The Macintosh Communications Toolbox, generally shortened to CommToolbox or CTB, was a suite of application programming interfaces, libraries and dynamically loaded code modules for the classic Mac OS that implemented a wide variety of serial and network communication protocols, as well as file transfer protocols and terminal emulations.
Using CommToolbox, one could write an application that would seamlessly work over AppleTalk, a modem or any variety of other connections, transfer files using XMODEM, Kermit or other file transfer protocols, and provide DEC VT102, VT220, IBM 3270 and other terminal emulation services.
Developers could also write plug-in communications modules known as "Tools", allowing any CommToolbox-aware application to use that connection method.
CommToolbox API's consisted of 4 managers: The CRM provided the Mac with its first centralized repository to register and enumerate serial devices.
In this manner, an application could be written "agnostically" without implementation-specific knowledge of any particular data connection, file transfer, or terminal emulation protocols.
[3] In addition, these tools also provided a set of system-wide standard UI implementations that could be automatically invoked and used for configuration.
The equipment set-up was a backlit Mac Portable using a PSI Integration internal fax modem (used in half duplex mode due to the nature of the shuttle air-to-ground voice links).