How can I access UART between HL7800 and nRF52840 on the Pinnacle 100?

Answer

The Pinnacle 100 is a cellular modem which is basically composed of a Sierra HL7800 cellular module (Altair ALT1250 chipset) mated with a nRF52840 Nordic BLE chipset. The nRF52840 being provided with 2 UARTs, one of them is used to command the Sierra HL7800 cellular chipset. This UART that both HL7800 and nRF52840 share is called UART1, it is only accessible via non populated connector J2 on the Pinnacle 100 PCB:

If actively using UART1 to command the HL7800 from another host, please make sure to disable nRF52840 UART end to avoid conflict between respective external host and nRF52840 UARTs.