Answer: The Hilo module has a full UART. That means you can connect all the 8 signals of the UART to your application. The advantages are that you can use:
- DTR: to manage the sleep mode (AT+KSLEEP), to come back from the data mode to the command mode (AT&D1).
- DCD: to know if you have an active data session.
- RI: to be notify of any event from the network (incoming call, incoming SMS, etc.) (AT+KRIC).
- CTS and RTS: to have hardware flow control. (AT&K3)
- DSR: to know when the UART of the module is fully started. Be careful, you also have to wait for the rise of the CTS.
- And of course, TX and RX to communicate with the module.
By the way, you can save some I/O in some cases:
- If you do not want to manage the sleep mode or the data mode by the DTR, you can force it to his active state (0V0). Anyway, there is a software way for this both functionalities.
- You can let the Hilo manage itself the sleep mode.
- You can use the +++ to switch from the data mode to the command mode.
- A data session always begin by a “CONNECT”. So you may save DCD if it is acceptable for your application.
- Every network event is also available on the AT link. So the RI signal is really useful if your application goes in sleep mode.
- You may use the software flow control instead of the hardware flow control (AT&K4).
- You may don’t need any flow control if few data be transmitted (AT&K0)
Conclusion: At least, you have to connect RX and TX to your application. This is the minimum configuration of the UART. All the others signals might be removed, it depends of the needs of your application.