The position of each axis is measured by an optical disc encoder which is attached to a shaft carrying the worm gear, drive motor and flywheel. The flywheels stop lockup in the gears. The encoder gives out a train of pulses which are counted (up or down) by a Hytec CAMAC module. There is one module for each axis.
Every tenth of a second, a strobe pulse comes from the time service and goes into the Hytec modules. This strobe causes a fresh copy of where the telescope is positioned to be copied into a register which in turn is readable by the computer.
This same strobe pulse goes into the computer which is interrupted from what it is doing and goes to look at the encoder position. The encoder position is compared with where the computer calculates that the telescope ought to be and it then applies a correction, as necessary, to the demanded drive voltages on the servo amplifiers. This procedure involves a CAMAC module which does a digital to analogue conversion. (DAC module). The voltage from the DAC is applied to the preamplifiers which in turn drive the main "Inland" power amplifiers.
The program that does the job above also drives the red Beckmann displays on the console which display the current Right Ascension, Declination, Zenith distance, Sidereal Time and Universal Time. Note that the Universal Time is effectively GMT whilst the Sidereal Time is local and has been corrected for the longitude of La Palma. The value used in the software is 1h 11m 30.3s West. The time is corrected at regular intervals by radio time signals and the clock is normally kept to UTC within an accuracy of a few millisecs.
The Beckmann displays do not work unless the control computer programs are fully running. This means that they will read random garbage until startup time. There are circles on the telescope to help you get within a fraction of a degree if necessary while in engineering mode. There is a diagnostic display unit on top of the control console. The two right hand displays therein give the current tracking errors . Ignore the most significant digit. The upper display is for the RA, the lower for the DEC. The units are 0.05 arcsec (colloquially called "Baldwins").