Commands For Bus Operation - HBM WE2108 Serie Referenzhandbuch

Scale electronic unit (NSW)
Fig. 7.1.4: Connection of several WE2108 to a RS485 bus
The HBM interface converter SC232/422A can be used for the PC−side conversion of
RS232 (COM interface) to RS485.
Since the RS−485 is a differential bus interface, the levels are also stated as differential volt-
age between the lines (and not ground−related). The following applies as idle level on the
RS−485 master line:
•
•
The maximum common mode range (raising both lines by the same potential) is "7 V. Com-
mon mode signals arise if the reference grounds of the connected units have different poten-
tial, e.g. due to large currents in the supply cable. If required, the potential equalization
between the bus subscribers should be produced through a separate ground wire (not the
cable shield). Shielded cables must be used for EMC reasons for the bus lines (in the HBM
product range, e.g. 6 x 0.14 m@).
9.2
•
•
•
WE2108
Ta
Tb
Ra
Rb
Control computer
TB − TA > 0.35 V (given by the terminating resistors at the bus end)
RB − RA > 0.35 V (given by the master terminating resistors)

Commands for bus operation

STR
ADR
Sxx
BUS RS 485
STR = 0
WE2108
(Switch terminating resistors)
Unit address
Set up communication to a unit (general command description)
500 Ohm
GND
+5V
STR = 1
WE2108
115
GND
+5V
HBM