JBUS/MODBUS
RS232 link configuration and connection
5 4 3 2 1
SG Txd Rxd
Pin | Function |
|
|
1 | Received data (input) |
2 | Not connected |
3 | Transmitted data (output) |
4 | Not connected |
5 | Signal ground |
SA2 |
SA1
SA2
1
8
ON
OFF
RS485 link configuration and connection
For proper operation, the polarity of EIA RS485
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
RS485
0 V
RD = (B')
RD = (A')
TD = (B)
TD = (A)
1
2
3
4
5
R-
R+
T-
T+
0 V
SA2 |
SA1 |
SA2
1
8
ON
OFF
SA1 
1
8
ON
OFF
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
34003906EN/AE - 9
