Sensoray 518 - Manual (Page 52)

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Command Processor
The command processor fetches and executes commands from the host processor. Top priority is
given to this task in order to minimize communication latency between the host and coprocessor.
This task is awakened when a command is received from the host. The command processor
decodes the opcode embedded in the first byte of the command string, and control is passed to
the appropriate command processing procedure.
Hardware
Although the coprocessor analog circuitry is proprietary to Sensoray, an overview of the circuitry
is provided here to give you some insight into the inner-workings of the board.
Coprocessor analog circuitry is partitioned into three sections: reference, measurement and
excitation. The reference section contains standards that are used to enhance the accuracy of all
sensor measurements. The measurement section includes analog switches, various amplifiers and
the A/D converter. The excitation section is responsible for stimulating resistive sensors.
Reference Section
The reference section consists of a set of reference standards that are used to enhance the
stability and accuracy of all sensor measurements. These standards are very stable over time and
temperature. The exact values of the reference standards are stored in EEPROM on the
coprocessor board when the board is calibrated.
Reference standards are measured periodically by the onboard CPU. These measurements are
interleaved with external sensor measurements. One reference standard is measured per
approximately every twenty external sensor measurements. Each digitized reference standard
value is stored in onboard RAM and used to null offset and gain errors in the external sensor
measurements.
A phenomenon known as warm-up noise occurs when the 518 is warming up from cold start or
when subjected to a thermal transient. When the coprocessor is exposed to changing ambient
temperatures, sensor data will appear somewhat noisy. This is caused by circuit gain and offset
characteristics changing faster than reference standards are measured. Warm-up noise will
subside when the 518 reaches thermal stability.
Measurement Section
The measurement section is responsible for selecting and digitizing the external sensor input
signals, as well as the internal reference standards.
The measurement section starts a digitization by selecting the channel to be measured. The
selected channel is switched through an eight-channel differential analog multiplexer. Reference