Manual

6. INPUT CIRCUITS
6.1 THE SIGNAL INPUT
The basic circuit of the input is shown below. Both the current and voltage
signals are fed to a data selector but only one signal is processed, depending
upon whether a current (4-20mA or 0-20mA) or a voltage (1-5 V or 0-10 V)
input configuration is selected. The signal is fed to a voltage to frequency
convertor and transmitted to the microprocessor via an opto-coupler.
The microprocessor uses a crystal reference to provide an accurate measurement
of the incoming frequency. Once every 10 minutes a stable and accurate internal
reference is sampled and used to compensate the input. This technique ensures a
highly accurate measurement and makes periodic calibration unnecessary.
Input Circuits 39
Voltage to
Frequency
Microprocessor
Regulator
DC to DC
Convertor
Reference
10K
250
Ohm
Isolation
9
14
8
Voltage
Current