Elpro Technologies 905U-D User Manual download pdf (Page 16)

905U-D Radio Modem User Manual

Man_905D_1.10.doc Page 16

Example cable drawings for connection to a DTE host (a PC) or another DCE host (or modem) are

detailed above.

DB9 Connector Pinout

Pin Name Direction Function

1 DCD Out

Data carrier detect

–

- on when link is established in controlled mode

- on always in transparent mode

2 RD Out

Transmit Data – Serial Data Output

3 TD In

Receive Data – Serial Data Input

4 DTR In

Data Terminal Ready - DTR can be configured to initiate low power

mode, or to force a link disconnection (“hang up” in controlled mode.

5 SG

Signal Ground

6 DSR Out

Data Set Ready - always high when unit is powered on.

7 RTS In

Request to Send - hardware flow control configurable

8 CTS Out

Clear to send - hardware flow control configurable

9 RI

Ring indicator - indicates another module is attempting to connect in

controlled mode.

2.4.2 RS485 Serial Port

The RS485 port provides for communication between the 905U-D 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.

As the RS485 communication medium is shared, only one of the units on the RS485 cable may

send data at any one time. Thus communication protocols based on the RS-485 standard require

some type of arbitration.

RS485 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 RS485 wires. 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.

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