Hi there!
The pinout of serial 2 for the APS-3 is as follows:
LEMO PIN
4 Tx (Data From Radio)
1 RX (Data to Radio)
3 Signal Ground
6 Signal Ground
7 DNC (USB -)
8 DNC (USB +)
5 RTS (From Radio)
2 CTS (From Radio)
Note that I've never used pin 7 and 8, which are indicated as being USB -/+ and they're marked as "do not connect (DNC)". So they should probably not be connected.
Importantly, note also that to enable serial 2 you have to send the following commands to the receiver, or put them in the receiver's boot config:
sgpf, GP1, Output, none, LevelHigh
sgpf, GP2, Output, none, LevelHigh
sgpf, GP3, Output, none, LevelHigh
The port will then be connected/referenced to the receiver board's COM2. So if you wish to enable RTCM3 input into the Serial 2 port, you could e.g. send the command:
setDataInOut, COM2, RTCMv3
Thank you sir!