“Open-collector”, “Open-drain” demystified

Friday, October 14th, 2005

You’ll come across the terms “open-collector” and “open-drain.” These are used sometimes to describe how a particular IC’s pins are implemented internally.

This and also this are good explanations of what those terms actually mean and why an IC’s pin would be designed as open-collector or open-drain.

Max232A RS-232 Driver/Receiver

Friday, October 14th, 2005

Maxim’s RS-232 level converters are sometimes necessary if a chip or a control board needs to communicate with the PC, especially if the specs for the chip/board indicates that it can only accept TTL/CMOS logic levels. The uControllers, the PIC and the BX-24, that we’ve been using are more tolerant of these voltage levels, so that’s why we can connect the TX/RX of the PC directly to these uControllers.

For situations where you need to shift the voltage levels so you can communicate with a PC(on the PC the voltage swings from +12 to -12 V), you’ll need the Max232A. This bumps up the voltage for serial communication TO the PC(because most chips/boards work on 0 to 5 volts) and lowers the voltage for serial communciations FROM the PC to a safer level for the chips/boards.)

If you are using the MAX232A, you’ll need five 0.1 uF capacitors. The datasheet has a connection diagram that shows 5 polarized capacitors. You can use non-polarized caps. I verified this with MAXIM technical support.