The measurement card for the indoor or outdoor thermometer* may be provided with the additional LED indicator shown in the diagram to check the operation of the software.
Every fifteen seconds, a pulse is placed via the background program TLOGGER on to line PBO, which is taken outside via connector K6. The level on this line is, of course. not switched irrespectively, but only at the instant that TLOGGER checks whether a temperature measurement is being carried cut. If that is so, the level on the line remains high for about (depending on the speed of the computer). When no measurement is being carried out, this time is appreciably shorter: of the order of 80 ms. The line is also used by the software to signal an error condition outwards.
If, for instance, TTRANS.CFG or TLOGGER.CFG is not found, or the path is not correct. or the disc is full, pulses at a frequency of 10 Hz are placed on it.
The diagram shows how the LED is connected to Ks via a short length of a flat cable. Power is supplied by the computer.
* Elektor Electronics. March 1991