How can BL65x module or dongle be used to receive data from a custom service running on another device?

Several Sample Applications are available from the corresponding BL65x-Applications folder on Laird Connectivity's GitHub repository, including an AT Interface sample application.  The AT Interface sample application enables a quick way to configure a smartBASIC BL65x module using the commands available in the AT Interface User Guide to scan, connect, read the GATT table and receive data from the connecting device via the appropriate UUID for the custom service. We recommend referencing the AT Interface Quick Start Guide v1 0.pdf to get started with the AT Interface Application, which walks you through loading the application and a few of the basic commands, including initiating a vSP connection, (vSP is Laird's custom Serial UART application.) A non-vSP connection can be established using AT+LCON  as per page 21 of the AT Interface User Guide.

Alternatively, a custom smartBASIC central application could be developed and loaded to the dongle to connect with the custom service by referencing the Core Functionality smartBASIC and modular Extension User Guides available on the corresponding Product Page for the module, for example:

User Guide - smartBASIC Core Functionality v3.4
BL654 smartBASIC Extensions Guide v29.4.6.0

Please note: the BL654 dongle comes in two variants, smartBASIC variant (451-00003) and Nordic/Zephyr variant (451-00004). If electing to work with the smartBASIC firmware and example applications available on the GitHub repository it will be necessary to purchase the 451-00003 smartBASIC variant, as smartBASIC applications cannot be loaded to the 451-00004 Nordic/Zephyr variant.