Sensoray 518 - Manual (Page 5)

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INTRODUCTION
Functional Description
The 518 interfaces eight process sensors directly to the PC bus. Each of the eight
sensor channels may be independently configured (via software) to accept
thermocouples, RTD’s, strain gages, thermistors, resistors, 4 to 20 milliamp current
loops, or DC voltage inputs. Each sensor channel makes use of differential inputs to
allow for wide common mode variation.
Pulsed sensor excitation is provided for thermistors, resistors, RTD’s, and strain
gages. Pulsed excitation minimizes sensor self-heating effects and helps to reduce
board power consumption. Excitation signals are electrically separated from
channel differential inputs so that three- or four-wire sensor connections may be
implemented.
Cold junction compensation is provided for thermocouples, and all inputs are
protected from high differential and common mode voltages.
The 518’s onboard microprocessor continuously scans the eight channels. As each
channel is scanned, the sensor signal is digitized, linearized, and converted to
engineering units appropriate for the sensor type. The most recent sensor data from
each channel is immediately accessible to the host processor.
Alarm limits are individually programmable (via software) for each channel. A
status flag is set when any channel limit violation occurs, eliminating the need for
the host processor to poll sensor data for limit violation detection.
Communication with the host processor takes place over two contiguous I/O ports
which may be mapped anywhere in the system I/O address space. Communication
and scanning tasks execute in an interrupt-driven multitasking local environment to
minimize communication latency while maximizing channel scan throughput.