HBM PAD4001A Operating Manual page 26

Interfaces
Bus termination
+5 V
500 Ω
500 Ω
+5 V
500 Ω
500 Ω
TB
T
TxD on/off RxD
PC = Master
24
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 an RS-232 interface, an
interface converter is required (e.g. from HBM, ordering
no.: 1-SC232/422B).
TB
TA
RB
RA
TA RB RA TB
T
R
TxD on/off RxD
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.
A4238-1.0 HBM: public
TA RB RA
...
R
Node 1
Bus termination
500 Ω
500 Ω
500 Ω
500 Ω
TB TA RB RA
T
R
TxD on/off RxD
Node 90
PAD4001A/4003A
+5 V
+5 V