In some cases a GPS unit may require a command sent to them before the GPS will start sending the GPS data down the RS232 connection. First, set up the GPS to send only the $GPGGA string.  If you send the entire constellation information it will fill the 858 memory in less than an hour and you don't want that.

Make sure the 858 and the GPS are set at 9600 baud.  The GPS may have other RS232 settings that can be defined.  If this is the case be sure to set the GPS RS232 parameters as follows: 8 Data bits, 1 Stop Bit, No Parity and No hardware flow control. You may or may not need a Null Modem. These settings are under the assumption that you are unable to see any GPS transmission in the Chat mode.  Here is the section of the manual from page 72.

Overview

The G-858 may be used to simultaneously acquire GPS positions while it is acquiring magnetic data. In addition, the G-858 may be programmed to send commands to the GPS receiver when certain keys are pressed. All communication with the GPS takes place through the RS232 port.

The RS232 port of the G-858 is extremely versatile. That feature, and the fact that setting up RS232 communication in general is often confusing, means that you will need to carefully study this section and experiment to get your external equipment to work with the G-858.

Before connecting your GPS