In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Zoot wrote:

Ugha and Shawn -- TIME TO START A THREAD JUST FOR YOUR SERIAL comm. questions. This thread is all over the place ("Learning SX/B??") :-). The need for a pullup has been mentioned several times.

The serial protocol used by JonnyMac and others is generally open true (not the same as "active-low" or "active-high"). This means when the device wants to send a 0, it drives the wire LOW. When it wants to send a 1 it sets the pin to INPUT, letting the pullup generate the high. Sometimes Jon cheats and has the SX drive the line high, and uses a series resistor to take care of possible shorts, but if OPEN TRUE (OT) is used on the Stamp or SX side with SERIN/SEROUT, you will get the line set to input for "1s".

The internal pullups on the SX are not strong enough for this purpose, unless your platform is remarkably free of noise and interference.

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