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Application Note 56 Revised September 2003
Interfacing to RS-485 Sensors
A few Sea-Bird instruments use the RS-485 protocol for transmitting setup commands to the instrument and
receiving data from the instrument. However, most personal computers (PCs) do not come with an RS-485 port.
This Application Note covers interfacing our RS-485 instruments with a PC by the following methods:
• Connecting the instrument to an external RS-485/RS-232 Interface Converter that plugs into an existing
RS-232 port on the PC.
OR
• Installing an RS-485 interface card (and associated software) in the PC, and then connecting the instrument
directly to the new RS-485 port in the PC.
External RS-485/RS-232 Interface Converter
RS-485/RS-232 Interface Converters are available commercially. These converters plug into the RS-232 port on the PC,
and allow an RS-485 device to be connected through the converter. Sea-Bird tested a converter from one manufacturer
with our instruments, and verified compatibility. The manufacturer and tested converter is:
Black Box (www.blackbox.com) –
IC520A-F with RS-232 DB-25 female connector and RS-485 terminal block connector
Other converters from this manufacturer, and converters from other manufacturers, may also be compatible with
Sea-Bird instruments. We recommend testing other converters with the instrument before deployment, to verify that there
is no problem.
Follow this procedure to use the IC520A-F Converter:
1. Connect the Converter to the PC:
• If the PC has a 25-pin male RS-232 connector, plug the Converter directly into the PC connector.
• If the PC has a 9-pin male RS-232 connector, plug the Converter into a 25-pin to 9-pin adapter
(such as Black Box FA520A-R2 Adapter). Plug the 25-pin to 9-pin adapter into the PC.
2. On the Converter, measure the voltage between XMT+ and ground and between XMT- and ground.
Connect whichever has the highest voltage to RS-485 ‘A’ and the other to RS-485 ‘B’. The ground terminal can
be left unconnected.
RS-485 Interface Card and Port in the PC
An RS-485 Interface Card installs in the PC, and allow an RS-485 device to be connected to the RS-485 port.
These Interface Cards are available commercially. When using with a Sea-Bird instrument:
• RS-485 Transmitter -
The Interface Card must be configured to automatically handle the RS-485 driver enable.
• Two-Wire Interface -
TX+ and RX+ on the Interface Card must be connector together and to ‘A’ on the instrument.
TX- and RX- on the Interface Card must be connected together and to ‘B’ on the instrument.
Note: Some Interface Cards have a jumper to make the connections internally, while for other Cards the
connections must be made in a jumper cable.
Sea-Bird Electronics, Inc.
1808 136th Place NE
Bellevue, WA 98005
US
Phone: (425) 643-9866
Fax: (425) 643-9954
E-mail: seabird@seabird.com
Web: www.seabird.com
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