APC 66123 - Manual (Page 9)

3402230800/AC - 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