Bus Operation Of Several We2108; Connection To The Rs485 Bus - HBM WE2108 Serie Referenzhandbuch

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9
The following commands are required only if several WE2108 (RS485 version) should
be controlled by one PC or PLC through the same line. The WE2108 with RS232 inter-
face is designed for a point−to−point connection to one computer. Therefore no com-
mands are required for addressing.
A prerequisite for bus operation is that all units have the same baud rate but different
addresses. The other settings such as filters, password access etc. are possible indepen-
dently of one another in all units.
The outputs of all WE2108 are connected in parallel to the transmission line to the control
computer. Therefore only one WE2108 may always transmit. The bus operation of the
WE2108 is designed as master−slave configuration, whereby the WE2108 implements a
slave. Thus all activities of the WE2108 are initiated by the master (control computer, e.g. PC
or SPS). The WE2108 remains inactive on its transmission line without request by the mas-
ter. A unit is addressed by software by the Select command Sxx. The unit the address of
which was last output with Sxx is always active on the bus.
A "broadcast command" (S98) is implemented for certain cases of communication. This
means that after such a command, all WE2108 execute the command, but no WE2108
answers. Examples of the use of these commands are described in Section 9.5.
9.1
All WE2108 (RS485 version) are connected in parallel, but the transmit and receive lines
must be changed over before the control computer (see Fig.). A correct termination of the
line impedance is important to avoid faults on the bus. For this purpose the WE2108 (RS485
version) has integrated terminating resistors (500 ohms) which can be activated by software
(command STR). The terminating resistors must be activated at the ends of the bus line, as
a rule therefore in the last WE2108 and in the bus converter of the control computer. They
must be switched off in the other units, since otherwise the bus drivers are overloaded.
HBM

Bus operation of several WE2108

Connection to the RS485 bus

Scale electronic unit (NSW)
WE2108