OMS MAXnet, User Manual

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REAL-TIME POSITION CAPTURE COMMUNICATION INTERFACE
MAXnet User’s Manual 3-5
3.7. REAL-TIME POSITION CAPTURE
The position capture commands control the real-time recording of axis position data and the management
of the captured position data. The captured position data includes the axis, the positive edge I/O bits, the
negative edge I/O bits, the home and encoder home events, and the encoder position of the axis. The
position data is captured when the conditions specified for the input bit are met. The capture conditions
for the home switch and general purpose input bits can be a rising/positive edge, a falling/negative edge,
or the event can be both the rising/positive and the falling/negative edge so data is captured on any
transition of the input bit. The real-time position capture feature is only available on an axis with
incremental encoders. See the MAX family command reference manual for more details on the real-time
position capture feature.
The MAXnet controller has a ring buffer in VME shared memory, which is used to transfer the real-time
position capture data to the host. When a capture event is recorded by the motor update cycle routine, it
transfers the capture table entry to the shared VME memory. The host is signaled that the data is
available via bit number 8, or hexadecimal value 0x00000100, in the controller status word 2 register at
offset address 0x00000FC8. The data available bit is also available at byte offset 0x00000FCA, and bit
number 0 or 0x01. The shared memory for the capture data is implemented as a ring buffer with an insert
index that the controller uses to insert data into the shared memory region, and a removal index that the
host uses to remove data from shared memory region. The controller places the capture data into the
ring buffer at the location specified by the insert index, and advances the insert index. If after being
advanced, the insert index equals the removal index, then the controller also advances the removal
index. If the controller has to advance the removal index, this means that the host is not removing data
fast enough, and capture data was lost by the host. The capture data is available in the shared VME
memory at offset addresses 0xD94 through 0xFBB. The format of the capture table data in shared VME
memory is defined in table 1-5 below.
TABLE 3-1 Real-Time Position Capture VME Shared Memory (Word Access Offset 0xD94)
Byte Offset Byte Offset (Hex) Byte length Description
3476 0xD94 1 Controller insert index
3477 0xD95 1 Host removal index
3478 0xD96 550 Table entries (10 bytes per entry, and 55 entries)
The number of entries can be greater than one for each axis, if capture events occur on back to back motor
update cycles, and if the host does not collect the data fast enough. The format of each table entry is defined in
table 3-2 below.
TABLE 3-2 Real-Time Position Capture Table Entry
Byte Offset Byte Offset (Hex) Byte length Description
0 0x00 4
Encoder position, offset 0x00 contains MSBs, offset 0x03
contains LSBs
4 0x04 1 Axis (X = 0, Y = 1, etc.)
5 0x05 1
Home event bits:
0x01 = Positive edge home switch
0x02 = Encoder home event
0x04 = Negative edge home switch
6 0x06 2
Positive edge I/O bits, offset 0x06 contains MSBs, and
offset 0x07 contains LSBs
8 0x08 2
Negative edge I/O bits, offset 0x08 contains MSBs, and
offset 0x09 contains LSBs