Functional Operation; Overview; Figure 4.1 Ppt Block Diagram - Honeywell PPT User Manual

4

FUNCTIONAL OPERATION

4.1

OVERVIEW

Honeywell's Precision Pressure Transducer (PPT) is based on a silicon piezoresistive sensor coupled
with a microprocessor and other electronic circuitry. The piezoresistive sensor contains both pressure
sensitive and temperature sensitive elements which, when modeled over a matrix of pressures and
temperatures at the factory, provides a digitally compensated pressure reading. The PPT has a
typical accuracy of 0.05% of full scale (FS) and the PPTR has a typical accuracy of 0.10% FS over a
-40 to +85°C temperature range. This accuracy is achieved by compensation algorithms in the
microprocessor. Because of the internal digital circuitry, having a digital output capability is straight
forward. The digital readings are transmitted via RS-485 signals or RS-232 signals—which are
commonly available on most personal computers. When connected to a computer, the user can modify
the pressure reading rate, integration times, units of pressure, thresholds for deadbands and more.
In addition, a temperature compensated analog output is available. Analog output parameters can be
modified by the user when the PPT digital communications port is connected to a computer, or host
processor. For example, the factory set range of 0 to 5 VDC for the analog output voltage can be
modified to 1 to 5 VDC operation or any voltage span within the 0 to 5 volt range in 1% increments.
Those who choose to use the factory set defaults may use the analog output without ever connecting
to a computer.
The analog output is driven by a 12-bit digital-to-analog converter (DAC). Because the PPT has an
internal EEPROM (Electrically Erasable Programmable Read Only Memory) to store configuration
settings, it can replace conventional analog transducers without connecting to a serial bus. Program
your settings once and use it as a three-wire device—Power, Analog Out, Ground.
Pressure
Input
Temperature
The PPT can be tailored to specific application requirements. Configuration information is sent to, or
read from, the PPT by user command messages over the serial port. Any configuration parameter
that can be changed by a command may be retrieved by use of that same command. Commands used
to change parameters must be preceded by an enabling command (WE or NE), but commands used
to retrieve information do not require an enable. All configuration changes are stored in the PPT
RAM for immediate use until power is removed. These configuration changes are only made perma-
nent in the EEPROM when the user executes the Write Enable (WE) command followed by the
EEPROM Store Parameters (SP) command. Exceptions to this are the A=, B=, C=, and D= com-
mands which are immediately stored to EEPROM if preceded by the WE command.
PPT
Pressure
Sensor
16 bit
Anaolg
to
Digital
Converter
Sensor
Figure 4.1—PPT Block Diagram
Voltage Regulator
EEPROM
Micro-
processor
RAM
1 3
DC
Power
COM
RS-232 or
(Digital
RS-485
Serial
Driver
Data)
0-5V
12 bit DAC
Output