User manual

Chapter Three Installation
MAN_905G_1.16 Page 34
Hardware handshaking using the CTS/RTS lines is provided, and are under the control of the
Host Comms Driver. Example cable drawings for connection to a DTE host (a PC) or another
DCE host are detailed below:
905U-G
DB9
905U-G
DB9
3.5.2 RS485 Serial Port
RS485 should not be used with the DF1 unit. The RS485 port provides for communication
between the 905G unit and its host device using a multi-drop cable. Up to 32 devices may be
connected in each multi-drop network. Note that the RS485 port is shared internally with the
RS232 port - make sure that the RS232 port is disconnected before using the RS485 port.
RS485 is a balanced, differential standard but it is recommended that shielded, twisted pair
cable be used to interconnect modules to reduce potential RFI. An RS485 network should be
wired as indicated in the diagram below and terminated at each end of the network with a
120-ohm resistor. On-board 120 ohm resistors are provided and may be engaged by operating
the single DIP switch in the end plate next to the RS485 terminals. The DIP switch should be
in the “1” or “on” position to connect the resistor. If the module is not at one end of the
RS485 cable, the switch should be off.
905U-G
INTERNAL
EXTERNAL RESISTOR
REQUIRED
TERMINATING
RESISTOR SWITCH
ON = TERMINATION
OFF = NO TERM.
RESISTOR