14
6193-3201
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-90 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 operat-
ing 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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