RS232, signal levels, MAX2xx chips, etc. | M68HC11

A few odds-and-ends notes to add to the discussion of RS232 and related topics: 1. As previously noted, RS232 signal levels range from -3 to -25V (mark/logic 1) to +3 to +25V (space/logic 0). A typical desktop PC will show mark/space levels typically at +/- 8V, sometimes as high as +/- 12V. Laptop PCs (and even some desktops) tend to run a bit lower, as low as +/- 5V. 2. The venerable MAX232 device is the orignal single-supply RS232 transciever. It uses a switched-capacitor charge pump and voltage inverter to generate the reference voltages used on the transmit side. Such circuits are not 100% efficient; the typical space/0 level is around +9V, and the mark/1 level is around -8V. These levels decrease considerably if the transmit outputs carry a significant load. 3. The original MAX232 device requires 4 (or better yet, 5) 10 uF capacitors, two used by the internal voltage doubler and another two used by the voltage inverter. Electrolytic caps work OK, although better results are realized if one uses low ESR (equivalent series resistance) caps, such as tantalum. The newer series MAX202 device has the same pinout as the MAX232, but can utilize much smaller 0.1 uF capacitors. 4. Maxim (and other manufacturers) now provide a wide line of devices that serve as RS232-to-logic-level transcievers. Some devices, such as the MAX208, provide more I/O capability (4 in/4 out), others incorporate low-power shutdown modes, lower-voltage (3V) operation, and even electrical isolation. A look at the parametric charts offered on the Maxim website will give you an idea of what is available. That's my $0.02 worth. -- Mark

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RS232, signal levels, MAX2xx chips, etc. - Mark Schultz - Nov 20 15:01:00 2004