RS232 Sniffer - Software and Hardware Solution

Olga Weis

RS232, short for ‘recommended standard 232’, is a physical interface for connecting serial devices such as sources of UPS, data storage devices, scanners, printers, etc. RS232 exchanges data at a relatively slow speed, that is why in modern PCs it is often replaced by USB connection. However, in certain areas RS232 is still indispensable - for laboratory automation or surveying and industrial machines. It is widely used to communicate to networking equipment - servers have neither monitor nor keyboard, and at the time of boot there is no network connection.

Serial Port Monitor

From time to time we need to monitor activities of our serial ports, say, developers might have to sniff rs232 port to code an application or create a driver for a serial device. Windows OS, however, does not have built-in RS232 monitor so you cannot really watch the data flow in and out of the port unless you employ one of these methods:

  • Use RS232 sniffer software, this would be the easiest solution to the problem. Special applications are designed to listen to activities at both ends of the communication and display the data on a screen or record it on a disk.
  • Hardware RS232 sniffer - although it has disadvantages, it is really handy in certain cases. For example, a full duplex communication on an RS232 channel does not allow one to tie both lines together, two serial ports are monitored separately.

RS232 monitoring software

The simplest way to sniff RS232 port is to install a special application capable of monitoring, displaying, recording logs and analyzing all activities of the serial port. Should a problem occur when you are developing a software app or a driver, it will be tracked and displayed immediately.

Serial Port Monitor

Serial Port Monitor is a system utility for RS232/422/485 COM ports monitoring. It is compact yet powerful enough to do its job really well and does not require any hardware.

Serial Monitor offers various filters and allows you to search for a specific data. It also has a built-in terminal and gives you multiple options to export data. Another advantage of the application is its simple and intuitive interface.

Serial Port Monitor enables you to:

  • Monitor all activities of a serial port
  • Watch multiple ports simultaneously
  • View data in different modes
  • Emulate data sent to devices

So how do you monitor a serial port with the help of this application?

It is easy, follow these steps:

1. Download and install the app on your computer, launch it.

2. Now you need to enable monitoring.

This is how you do it - on starting a new session either choose ‘Monitoring -> Start’ in the Main menu or click ‘Start monitoring’ in the main toolbar or press F5. This should start a monitoring session immediately.

Check if “Start monitoring” button is deactivated now, but “Pause monitoring” and “Stop monitoring” buttons are enabled. This would mean that monitoring is in progress.

3. Open the port via the application.

Hardware solution to sniff RS232 port

Another way to track RS232 port data is to use a special cable that facilitates a one-on-one communication between DTE and DCE devices. DTE stands for Data Terminal Equipment, a PC or a printer, for example; while DCE is Data Communications Equipment, usually a modem. To connect a DTE device to another DTE or a DCE device a cable where transmitting and receiving lines cross is used, it is called a null-modem cable. To listen to the incoming and outgoing data one needs a special cable.

Serial Port Monitor

Transmit and receive lines in RS232 connection are separate, which makes it full-duplex, i.e. one can send and get data simultaneously.

The majority of protocols known as ‘master-slave’ or ‘query-reply’ are half-duplex. If devices on the opposite ends of the connection send data at the same time, the signals will collide on the receive line. A splitter cable can re-route the transmit lines of both devices to the read line for the computer that displays monitoring data. A device that is currently not sending data pulls its transmit line low, in the range of -12 V to -5V. If both transmit lines are low, i.e. none of the devices is sending data, the receive line of the split-off cable is pulled low as well. When both devices are sending out data, the receive line of the split-off cable is pulled up to the range of +5V to +12V. There might be some cross-talk between the transmit lines of the devices, but this should not be a problem.

A PC involved in monitoring data needs an application such as HyperTerminal, a Windows app, or its alternatives. The monitoring PC cannot send out or emulate sending out data as it transmit line is not connected.

As you see, the second solution requires additionally not only hardware, but an app too. While Serial Port Monitor does not need any to sniff rs 232 and allows monitoring several ports simultaneously.

Serial Port Monitor

Requirements: Windows XP/2003/2008/Vista/7/8/10/Server 2012 , 9.16MB size
Version 7.0.312 () Release notes