NMEA 0183

PC Opto-Isolator Cable


Isolate an RS232 (PC) port to

protect against ground loops...




Does the Opto-4 need external power?

No, it takes power from the RS232 handshaking lines. Please note that this requires the RS232 port to have these handshaking lines both connected and enabled. Most full RS232 ports (provided through DB9 connectors) on PC’s will comply with this. If there are no handshaking lines on the RS232 port then see the instructions on how to connect external power in the Quick Start Guide on the OPTO-4  downloads page.

Why is isolation needed for my RS232 connection?

The NMEA 0183 specification, from version 2 and higher, requires devices to be isolated on their input lines. The OPTO-4 allows an RS232 input connection to meet these NMEA 0183 Listener specifications.
Without this protection you risk ground loop damage to the RS232 device and/or the NMEA Talker. This might result in the need to replace expensive equipment.

Can I use the NMEA Opto-isolator for other non-NMEA uses?

The OPTO-4 cable converts the RS422 / RS485 (NMEA) voltage levels to RS232 (PC) voltage levels. Any data type can be transferred by the OPTO-4 using these voltage levels.
An example of an alternative use is to extend an RS232 cable run beyond the normal length of 20 meters. A customer has reported that using two OPTO-4 cables connected together, a cable run of 200 metres was achieved.
Two OPTO-4 cables back to back (with the RS232 connectors at each end) will also provide bi-directional isolation.

What if my NMEA 0183 Listener does not have an isolated input or uses ground for the negative data connection?

Isolation is provided between the NMEA Talker ground and the Listeners RS232 ground.
The OPTO-4 uses the same ground as the RS232 connection for the connection to the NMEA Listener. Connecting an NMEA Listener that is not isolated or uses ground for the return line will negate the isolation, making the OPTO-4 into a normal serial cable. To maintain the isolation that protects the devices from damage, the NMEA listener MUST have an isolated input.

Is there bi-directional isolation?

No, only the RS232 (PC) input is isolated from the NMEA 0183 Talker. An NMEA 0183 Listener connected to the RS232 (PC) output must have an isolated input. For bi-directional isolation, two OPTO-4 cables can be connected back to back

Can I use the OPTO-4 when the RS232 port does not provide power on the handshaking lines?

Yes, but you will need to provide power to the 9 pin connector (DB9) independently. Instructions for this are explained in the Quick Start Guide on the OPTO-4  downloads page.

What are the connections on the OPTO-4?

The RS232 end is a female 9-pin D-Sub DB9 connector following the standard 9-pin RS232 pin out specification.
The other end has bare wires. An isolated input pair A/+ and B/- for connection to an NMEA Talker, using RS422 voltage levels. A non-isolated output pair A/+ and ground for connection to an isolated listener, using RS232 voltage levels. There is also a shield wire at this end which connects through to the DB9 shield.

Can I connect two OPTO-4 cables together to get bi-directional isolation?

This is possible when connecting the wire ends together, but not when connecting the DB9 connectors together. Connecting the DB9 9 pin connectors together will remove the isolation, connecting the ground of both devices together.
Connecting the wire ends together will preserve the isolation. Both OPTO-4 cables will require power on the 9 pin RS232 connector, typically provided by the RS2323 port handshaking lines.
This configuration can also be used to extend an RS232 cable from the standard 20 metres potentially up to 200 metres as was demonstrated by one customer.

Can I use the OPTO-4 with a USB to serial converter cable?

Yes, simply connect the OPTO-4 female 9 pin connector to the male 9 pin connector of the USB to Serial Cable. The OPTO-4 will continue to provide isolation between the NMEA 0183 Talker and the PC. The USB to serial converter must support hardware handshaking (this is normally standard) and it must be enabled.

Is the OPTO-4 a direct replacement for the PC-OPTO-3?

Yes, the OPTO-4 is a direct replacement for the PC-OPTO-3. The functionality is the same, please see the data sheet for the physical differences on the OPTO-4  downloads page.


end faq

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