Novanta Precession Elephant 2, Руководство пользователя

Страница: 95 / 103

PRECESSION ELEPHANT 2 Scan Head 2 Instruction Manual 2021-09-22
The first 4 bits in a sub frame contain the preamble (PA), which is necessary to
synchronize the receiver and to distinguish different sub frame types. The preamble is
recognized by containing no change in state between the bits at 2 positions in the data
stream.
There are 3 preambles that may occur in 2 forms each. Which of these 2 forms is used is
dependent on the last cell (parity) of the previous 32-bit sub frame; see also the
following table.
Preamble Previous cell Description
0 1
B 11101000 00010111 tags a sub frame, which contains data for the X-
channel and which stands at the beginning of a data
block at the same time
M 11100010 00011101 tags a sub frame, which contains data for the X-
channel and stands not at the beginning of a data
block
W 11100100 00011011 Tags a sub frame, which contains data for the Y-
channel. If more than 2 channels are used then this
may be any other channel than the X-channel
Within the next 20 bits the position data is transmitted with the most significant bit
transmitted first of all.
4 extension bits follow. These bear different meanings depending on the device
controlled:
Data to
the scan
head The 20 position data bits set the nominal position for the channel; the four
extension bits shall contain the pattern 0001.
95