Horner APG RX371 OCS HERX371C101 - Manual (Page 27)

MAN0924-01-EN CH.5
February 8, 2010 Page 27 of 124 # 1018
5.4
RS-485 Termination
Proper RS-485 termination minimizes reflections and improves reliability.
Both serial ports allow an internal RS-485 termination resistor to be placed across pins 1 and 2 by DIP
Switch Setting.
Only the two devices physically located at the endpoints of the RS-485 network should be terminated.
5.5
RS-485 Biasing
RS-485 biasing passively asserts a line-idle state when no device is actively transmitting, which is useful
for multi-drop RS-485 networking.
Both serial ports allow internal RS-485 bias resistors to be switched in, pulling pin 1 up to 3.3V and pulling
pin 2 down to ground. The Set Serial Ports item in the System Menu can be used to enable RS-485
biasing. Also, an application graphics screen that writes to %SR164 can do the same thing. Setting
%SR164.1 enables MJ1 biasing and setting %SR164.2 enables MJ2 biasing.
If biasing is used, it should be enabled in only one of the devices attached to the RS-485 network.
5.6
Cscape Programming via Serial Port
The RX-371 OCS MJ1/MJ2 serial port supports CsCAN Programming Protocol. If a PC COM port is
connected to the RX-371 OCS MJ1/2 serial port, Cscape can access the RX-371 OCS for programming
and monitoring.
5.7
Ladder-Controlled Serial Communication
Using Serial Communication function blocks, both MJ1 and MJ2 support Generic, Modbus Master and
Modbus Slave Protocols. In addition, external modems can be connected and accessed using Init, Dial
and Answer Modem function blocks.
The DIP Switches are used for
termination of the RS-485 ports. The
RX-371 is shipped un-terminated.
To terminate, select one of the DIP
Switches and configure it based upon
the option that is desired.
SW1 -
ON enables MJ2 RS485 port
termination (121 Ohms).
OFF disables MJ2 RS485 port
termination.
SW2 & SW3 - ON places MJ2 RS485 port in
half-duplex mode.
OFF places MJ2 RS485 port in full-
duplex mode.
SW4 -
ON enables MJ1 RS485 port
termination (121 Ohms).
OFF disables MJ1 RS485 port
termination.