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15
6193-5201
DB9 Connector Pinouts
Pin
Name
Direction
Function
1
DCD
Out
Data carrier detect
–
2
RD
Out
Transmit Data
– Serial Data Output
3
TD
In
Receive Data
– Serial Data Input
4
DTR
In
Data Terminal Ready
–
5
SG
Signal Ground
6
DSR
Out
Data Set Ready
– always high when unit is powered on.
7
RTS
In
Request to Send –
8
CTS
Out
Clear to send –
9
RI
Ring indicator
–
2.4.2 RS-485 Serial Port
The RS-485 port provides for communication between the RM-240 unit and its host device
using a multi-drop cable. Up to 32 devices may be connected in each multi-drop network.
As the RS-485 communication medium is shared, only one of the units on the RS-485
cable may send data at any one time. Thus communication protocols based on the RS-485
standard require some type of arbitration.
RS-485 is a balanced, differential standard but it is recommended that shielded, twisted pair
cable be used to interconnect modules to reduce potential RFI. It is important to maintain
the polarity of the two RS-485 wires. An RS-485 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 RS-485 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 RS-485
cable, the switch should be off.
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