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It also has a 1 second Slow and a.125 second Fast time constant circuit. Present OSHA requirements
specify Slow.
Microcomputer Modes
The M-27 is controlled by a MC146805 CMOS Microcomputer. The system has 8192 bytes of
RAM, 6144 bytes of ROM, a 33-segment LCD, 24 programming switches, serial and parallel printer
outputs, a V/F converter for continuous dose integration, and power down circuits to retain data when off.
The M-27 operates in one of the following 6 modes.
Reset Mode
The M-27 enters the reset mode when the unit is reset or a battery is installed. In this mode power
is applied to the unit, all previous data is cleared, the internal switches are read, the counter is cleared, the
pause register is set, three dashes are placed on the display, and the unit enters the wait mode.
Wait Mode
The microcomputer waits for an interrupt to occur. During the wait mode it continues to gather
data from the voltage to frequency converter. Otherwise the microcomputer is in a low power state. When
the 1/16 second interrupt occurs the unit enters the pause or run mode.
Pause Mode
In this mode, the Slow or Fast time constant sound level is computed and pause time and the
elapsed or real time clocks are incremented. At every interrupt the keyboard is checked to see if any key is
pressed - unless it is doing a previous calculation such as sound level when it may delay for up to 1 second.
If a key is pressed the computer enters the compute mode. Otherwise it returns to the wait mode.
Run Mode
The same as the pause mode except the data necessary to compute Lavg, Leq, Dose, TWA, SEL,
Peak, Max, Run Time, Upper Limit, as well as Event data is accumulated.
Compute Mode
If a key has been pressed in the pause or run modes, the M-27 will compute and display the
desired function. If the key is held down it will recompute it one second later using the data accumulated
during that second. If the data will not compute (as an example, Lavg will not compute if the sound level
never exceeds the threshold level) the display will either show three dashes or display LO.
If the print key has been pressed, the processor will compute one line of print and send the data to the
printer output. The printer output may be terminated by pressing the ON/OFF key until 0-5 is displayed.
Pressing PAUSE will cause the printer to stop. Press PAUSE to continue. When PSE is displayed, the unit
draws extra current and should not be left in this state. ON/OFF will exit PSE. The computer continues to
accumulate data during a printout. For best results the computer should be placed in the pause mode during
a printout.
When computing is completed, the processor returns to the wait mode.
Stop Mode
If the ON/OFF key has been depressed for 5 seconds, the M-27 enters the stop mode. In the stop
mode the processor stops accumulating, but does not erase, data. Power is removed from the analog
circuitry and the display circuitry. The oscillator is stopped. Only a few microamps of current are drawn
from the battery to maintain the microcomputer's memory and to power the ON/OFF key.
When the ON/OFF key is then pressed the computer returns to where it was before it was turned
off except it will be in the pause mode.