LS SERIES SERVICE MANUAL
ISSUE 4.0 LOAD MEASURING SYSTEM 4
Note that a RAM DUMP will not erase the calibration data from the loadcell EEPROM.
4.7 Analogue to Digital Converter
See diagram 04/1455 sheet 2 in section 17
The loadcell preamplifier output is fed to a low drift unity gain buffer IC11 on the Main PCB.
The buffered load output is fed to the multiplexer IC3, which may also be fed with input
signals from one or more analogue external extensometers. The Load and Extension signals
are sequentially selected by the multiplexer by the ANSW0, ANSW1, ANSW2 and ANSW3
control signals, which are produced by the Programmable Logic Device IC26.
The multiplexer output is buffered by IC5 then converted into 16 bit data by the ADC IC6 and
the data is fed to the microprocessor via the Programmable Logic Device IC26.
The ADC has an accurate internal +2V5 reference voltage available on pin 5 and this voltage
is used to produce the +4V excitation voltage for the loadcell bridge. The +2V5 is reduced to
+2V by R35 & R29 then multiplied by 2 by IC29 & IC7. The ADC circuit contains NO presets
and no electrical calibration is required.
4.8 Overload Detector
The load output from pin 6 of IC11 is normally in the range of + / - 8V. If the loadcell is
overloaded in tension mode, then the voltage will increase above +8V. If the loadcell is
overloaded in compression mode, then the voltage will fall below -8V.
If the load output from pin 6 is positive, this voltage is fed to the overload comparator IC32A
via one diode in D1 (dual common anode diode). If the load output from pin 6 is negative,
this voltage is inverted by the unity gain, inverting amplifier (IC32B) then fed to the overload
comparator IC32A via the second diode in D1. The inverting input of IC32A pin 2 is held at
+4V so the output on pin 1 is -15V when the input to IC32A pin 3 is below +4V.
The voltage at the junction of the 2 diodes in D2 (dual common anode diode) is held at
+200mV so the "Overload0" signal fed to pin 118 of IC26 (ISP) is held low. This is the normal
condition.
If the loadcell is overloaded in tension mode, the overload detector will switch when the input
to IC32A pin 3 reaches +4V, i.e. the +ve output from IC11 pin 6 reaches approx. +10V2
(allowing 200mV drop across one diode of D1). This is 127% of the loadcell capacity.
If the loadcell is overloaded in compression mode, the overload detector will switch when the
input to IC32A pin 3 of IC32A reaches +4V, i.e. the -ve output from IC11 pin 6 reaches
approx. -10V2 (allowing 200mV drop across one diode of D1). This is 127% of the loadcell
capacity.
When an overload condition occurs, pin 118 of IC26 (ISP) is allowed to rise and the
microprocessor will stop the machine with a "Loadcell Overload" error message.
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