HBM PW15iA Mounting Instructions page 30

Interfaces
Bus termination
+5 V
500 Ω
500 Ω
+5 V
500 Ω
500 Ω
TB
T
TxD on/off RxD
PC = Master
28
nect as many as 90 digital load cells to an RS-485 inter­
face. All the load cells are connected in parallel on a line,
the total length of the line can be as much as 500 m. The
software uses the different addresses to differentiate
between the load cells. 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. 5.1 Connecting several nodes to a PC via an RS‐485
4‐wire bus
The correct assignment of the transmit and receive lines
can be seen in Fig. 5.1 (bus line Ra to Ta of the con­
verter, etc.). The PW15iA already includes bus termina­
tion resistors (line termination), that can be activated with
the software command STR. So no additional bus termi­
nation resistors are required for RS-485.
A4359-1.0 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
+5 V
+5 V
PW15iA