Clock Port

The A1200 clock port is an expansion header intended for use with an add-on real-time clock module. It provides all the standard signals to map the module to the same address as the built-in or optional real-time clock on other Amigas (64KB starting at $DC0000). This is normally occupied by the RTC on accelerator cards on the A1200. The port also provides a second chip select signal designated SPARE_CS, which maps to 64KB starting at $D80000. This range has no other use and so is used for a number of expansions that can be attached to the clock port.

To facilitate the use of more than one peripheral, this space can be divided up into 16KB chunks as detailed below. Not every peripheral or driver software supports this method - some will only work at the original address. Address lines A14-15 are required to decode these addresses and must be obtained from elsewhere on the board as only lines A2-A5 are available on the port itself.

The port is unique to the A1200, although various expansions can replicate its functionality on other Amiga models.

All accesses must be in 16-bit words.

Address	Port
D80000	Original Port
D84000	Second Port
D88000	Third Port
D8C000	Fourth Port
D90000	Non-standard port used on some expansions, mapped to unused Chip Select NET_CS.

Note that due to the restrictions imposed by the limited data (D16-D23) and address (A2-A5) lines available, all accesses are at odd addresses, and at 4-byte intervals, e.g. $D80001, $D80005, $D80009 and so on.