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Keysight Technologies
RS-232 Troubleshooting

Application Note

In the course of dealing with personal computers, you may use the RS-232 serial interface. This
application note will describe RS-232 at a basic level, with an orientation towards Windows-based
instrument programming.

1. An Overview of RS-232
The irst question that needs to be addressed is: what precisely is a "serial" interface?

Consider a computer connected to an instrument or other "remote" device. One of the simplest
possible communications schemes is shown in Figure 1.

Each device sends data bits coded as electrical pulses, with a "0" corresponding to a low voltage and a "1" a
high voltage, to the other over a dedicated line, using a shared ground line. Using separate lines to transmit
and receive data allows both devices to send data simultaneously without interference, at least in principle.

For example, to send a byte to the remote device, the computer would have to send as shown in Figure 2.

There are a wide variety of serial communications schemes. The most popular is RS-232, which is in fact
so universal that it is often simply referred to as "serial." RS-232 deines various mechanical and electrical
specs for serial communications.
Output Input
RS-232 deines legal voltage levels as follows: 0 (mark) +5 to +3 to
+15 volts DC +15 volts DC
1 (space)