HBM PAD4002A - Page 30

Interfaces
28 A4243-1.0 HBM: public PAD4002A
line can be as much as 500m. The software uses the
different addresses to differentiate between the PADs. If
the control computer only has one RS-232 interface, an
interface converter is required (e.g. from HBM, ordering
no.: 1-SC232/422B).
Bus termination
Bus termination
TB
TA
RB
RA
PC = Master Node 1 Node 90
...
500 Ω
500 Ω
500 Ω
500 Ω
500 Ω
500 Ω
500 Ω
500 Ω
+5 V
+5 V
+5 V
+5 V
TxD
on/off
RxD
TB TA
T
R
RB RA
TxD
on/off
RxD
TB TA
T
R
RB RA
TxD
on/off
RxD
TB TA
T
R
RB RA
Fig. 7.1 Connecting several PADs to a PC via an RS‐485
4‐wire bus
The correct assignment of the transmit and receive lines
can be seen in Fig. 7.1 (bus line Ra to Ta of the con
verter, etc.). The electronics already include bus termina
tion resistors (line termination), that can be activated with
the software command STR. So no additional bus termin
ation resistors are required for RS-485.