10755A Compensation Interface Block Diagram - HP 5501A Operating And Service Manual

3.22 10755A Compensation Interface
Changing environmental conditions that result in a change in the index of refraction of air
cause a change in the velocity of light and therefore in the wavelength of laser light. Accurate
measurements require compensation for this effect.
The 10755A Compensation Interface Card (Figure 3-79) places the required compensation factor
on the coupler backplane when commanded by the system controller. Most coupler-based
systems will include one compensation interface card.
5510A OPT 0101
I
b - %
1
Set Thumbwheels
COU",P,
Front Edge of Card
Backplane
Figure
3-79. 70755A
Compensation lnterface
Block
Diagram
The compensation interface gets the compensation factor either from the 5510A Opt 010 Auto-
matic Compensator, a separate unit, or from the 10756A Manual Compensation module, which
plugs into the coupler. The automatic compensator measures the temperature, pressure, and
humidity of air and computes the index of refraction of air, n, and the compensation factor, l/n,
which is between 1.0000000 and 0.9990000 for all earth-type atmospheres. Because only the four:
least-significant digits change, the compensator presents only these digits to the compensation
interface card in binary-coded-decimal format on 16 parallel lines. The compensation interface
converts the data to a form usable by the laser transducer system and outputs
it
to the coupler
backplane when commanded to do so.
In typical installation (workshops, laboratories, machine shops, etc.) the compensation factor
can change over a period a s short as a few minutes. The 5510A Opt 010 Automatic Compensator
can compute a new number a s often as twice each second. Where the environment i s closely
controlled, such a s in metrology labs, the compensation factor changes very little during a day,
and the data can be entered instead from a 10756A Manual Compensation module. This module
presents the compensation factor, computed from a formula or derived from tables in a hand-
book, to the compensation interface in BCD bit-parallel, digit-parallel format identical to that
of the 5510A Opt 010. A small cutout in the front panel of thecoupler allowsaccessto the thumb-
wheel switches that set the number, and an update pushbutton tells the compensation interface
that a new number has been entered and should now be converted into the proper format for
the backplane.