RS-485 User's Manual MEH448d - Fuji Electric America

Terminal resistance
insertion switch
(RS-485
communications
port 1)
Terminal resistance
insertion switch
(RS-485
communications
port 2)
(f) Printed circuit board (FRENIC-MEGA)
Figure 2.6 (2) Switch arrangement for insertion of a terminal resistance
[3] Connection with a four-wire host
2-14
Default
setting
-
SW2 SW3
OFF OFF
Although FRENIC-Mini/Eco uses two-wire cables, some hosts adopt only four-wire cables.
Connect to such a host by connecting the driver output with the receiver input with a crossover
cable on the host side to change the wiring method to two-wire.
Four-wire host
(master)
Figure 2.7 Connection with a four-wire host
- The driver circuit on the host side must have a function to set the driver output to
high impedance (driver enable: OFF). Though products conforming to RS-485
normally has this function, check the specifications of the host.
- Keep the output of the driver circuit on the host side in the status of high
impedance except when the host is transmitting data (driver enable: OFF).
- Keep the receiver circuit of the host device deactivated (receiver enable: OFF)
while the host is transmitting data to prevent the host from receiving the data it
transmitted. If the receiver cannot be deactivated, program the host so that the
data transmitted by the host is discarded.
ON
FRENIC series
[two-wire]
ON