Theory of Operation
Publication 21555, Rev. E, September 2002
3-7
3.4.6
Correction Thermistor Circuit
The compensation thermistor is mounted near CR13 to sense the temperature of the 20 dB attenuator
section that produces the 0 dBm output. This is the only absolute power specified. All other power
levels are measured by the software relative to 0 dBm.
3.4.7
Calibrator NVRAM Control Circuit
The calibrator serial number and the correction constant for the 0 dBm output level, as well as the date
of calibration and password for rewrite access, is contained in a Non-Volatile RAM. The read and write
for it is provided by the parallel peripheral interface (PPI) U26. Before allowing access to the NVRAM,
the software looks for a logic 1 on port A, bit zero of the PPI and, if that is present, it asks the operator
for the password. If the correct password is supplied, the collected data will be written into U18. If the
jumper W1 is set to supply a logic 0 to the PPI, the operator will have write-access to U18 without
needing a password.
3.4.8
Sensor NVRAM
Each sensor has a NVRAM to store all of the calibration constants, the date of calibration, place of
calibration, etc. This NVRAM is also password protected but has no hardware switch to defeat it. The
read/write control for it is furnished by U24A/B/C, U24D/E/F, Q2 and Q3. Q1 and Q8 control the 5 V
supply to reduce the amount of heat in the sensor, as well as reducing the noise from the supply.
3.4.9
Sensor Interrupt
Each time a sensor is connected or disconnected from the 58542, a CPU interrupt is generated by
causing the thermistor voltage change to set a latch, which signals the CPU that it needs to check for a
sensor change. The latch is driven from a “window” comparator, U35D and U36A & B. This
comparator is driven from capacitors which are connected to each of the thermistor lines from the
sensors. The latch is enabled or cleared by a signal from the PPI, U26.
3.4.10
Digital Control Circuit
The digital control circuit is the interface between the CPU and the preceding functions.
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