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Theory of Operation M703E Calibrator Operator’s Manual
A variety of sensors report the physical and operational status of the calibrator’s major components, again
through the signal processing capabilities of the motherboard. These status reports are used as data for the
concentration calculations and as trigger events for certain control commands issued by the CPU. They are
stored in memory by the CPU and in most cases can be viewed but the user via the front panel display.
The CPU communicates with the user and the outside world in a variety of manners:
Through the calibrator’s keyboard and vacuum florescent display over a clocked, digital, serial I/O bus
(using a protocol called I2C);
RS 232 & RS485 serial I/O channels;
Via an optional Ethernet communications card:
Various digital and analog outputs, and
A set of digital control input channels.
Finally, the CPU issues commands via a series of relays and switches (also over the I
2
C bus) located on a
separate printed circuit assembly to control the function of key electromechanical devices such as heaters,
motors and valves.
9.2.2. CPU
The CPU is a low power (5 VDC, 0.8A max), high performance, 386-based microcomputer running a version of
the DOS operating system. Its operation and assembly conform to the PC-104 specification, version 2.3 for
embedded PC and PC/AT applications. It has 2 MB of DRAM memory on board and operates at 40 MHz clock
rate over an internal, 32-bit data and address bus. Chip to chip data handling is performed by two 4-channel,
direct memory access (DMA) devices over data busses of either 8-bit or 16-bit bandwidth. The CPU supports
both RS-232 and RS-485 serial protocols. Figure 9-3 shows the CPU board.
The CP
U communicates with the user and the outside world in a variety of ways:
Through the calibrator’s keyboard and vacuum fluorescence display over a clocked, digital, serial I/O bus
using the I
2
C protocol (read I-square-C bus)
RS-232 and/or RS-485 serial ports (one of which can be connected to an Ethernet converter)
Various analog voltage and current outputs
Several digital I/O channels
Figure 9-3: M703E CPU Board Annotated
138 05744 Rev B