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Appendix A – Connecting Peripheral Instruments
A-13
Important: when performing the above connection, be sure you
connect the devices directly; do not use the vessel’s DC ground circuit
to carry the Transmit (negative) signal. You may encounter devices
that do not properly implement the NMEA specification, and share
their Transmit (negative) lead with the device’s DC power ground
circuit. In this case, you must still run a cable directly between the
Transmit (negative) and Receive (negative) leads as previously stated
— do not share the DC power ground circuit.
RS-232 Basics
Most desktop and laptop computers have RS-232 ports. An RS-232
port uses a 3-wire interface, in which the transmit and receive leads
reference the same signal ground. It is important not to confuse the
RS-232 cable’s signal ground with the vessel’s power ground circuit.
While the vessel’s power ground may coincidentally be at the same
level as the RS-232 signal ground, there is no industry standard that
requires this. Since the RS-232 transmit and receive data signals
reference a common signal ground to maintain their integrity,
you must connect the signal ground circuit directly between the
two RS-232-linked devices.
To wire an RS-232 device to another RS-232 device, connect:
1. Device A’s Tx lead to Device B’s Rx lead.
2. Device A’s Rx lead to Device B’s Tx lead.
3. Device A’s Signal Ground (SGnd) lead to Device B’s
SGnd lead.
RS-232 uses a common ground, which differs in electrical
specification from the NMEA’s transmit/receive-pair arrangement.
Because of these signal level differences, the way you interconnect
an RS-232 device and an NMEA device varies with the application.
Note: You may also encounter voltage differences
when interconnecting older RS-232/NMEA
hardware - use care.