Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 13
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You will note that 19-bits of data per axis (and per returned status) are communicated
using this protocol, although the entire sequence requires 20 clocks. And although
parity is not used by this protocol, some data integrity is provided by “Control Bits”.
You will also note that the first three bits received by the Mach-DSP are designated “C2,
C1, C0”. These first three bits are called “control bits”, and they designate the function
of the data that is contained in the bits that follow. The table below describes the control
bits and associated functions:
Table 3.2c: Control Bit Definitions
Control Bits
[2:0]
Function
000
(reserved for future use)
001
16-bit position data follows
010
(reserved for future use)
011
(reserved for future use)
1xx
18-bit position data follows
As shown in the table, only Control Bit patterns 001, and 1xx are presently used,
designating either 16-bit or 18-bit position commands, and therefore there is room for
future expansion as well as flexibility to implement custom protocols for individual
clients.
For 16-bit position data, a value of 0 (all bits cleared) normally represents the far-left
and far-bottom of the scan field; a value of 32768 represents the middle of the scan
field; and a value of 65535 (all bits set) normally represents the far-right and far-top of
the scan field.
For 18-bit position data, a value of 0 (all bits cleared) normally represents the far-left
and far-bottom of the scan field; a value of 131072 represents the middle of the scan
field; and a value of 262143 (all bits set) normally represents the far-right and far-top of
the scan field.
Of course the relationship of the data to the polarity of axis motion (insomuch as far-left
/ far-right as well as far-bottom / far-top) depends on whether you are using a left-
handed or right-handed X-Y mount, and each axis may be inverted by changing the
“Input Scale” on the Mach-DSP application software, as described in section 7.3.
XY2-100 Status Return signal
The Mach-DSP includes a single serial port line (the RST signal located on the Control
Panel Serial Interface connector), which may be used to communicate status data back
to the microprocessor-based equipment. Presently this is not implemented and is
reserved for future use. Note that there is great inconsistency among XY2-100-based
equipment on the use (and even presence) of the Status signal.
