HP 5501A Operating And Service Manual page 112

The line receivers accept two differential R F signals, one from the laser source (reference
signal), and one from the remote receiver (measurement signal), and convert them to TTL
signals. The input signals are then amplified by the line drivers for use by other cards in the
system. For example, the reference signal must be sent to each counter card and must be
daisy-chained because the line impedance matching requirement and available line driving
power do not allow parallel connection. The measurement signal is also reproduced, and may
be used for other signal processing, such as differential measurement using one measure-
ment signal at the reference input.
The resolution extension circuit allows a user to select normal resolution of 0.16 micrometre
(6 microinch) or extended resolution of 0.016 micrometre (0.6 microinch) under program con-
trol. The only drawback of the resolution extender is the 1.2-inch-per-second slew rate limita-
tion imposed when resolution extension is in use. This is more than compensated for by the
real time extenstion in resolution and resulting increase in positioning accuracy.
After the measurement signal has been resolution extended (or not, depending upon program
control), the measurement and reference signals are converted to displacement information
in the form of up/down pulses, which are fed to a counter. This is done by the pulse converter
circuit.
The next functional block on the counter card is a 28-bit binary counter that counts the up/
down pulses from the converter to give total displacement information. The delay between a
position change on the measurement axis and a change in the counter i s on the order of 0.4
microseconds depending on the length of cable between the receiver and the counter card.
In addition to these signal processing blocks, the counter card also has several control and
error functions. The error circuits constantly monitor the status of the counter and the con-
ditions of the reference and measurement signals at the input to the pulse converter. If the
reference signal i s interrupted for more than 6 microseconds a reference error bit is set. If the
measurement signal i s interrupted or becomes invalid because of excessive slew rate, a mea-
surement error bit is set. The error circuits monitor the frequencies of the two signals and set
the respective error bit if prescribed limits are exceeded.
The counter status error bit is set whenever the counter chain overflows or underflows. This
error is recoverable, that is, it can be corrected by moving the retroreflector in the direction
opposite to that which caused the overflow or underflow until the condition is reversed. Thus
no counter information is lost. This i s not the case for reference or measurement signal errors.
Should one of these occur, the measurement axis must be returned to some gage point to re-
define the zero point.
The control circuits interpret system instructions and cause the counter to perform the required
operation. Test functions are included on the counter card to allow the system controller to
check the operation of the counter chain and error bits, the output buffer, and the pulse con-
verter clock. For additional information and schematics, refer to the 10760A Counter Oper-
ating and Service Manual.
3 . 1 7 1076214
Comparator
In a comparator system, the 10762A Comparator cards, one for each axis, replace the general-
purpose counter cards. A compensation interface card, a binary I/O card, and a fast pulse
converter complete the electronics package in the 10740A Coupler. Also required for each
axis are an external digital-to-analog (D/A) converter (not supplied with the system).