Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 241
A triggered time-base entry in Turbo PMAC automatically computes the 1/T count extension of the input
frequency itself before the differentiation. It computes this to 1/256 of a count. This is compared to the
1/32 of a count that the separate 1/T encoder extension uses.
The extra fractional information can reduce the quantization noise created by the differentiation and
provide smoother operation under external time base.
Note:
The intermediate result in the first line of a triggered time-base entry contains the
undifferentiated 1/T extension of the source encoder position, in units of 1/256 of a
count. This value can be used as feedback data or master position data, with more
resolution than the standard 1/T extension.
In use, the method digit (comprising bits 20-23 of the first line) is changed as needed by setting of the I-
variable. Triggered time base has three states, frozen, armed, and running, all of which must be used to
utilize the triggering feature.
First, the method digit is set to $9 (e.g. I8010=$978008) before the calculations of the triggered move are
started, to freeze the time base (and therefore the motion) while the move calculations are done. This is
typically done in the user’s motion program. When this entry is in the frozen state, the table reads the
channel’s capture position register each servo cycle to ensure the triggering logic is reset for the next
capture. The final result of the entry is always 0 when frozen.
Note:
In a Turbo PMAC application with a light computational load, it is possible that
the entry will not be in the frozen state during a servo interrupt, and the table will
not get a chance to reset the trigger logic. Therefore, it is advisable to reset the
triggering logic explicitly in the user program with a dummy read of the channel’s
captured position register, which is the X-register with an address 3 greater than
the address specified in the entry (e.g. X:$07800B if the entry specifies $078008).
The suggested M-variable for the captured position register is Mxx03.
Next, the method digit is set to $B (e.g. I8010=$B78008) after the calculations of the triggered move are
finished, to arm the time base for the trigger. Typically, this is done in a PLC program that looks to see if
the entry is frozen and changes it to the armed state. The final result of the entry is always 0 when armed.
In the armed state, the Table checks every servo cycle for the channel’s trigger bit to be set. When the
Table sees the trigger (the capture trigger for the machine interface channel as defined by I7mn2 and
I7mn3 for Servo IC m Channel n, or by I68n2 and I68n3 for MACRO IC 0 Channel n), it sets the method
digit to $A for running time base automatically. It uses the position captured by the trigger as the starting
position (time zero) for the running time base. (Those using this method for the reduced quantization
noise may simply leave the method digit at $A.)
The following tables show the possible first-line entries for triggered time base (running mode):