34003906EN/AE - 9
RS232 link configuration and connection
RS485 link configuration and connection
For proper operation, the polarity of EIA RS485 2-wire and 4-wire lines must be set at only one point
and the lines terminated at the end.
Polarity
Normally, the master of the network sets the polarity of the line. The receiver inputs have a true failsafe
feature wich eliminates the need for external bias resistors and ensures a logic high output level when
the inputs are open or shorted. This guarantees that the receiver outputs are in a known state before
communication begins and when communication ceases.
Termination
Termination is used to match impedance of a node to the impedance of the transmission line being
used. When impedance are mismatched, the transmitted signal is not completely absorbed by the load
and a portion is reflected back into the transmission line. The termination line is not necessary if the
speed on the line is much less than 115Kbauds
The default setting of the RS485 is a 4 wires configuration without polarity and without termination.
SA1 switches are used to make the termination and the topology of the line (2 or 4 wires). The
termination resistance value is 166
Ω.
SA1 description:
1
: reserved
2
: reserved
3
: link termination between T- to R- (2 wires configuration) if set to ON
4
: connection T- to R- (2 wires configuration ) if set to ON
5
: connection T+ to R+ (2 wires configuration ) if set to ON
6
: reserved
7
: reserved
8
: link termination between R+ and R- if set to ON
JBUS/MODBUS
5
Txd
Rxd
SG
4
3
2
1
SA1
SA2
SA2
Pin Function
1 Received data (input)
2
Not connected
3
Transmitted data (output)
4
Not connected
5
Signal ground
SA2
OFF ON
1
8
RD = (A')
TD = (B)
TD = (A)
0 V
1
2
3
4
5
RS485
RD = (B')
R-
R+
T-
T+
0 V
SA1
SA2
SA1
OFF ON
1
8
SA2
OFF ON
1
8
SA2
SA2
SA2
SA1
SA1
SA1
34003906EN_AE.fm Page 9 Vendredi, 19. décembre 2008 1:49 13
