What you didn’t know about RS-232 Serial Port

Olga Weis

RS (recommended standard) was developed by Electronic Industries Association back in the 60’s to facilitate communication between a modem and computer terminals. We are going to look at the limitations of this communication system, its relevance in modern personal computers, the relationship with other serial standards.

The low transmission speed of RS-232 serial port was the downside of this once-popular feature of PC. USB port has almost replaced the RS-232 in modern computers taking over its peripheral interface roles. Despite the obvious replacement by USB in most computers, RS-232 devices continue to provide an irreplaceable service to various industries and businesses. This defiant stance of RS-232 has forced most modern computers to either use an internal expansion card with several serial ports to connect to RS-232 peripherals or use a USB-to-RS232 converter.

USB-to-RS232 converter

What are the known issues with the RS-232 serial port?

Using the standard COM port comes with a bunch of limitations that you have to deal with. Here are the obvious limitations of the standard:

  • Increased consumption of power due to large voltage swing is a huge complication for the power supply design.
  • A lot of devices don’t use the handshake lines for flow control thereby making RS-232 unreliable.
  • Although the multi-drop connection issue has been addressed with more reliable alternatives, it still doesn’t compensate for the compatibility and speed limitations of RS232 port.
  • The need for a null modem or crossover cable whenever connecting a peripheral to a computer.
  • RS-232 doesn’t solve the problem presented by single-ended signaling.

Does RS232 play any role in
modern computers?

Obviously, nowadays USB is the most used protocol for local communication. However, if USB is that good, why do we still need the RS232 port? Let's start with a comparison. USB is faster and consumes less power. But then USB design is more complicated compared to the design of RS232 standard. A USB interface needs the help of a software to decode the serial data whereas RS232 doesn’t. For instance, serial ports of some computers help in direct control of several industrial hardware devices like UPS. In cases like this, any application can alter the condition of RS 232 control lines in the registers of a UPS using simple input/output instructions. USB, on the other hand, will employ the help of a software program in order to do this.

This is why most industrial automation and surveying laboratories continue to use serial port technology in spite of its limitations. The re-introduction of DB-9M connector on the Tecra personal computer by Toshiba proves that these standards are here to stay for now. Despite their differences both USB and RS-232 standards support most software programs in major operating systems.

Serial Port Device

Developers are also actively bridging the gap between the vintage RS232 enabled computers and modern counterparts. One of the solutions is Serial to Ethernet Connector. This software allows you to access an RS232 port of an old computer in a remote location from your modern computer over network (LAN/WAN/IP).

Relationship between RS232 and
other standards

RS-232 compliant ports might not necessarily operate with several other serial signaling standards like RS-422, RS-423, RS-449, RS-422, 423, RS-485 and so on. For GPS receivers and depth finders using a TTL level close to +5 and 0 voltage, the mark level moves to an undefined area of the standard. You will need a current translator to use the RS-232 standard in an environment like this.

How they relate:

  • RS-422 has a similar speed with RS-232 but differs in signaling
  • RS-423 speed is the same without a balanced signaling
  • Rs-449 decommissioned
  • MIL-STD-188 is similar to RS-232, but has a great rise time control with a better impedance.

Thinking of ditching your RS-232 device? Not so fast! As you can see this serial protocol continues to defy all claims that it has been completely replaced by USB. Though modern communication systems require a more sophisticated system like the USB, we are going to continue using the standard serial ports.

Third party applications have done well in enhancing the way we work with RS-232 serial port. An example is Serial to Ethernet Connector developed by Eltima Software. You can find interesting usage scenarios in the User Guide.