The KISS TNC (Terminal Node Controller) provides direct computer to TNC communication using a simple protocol as described briefly here. For more detail see [K3MC \& KA9Q 85].
Standard TNC software was written with human users in mind; unfortunately, commands and responses well well suited for human use are ill-adapted for host computer use, and vice versa. This is especially true for multi-user servers such as bulletin boards which must multiplex data from several network connections across a single host/TNC link.
The KISS TNC solves these problems by eliminating as much as possible from the TNC software, giving the attached host complete control over and access to the contents of the HDLC frames transmitted and received over the air. This is central to the KISS philosophy: the host software should have control over all TNC functions at the lowest possible level.
The AX.25 protocol is removed entirely from the TNC, as are all command interpreters and the like. The TNC simply converts between synchronous HDLC, spoken on the full- or -duplex radio channel, and special asynchronous, full duplex frame format spoken on the host/TNC link. Every frame received on the HDLC link is passed intact to the host once it has been translated to the asynchronous format; likewise, asynchronous frames from the host are transmitted on the radio channel once they have been converted to HDLC format.
This means that the bulk of AX.25 or another protocol i.e DUAL must now be implemented on the host system. This is acceptable, however, considering the greatly increased flexibility and reduced overall complexity that comes from allowing the protocol to reside on the same machine with the applications to which it is closely coupled.