IM 12B6C2-E-H
2-2. Functional description
The EXA PH200 is a real time micro-control-
ler operated pH analysing system. It uses a
dedicated micro-controller with exclusively
developed software to control all functions
necessary in such a system. The input and
output functions are concentrated in the
analogue section of the instrument. Even
these functions are operated through special
interfaces designed to give a minimum of
interference problems to the digital func-
tions. The digital and logic functions are
designed to operate securely. The unique
FAIL-function can give a warning when the
micro-controller has found a fault in the
measuring loop. This is accomplished by
sending a discrete 22 mA signal on the out-
put. The micro-controller checks the working
of the analogue to digital input converter cir-
cuit and watches over the conversion to the
output signals. By using non-volatile memory
(EEPROM) for the essential in formation the
operating parameters are fixed in memory
without the need for a battery. The software
is designed with the user in mind. It uses a
simple step by step, question and answer
style to communicate with the operator by
giving messages on the second line of the
display and indicating which keys are to be
pressed in the display too.
The user-interface is limited to a basic set of
6 keys accessible through the flexible win-
dow cover. The keys are scanned conti-
nuously and actions are taken immediately.
An extensive system of checking va lues and
parameters is implemented in the software .
The 2-wire design gives maximum flexibility
of operation with other industrial instruments
and controlling systems. An intrinsically safe
version is available for hazardous areas. The
rugged housing is designed for outdoor use
(IP65). In addition it is shaped in a way that
indoor panel mounting is also possible. The
optional universal mounting kit includes
brackets for all mounting methods. The
commissioning of the instrument can be
performed on site or in advance. A three
level operating system keeps the approach
clear and distinguishes between every day
operation and maintenance against one time
operations like commissioning or fine-tuning.
All this makes the EXA PH200 operate like a
normal analogue transmitter with a number
of additional functions.
These extra functions are only possible by
using a micro-controller at the heart of the
system:
• Sensor checking durig measurement (1)
• Simplified calibration through a semi-
automatic sequence.
• Additional process temperature compen-
sation can be activated.
• Simple range adaptation making it a ver-
satile instrument.
• Wide selection of temperature sensors.
• A HOLD-function for the output.
• Three buffer tables can be programmed.
• Programmable isothermal point of inter-
section.
• Protection against unauthorized access
by a passcode.
(1) European patent No. 0241601
US patent No. 4777444
3
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