The 6809 IO mezzanine board is a supplemental board to the 6809 host processor.  It allows the 6809 host processor to act as a stand alone computer system separate from the Z80 and ECB.  The 6809 IO mezzanine implements a power interface, serial port (ACIA), programmable counter/timer (PTM), and dual parallel IO with timers (VIA).  The 6809 IO mezzanine also provides independent bus expansion for the 6809 processor. 

The 6809 host processor and 6809 IO mezzanine board can operate together as a stand alone computer or as an ECB peripheral.  However as an ECB peripheral it provides a much greater degree of autonomy for the 6809 CPU to interface to the outside world independently of the Z80 CPU ECB bus controller.  The 6809 host processor retains its ability to access the Z80 through its interrupt system.