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M400E Ozone Analyzer Operator’s Manual 357BRemote Operation Of The M400E
8.5. USING THE M400E WITH A HESSEN PROTOCOL NETWORK
8.5.1. GENERAL OVERVIEW OF HESSEN PROTOCOL
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.
The following subs 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 Instruments
web site: http://www.teledyne-api.com/manuals/index.asp .
8.5.2. HESSEN COMM PORT CONFIGURATION
Hessen protocol requires the communication parameters of the M400E’s COMM ports to be set differently than
the standard configuration as shown in the table below.
Table 8-6: 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 M400E’s COMM ports, See Section 8.1.3.
To change the rest of the COMM port parameters. See Section 8.1
Note
Make sure that the communication parameters of the host computer are also properly set.
Also, 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 frequently.
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