-synchronous formatting
Syncdrive's bit-synchronous mode generates a frame formatted
according to the scheme shown below.
Figure 9 - Syncdrive Plus bit-synchronous frame format
flag
data
CRC
flag
The hardware uses the frame's start flag and end flag characters for
synchronization purposes. When the application transmits a block of
data, Syncdrive Plus automatically appends these flag characters to
the frame. When receiving a frame, the driver automatically removes
these flag characters from the frame before returning it to the
application.
Syncdrive Plus uses the frame's CRC bytes for error detection
purposes. When transmitting a block of data, it automatically
generates and transmits the CRC after it sends all of the data. When
receiving a frame, Syncdrive Plus automatically checks the CRC to
determine if the frame was corrupted. If a CRC error occurs,
Syncdrive Plus returns an error code (see MPA_GetData() return
values).
Note: Syncdrive Plus uses
synchronous communication
for faster data transfer rates
with less overhead.