APPLICATION NOTE NO. 56

Interfacing to RS-485 Sensors
September 2003

Application note in pdf format

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.

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.

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.

• Terminal Program Compatibility --
  If the Interface Card uses shared interrupts, SEATERM (our Windows terminal program) must be used to communicate with the instrument.
  If the Interface Card is configured as a standard COM port, either SEATERM or our DOS-based terminal programs may be used to communicate with the instrument.

Sea-Bird tested two Interface Cards from one manufacturer with our instruments, and verified compatibility. The manufacturer and tested cards are:

National Instruments (www.ni.com) --
AT-485/2
PCI-485/2

Other Cards from this manufacturer, and Cards from other manufacturers, may also be compatible with Sea-Bird instruments. We recommend testing other Cards with the instrument before deployment, to verify that there is no problem

Follow this procedure to use the AT-485/2 or PCI-485/2 Interface Card:

1. Install the RS-485 driver software (provided with Interface Card) on your PC before installing the Interface Card.

2. Install the RS-485 Interface Card

3. Configure the RS-485 Interface Card in your PC (directions are for a PC running Windows XP):

1. Right click on My Computer and select Properties.

2. In the System Properties dialog box, click the Hardware tab. Click the Device Manager button.

3. In the Device Manager window, double click on Ports. Double click on the desired RS-485 port.

4. In the Communications Port Properties dialog box, click the Port Settings tab. Click the Advanced button.

5. In the Advanced Settings dialog box, set Transceiver Mode to 2 wire TxRdy Auto.

4. Make a jumper cable (do not use a standard adapter cable) to connect the Interface Card to the instrument’s I/O cable. Pin outs are shown for a Sea-Bird 9-pin (current production) or 25-pin (older production) I/O cable.

5. Run SEATERM (these Cards use shared interrupts, so the DOS terminal programs cannot be used).

1. In SEATERM's Configure menu, select the desired instrument.

2. In the Configuration Options dialog box, set Mode to RS-485 and set COMM Port to the appropriate RS-485 port.

Last updated: 06 Apr 2007

Sea-Bird Home     Phone: 425-643-9866     Fax: 425-643-9954     E-mail: seabird@seabird.com