Serial data communication is in many respects similar to COBOL programming. There was a time when the COBOL computer language ruled the business programming world just as there was a time when most data communication used serial connections and the RS232, RS422 or RS485 protocols. But just like newer programming languages did not completely replace COBOL and there are still hundreds of millions of lines of mission-critical COBOL code in use all over the world, USB and other high-speed data communications protocols did not simply replace RS232-style serial communication

So for old time’s sake and to keep from getting rusty, let’s revisit what all is involved in designing or analyzing a simple serial cable that takes advantage of all the various aspects of the RS-232 standard (or RS-422 or RS-485).

First, there’s speed, and here there are two aspects. One is that the speed of the sending and receiving devices must match. Buffering and flow control may come into play here. The other is maximum transmission speeds and how they relate to maximum cable length.

Second, there’s flow control. This comes into play when one of the devices is much slower or may occasionally want to stop receiving. Flow control can be either via software (by using XON/XOFF or ETX/ACL signals) or via hardware by using one of the RS232 pins.

Third, there are parity (a simple way of error control), stop bits (enclosing characters with start and stop bits), and character length (there are different codes, and they may or may not include parity bits).

Fourth, serial communication can be in full-duplex, half-duplex or just simplex. The chosen mode can influence which connector pins are being used. Note that there is also “echoplexing,” which is echoing characters back to the sender. Setting this wrong may result in no characters displayed or them being displayed twice.

Fifth, make sure line feeds are set properly so that text is properly formatted. This remains a big issue to this day, with different computers and operating systems handling line feeds in a different way.

Sixth, differentiate between synchronous and asynchronous data transmission. Synchronous uses a clock whereas asynchronous uses start and stop bits. Sometimes there is a combination of the two. What’s important is that both ends are set the same way.

Finally, there’s polarity. This refers to signals being either positive or negative. For example, hardware flow control may expect a positive signal on a pin, and it won’t work if the signal is negative.