I bought a Motorola MC68681 duart and discovered that it should have been an XR68681 duart. The Motorola part is an older design and can't run at the speeds that the newer chip can. I was able to get it to run, but at 38400 instead of 230400. When you're loading large programs via the serial interface, that makes a *huge* difference, so don't be like me, get the correct chip first time.
## PI/T
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The PI/T has three 8-bit ports in total, two of which (ports A and B) are general purpose I/O. The third port (port C) can be configured to present some of the Timer/Counter internals to the outside world.
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On the sbc-3, two pins of port C (0 and 1) are used to implement a software I2C bus to talk to the on-board real time clock, a Dallas Semiconductors ds1307.
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### Links
- [Good Example](https://people.engr.tamu.edu/rgutier/web_courses/ceg453_s00/l2.pdf) of how to program the device and handle the interrupts in C.