HP 5501A Operating And Service Manual page 118

The heart of the pulse output card is a 28-bit comparator, which compares the 2 8 bits of back-
plane displacement information with the contents of a 28-bit up-down counter. Should the
displacement number increase, the comparator enables up counts into the up-down counter
until its contents are equal to the displacement. These up counts (or down counts for a nega-
tive displacement) each represent 0.1 micrometre (10 microinches in the case of English units).
These same count pulses are routed through a divide-by-N counter that allows the scaling
factor to be selected by the user, so that each output pulse can represent any even 10th micro-
metre from 0.1 to 25.6 micrometres (or 10 to 2560 microinches). A quadrature coding circuit
allows the user to select A-Quad-B output coding instead of the weighted up and down pulses.
The A-Quad-B output consists of two square waves in quadrature, with up/down information
supplied by their relative phase: wave A lags wave B for an upward displacement and vice
versa.
An algorithmic state machine (ASM) controls the internal functions of the card and generates
the proper sequence of backplane instructions: synchronous sample; counter card output;
multiplier card output; and return. If there are two, three, or more axes, the multiplier card is
shared by the several pulse output cards, and control of the backplane is transferred from one
pulse output card to the next in a regular sequence that is set during initial configuration.
A feature of the pulse output card is its ability to enter a preset number into the general-
purpose counter card. This is used to allow for environmental compensation of the deadpath
of the measurement axis. Deadpath is measured during machine installation and converted
to a code that is entered into switches on the pulse output card. During initialization, the con-
tents of these switches are preset into the up-down counter portion of the pulse output card.
At the proper time in the initialization sequence, the pulse output card sends a count up in-
struction to its assigned counter card. When the pulse output card counts down to zero, it
stops the counter, which now contains the deadpath, measured in quarter-wave fringe counts.
It is necessary to preset the pulse output card to a displacement count equivalent to the fringe
count in the counter. This is easily done by causing the counter to output to the multiplier,
and the multiplier to convert the fringe count to a compensated deadpath displacement and
output it to the pulse output card. The pulse output card clocks this displacement into its 28-
bit data register, and begins a special up count to equalize the up-down counter and the data
register. During this time no pulses are sent to the external controller. When the special up
count is complete the normal displacement data transfer begins. Should environmental con-
ditions change, the new compensation factor will operate on both the measured displacement
and the preset deadpath. This additional accuracy can be useful in some installations where
physical limitations make it impossible to reduce the deadpath to thedesired absolute minimum.
During each data transfer the pulse output card currently in control of the backplane looks at
the four error lines on the backplane. Should one of these be true, an error latch is set and an
LED lamp is lighted on the pulse output card to show which card has the error state and what
kind of error it is. A user option allows the system to ignore errors on axes not currently being
used, or to shut down the axis at the first error on any axis in use, or any combination. Each
pulse output card has an ERROR status line brought out to the front of the card; these may be
looked at individually for each axis or wire-ORed together. A backplane reset resets the error
latch.
The pulse output card also allows the user to select slow or fast maximum output pulse rate, and
normal or extended resolution (x6 or x10) for each axis individually.
Because each axis does a complete data input to pulse output cycle in approximately 11 micro-
seconds (33 ps for a three-axis system) the output pulses are a real-time measure of the dis-
placement. For additional information and schematic, refer to the 10763A English/Metric
Pulse Output Operating and Service Manual.