Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 15
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event, Frame Sync must be brought high by clock number 32, as shown in the diagram
above, otherwise a Frame Sync Error will occur, and data will not be communicated
correctly. In essence, all of this means that the data is “framed” in 32-clock sequences.
The Clock line can be any frequency up to a theoretical maximum of 25MHz, with any
reasonable duty cycle. The data frames described above may occur in sequence, one
after another (called “coherent frames”), or they may be sporadic (called “isolated
frames”). If they are sporadic, the Clock line may continue clocking in between data
frames, or it may be quiescent (preferably held in a low state). In all cases, whether the
clock is operational or not, the Frame Sync line must be held high when data is not sent.
The “frame rate” may be anything that the Clock and Frame Sync signals allow –
ranging from slower than one frame per hour up to a theoretical maximum of 781,250
frames per second. The data communicated via this serial port is absorbed by the
Mach-DSP at the rate that it is sent, and – unless Synchronous Sampling is selected (as
described below), there is no attempt to synchronize the Mach-DSP’s servo sample
clock (typically 100kHz to 300kHz) to the serial port signals.
You will note that 16-bits of data per axis are communicated using this protocol, and the
entire sequence requires 32 clocks. X-axis data is first with the MSB during clock
number 1. Y-axis data is next with MSB during clock number 17.
FB4 Position return data
The FB4 protocol uses one of the data lines to return the XY position that was read by
the Analog to Digital converters. And unless Synchronous Sampling is selected (as
described below), there may be a several sample delay between the instantaneous
position and the value of the position that is communicated by the return signal.
Synchronizing the servo sampling with data communication
It may be desirable to synchronize the servo sampling with the actual serial data
communication. This allows for consistent and fresh position data to be returned from
the servo, and prevents aliasing of the digital command and position signals that
otherwise occur when the servo sampling happens asynchronously to serial data.
Before synchronous sampling can be enabled, the
Digitizing sample rate
must be set
in accordance with the expected serial data frame rate, and the servo tuning must be
optimized for that particular sample rate. The
Digitizing sample rate
is found in the
Digitizer parameters
section of the
Pangolin only
tab. We would also recommend
testing bi-directional data communication, and making sure that everything is working
perfectly, before enabling synchronous sampling.
When you are ready, check the box
Synchronize sampling with Serial Input Frame
Sync.
When this is done, the Analog to Digital Converters on the Mach-DSP will start
their conversion cycle on the rising edge of Frame Sync, and serial communication
begins on the falling edge of Frame Sync.
