APPLICATION NOTE NO. 56
Interfacing to RS-485 Sensors
Revised February 2009

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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 a USB RS-485 Adapter that plugs into an existing USB port on the PC.

OR

• 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.

The use of these adapters/converters/cards is described below.

Note: Sea-Bird is providing information on adapters / converters / cards that we have tested with our instruments. Other devices from these manufacturers, and devices from other manufacturers, may also be compatible with Sea-Bird instruments. We recommend testing other devices with the instrument before deployment, to verify that there is no problem.

A USB RS-485 adapter plugs into a USB port on the PC, and allows an RS-485 device to be connected through the adapter. Sea-Bird tested an adapter from one manufacturer with our instruments, and verified compatibility:

National Instruments (www.ni.com) –
USB-485 with USB connector and RS-485 DB-9P connector

Follow this procedure to use the USB-485 Adapter:

1. Install the RS-485 driver software (provided with Adapter) on your PC before installing the Adapter.
2. Install the USB RS-485 Adapter.
3. Configure the USB port 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 USB port.
4. In the 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 Auto. Verify that Bias Resistors Enabled is not checked. Click OK. Then click OK in the Port Properties dialog box.

4. Disconnect the USB RS-485 Adapter from the PC, then plug it back in again. The new settings from Step 3 should now be in effect.
5. Make a jumper cable (do not use a standard adapter cable) to connect the USB-485 Adapter 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.

6. Run the terminal program software (see instrument manual for correct software for your instrument):

1. Seaterm: In Seaterm's Configure menu, select the desired instrument. In the Configuration Options dialog box, set Mode to RS-485 and set COMM Port to the appropriate RS-485 port.
2. SeatermV2: In SeatermV2's Instruments menu, select the desired instrument; Seaterm485 opens. In Seaterm485's Communications menu, select Configure. Set the Port to the appropriate USB port.

An RS-485 / RS-232 Interface Converter plugs into an RS-232 port on the PC, and allows an RS-485 device to be connected through the Converter. Sea-Bird tested a converter from one manufacturer with our instruments, and verified compatibility:

Black Box (www.blackbox.com) --
IC520A-F with RS-232 DB-25 female connector and RS-485 terminal block connector

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 allows an RS-485 device to be connected to the RS-485 port. 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 tested two Interface Cards from one manufacturer with our instruments, and verified compatibility:

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

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. Click OK. Then click OK in the Communications Port Properties dialog box.

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 the terminal program software (see instrument manual for correct software for your instrument).

1. Seaterm: In Seaterm's Configure menu, select the desired instrument. In the Configuration Options dialog box, set Mode to RS-485 and set COMM Port to the appropriate RS-485 port.
2. SeatermV2: In SeatermV2's Instruments menu, select the desired instrument; Seaterm485 opens. In Seaterm485's Communications menu, select Configure. Set the Port to the appropriate USB port.

Last modified: 23 Feb 2011

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