User's Manual

PMAC User Manual
Setting Up a Motor 71
Sixth Acc-14 Port B (J15): $FFF9
A typical setup word for this type of feedback is $20FFD0, which provides for non-filtered conversion of
the parallel data word fed into Port A of the first Acc-14 connected to PMAC.
Bit-Enable Mask Word
Parallel-feedback conversion requires a double (for non-filtered) or triple (for filtered) entry in the
conversion table. The second entry – filtered or non-filtered – specifies the size of the feedback word
used. The entry is a 24-bit word in which each bit actually used for the parallel feedback is a one; the
unused bits above are zeros (parallel feedback is typically connected starting at bit 0 of the data word).
For a 12-bit absolute encoder, this entry would be $000FFF; for 14 bits, it would be $003FFF. For the
standard shifted conversion, the maximum effective mask word is $07FFFF, unmasking the low 19 bits;
any data in the high five bits is shifted out of the word. For the unshifted conversion, up to 24 bits of real
data can be used with a mask word of $FFFFFF. The mask word allows PMAC to detect rollover in the
source data, so it can extend the count properly in software.
Filter Word
If the conversion format is $3x or $7x, the parallel data word is filtered. The filter simply sets a
maximum amount the data word is permitted to change in a single servo cycle. If PMAC sees a change
larger than this in the source data word, the converted data only changes by the maximum amount. There
is no permanent loss of position information if the filter kicks in.
Purpose of Filtering
This filtering permits protection against spurious changes on high-order data lines, while not delaying
legitimate changes at all. This maximum amount is the third setup entry for the encoder in the Y-column
of the conversion table. It should be set slightly greater than the maximum actual velocity expected on
the sensor, expressed in counts (bits) per servo cycle.
Converted Data
The converted data from the parallel word is put in the X data word matching the last (second or third)
setup word for the entry. This is the address that should be used by the motor I-variable that picks up
position (Ix03, Ix04, or Ix05). For instance, if the first setup entry (address Y:$0720) in the conversion
table were $30FFD0 (filtered parallel data), the size entry would be in Y:$0721, and the maximum change
entry would be in Y:$0722. The converted data would be placed in X:$0722. If this were the position
feedback for motor #1, Ix03 would be set to $0722 (1826 decimal). For incremental parallel feedback, bit
16 of Ix03 should be set to 1 for proper homing search moves.
Unshifted Conversion
If bit 19 of the source and process word for a parallel data conversion is set to 1, the converted data
contains no fractional bits. Entries of this form would have the conversion formats (bits 16-23 of this
word) $28, $38, $68, or $78, as opposed to the standard entries $20, $30, $60, and $70, which provide
five fractional bits in the converted data.