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7 Functional description
The functioning of the ULTRASONIC instrument is described below. Due to the limited number of
plug connections some functions exclude the simultaneous operation with other functions. Such
dependency is described in each case. For example, in half duplex mode independent telegram
output is not permissible. There are also restrictions regarding the functional definition of the cable
connector. This is due to the double assignment of individual PINs.
7.1 Serial communication
The ULTRASONIC provides an RS485 / RS422 interface for serial communication. It can be
operated either in full or half duplex mode and at different baud rates.
A standard terminal program, for ex., can be used for communication with the ULTRASONIC. With
a Windows-based operating system Hyper Terminal is included in the scope of supply. It has to be
installed subsequently if required.
The manufacturer, generally, is not aware of a possible use of the Ultrasonic Anemometer in a bus
formation. Therefore, the instrument has a wave terminating resistor, which can be connected by
the software, for half-duplex-operation. See command: BT (bus termination).
When starting the ULTRASONIC, the communications parameters are output to the serial
interface. Output takes place at 9600.8N1. The baud rate, the duplex modus and the ID are output:
Example:
THIES ULTRASONIC
!00BR00005
!00DM00001
The ULTRASONIC starts with ID 0, with a baud rate of 9600.8N1 and full duplex mode.
7.1.1 Duplex mode
Duplex mode decides the type of physical connection of the serial interface. In full duplex mode the
send and receive signals are each transmitted via separate pairs of cables. This means it is
possible to send and receive signals at the same time.
In half duplex mode transmission of the send and receive signals is via the same pair of cables in
the time division (successively) (: see Command DM.
For a bus operation in the half-duplex-mode (RS485), where the ULTRASONIC, in general, is
operated as “slave”, it is necessary to switch the line-transmitter into the “high–impedance-state”
during the intermission, so that the replies of the other bus parties are not suppressed.
It might be important with point-to-point-connections in the full-duplex-mode (RS422), depending
on the disturb-ratio on the communication lines, that the line-transmitter remains active during the
intermissions. So, a maximum differential input level leads to a maximum signal/noise ratio.
A half-duplex-mode can be selected via the command DM (duplex mode). With this mode, on
principle, the line-transmitter is switched on only when sending. For the full-duplex-operation there
are two modes: one for bus operation (RS485), where the line-transmitter is controlled as in half-
duplex mode, and another one (RS422), where the line-transmitter remains active even in case of
reception. See command DM.
For the ULTRASONIC there are restrictions on the parameter combination or function of the
terminals depending on the transmission type selected. Due to the limited number of plug
connector contacts multiple assignment of the connections are necessary. The following table
shows the functional options for the modes full and half duplex.