Philips PM2528 Operating Manual page 40

40
4.1.3,
4,2,
4.3,
4A.
ADC input control
'The ADC input control is an anajog multiplexer consisting of a number of analog switches. Using these
switches, one of the input signals of the ADC input contro! can be connected to the ADC. The switches are
controlled by the contral section.
ANALOG DIGITAL CONVERTER {ADC)
The conversion of analog signals into a digital code is based on the dual-slope integration principle.
The conversion consists of two states: the ramp-up state and the ramp-down state.
During the ramp-up state the analog input signal is supplied to an integrator for a definite time. This time is
determined by a fixed number of clock puises. During this state an integration capacitor is charged to a voltage
which is proportiona! to the input voltage.
During the ramp-down state the integration capacitor is discharged by a constant current, The discharging time
is proportional to the voltage on the integration capacitor, It is measured by counting the number of clock
pulses during this time.
The output code of the counter at the end of the ramp-down state is the value of the input signal.
The ADC control is responsible for the communication between the analog part of the ADC, the counter and the
control section.
CONTROL SECTION
In the controt section, both input and output data are processed,
Input data are derived from:
keyboard ~ function selection, ranging commands, start mode and start commands,
offset mode, high resolution mode or normal mode.
interfaces -~ 1EC-bus interface (control commands)
BCD parallei output (start command)
ADC — measured value
external start input — start commands
Output data are sent to:
keyboard — LED's in push-buttons
display — measuring result, unit indication
interfaces — 1€C-bus interface and BCD parallel output (measuring result, etc.)
analog output (measuring result)
analog section — relays, multiplexer.
ADC — start command etc.
The control section consists of a 8035 microcomputer, 4k EPROM and a number of I/O ports (analog control).
Input commands are recognised on an interrupt basis, The priority of the interrupts is software controlled.
When an interrupt is caused e.g. by pressing one of the front-panel push-buttons of the PM2528, the micro-
processor detects which command has been given and takes the desired action.
DISPLAY AND KEYBOARD SECTION
The display section consists of six 7-segment LED displays, 2 unit-indication part and the display control.
Data which have to be displayed are sent in a serial form from the microprocessor to the display control.
In the display control serial data are converted in bit-parallel data that is suitable for the display.
The keyboard section consists of 18 push-button switches and a keyboard control. Via the keyboard contro}
an activated push-button is detected.