Teledyne API – Model T300/T300M CO Analyzer
Communications Setup and Operation
147
6.7.2.
HESSEN
The Hessen protocol is a multidrop protocol, in which several remote instruments are
connected via a common communications channel to a host computer. The remote
instruments are regarded as slaves of the host computer. The remote instruments are
unaware that they are connected to a multidrop bus and never initiate Hessen protocol
messages. They only respond to commands from the host computer and only when they
receive a command containing their own unique ID number.
The Hessen protocol is designed to accomplish two things: to obtain the status of remote
instruments, including the concentrations of all the gases measured; and to place remote
instruments into zero or span calibration or measure mode. API’s implementation
supports both of these principal features.
The Hessen protocol is not well defined, therefore while API’s application is completely
compatible with the protocol itself, it may be different from implementations by other
companies.
Note
The following sections describe the basics for setting up your instrument
to operate over a Hessen Protocol network.
For more detailed information as well as a list of host computer
commands and examples of command and response message syntax,
download the Manual Addendum for Hessen Protocol from the Teledyne
API web site: http://www.teledyne-api.com/manuals/.
6.7.2.1.
HESSEN COMM PORT CONFIGURATION
Hessen protocol requires the communication parameters of the T300/T300M Analyzer’s
COMM ports to be set differently than the standard configuration as shown in Table 6-4.
Table 6-4:
RS-232 Communication Parameters for Hessen Protocol
PARAMETER
STANDARD
HESSEN
Baud Rate
300 – 19200
1200
Data Bits
8
7
Stop Bits
1
2
Parity
None
Even
Duplex
Full
Half
To change the baud rate of the T300/T300M’s COMM ports, see Section 6.2.2.
To change the rest of the COMM port parameters listed in Table 6-4, see Section 6.2 and
Table 6-1.
I
MPORTANT
I
MPACT ON
R
EADINGS OR
D
ATA
Ensure that the communication parameters of the host computer are also
properly set.
Note
The instrument software has a 200 ms latency before it responds to
commands issued by the host computer. This latency should present no
problems, but you should be aware of it and not issue commands to the
instrument too quickly.
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