In some cases, you want to send RTK correction data in to a GPS receiver from one device, and then get NMEA position data from the GPS receiver into a third device. If the correction data is coming from a device doing NTRIP, the NTRIP device may also need NMEA GGA data. Effectively you need to have RS-232 serial data coming from the receiver going to two devices. This requires a "Y" or "Splitter" cable.
I left out the pin numbers out on purpose because that depends on what type of plug you use (DE-9 for your typical PC) and if it is a DTE or DCE device.
Note: you cannot hook two TX lines together, even if one is only going to occasionally have data coming out of it. Even when there is no data flowing out a TX port, the line is being pulled high or low. The RS-232 driver does not allow it to float. Two TX pins on the same circuit will fight each other to put the wire at the voltage they want, which means the RX pin won't be able to clearly see bits coming in.
Last updated: March 6, 2011