RS-232 Basics (Part 1): Scale Pinout and Cable Wiring

RS-232 has been around since the 1960s, so it seems appropriate to start this article with a communication method that’s even older: the tin can phone.

The kid on the left is speaking into a can held in front of their mouth (transmitting data, TD), while the other is listening through a can held to their ear (receiving data, RD).

To enable both kids to speak (TD) and listen (RD) at the same time, they would require a second set of cans connected by a string, represented by the red arrow1. Let’s also mark the ground (GND) in green:

At this point, the illustration looks similar to what you might find in the RS-232 section of your scale’s user manual2:

RS-232 schematic: Scale to PC connection using a crossed cable

To establish bidirectional communication with this scale, you would need to connect the transmit data pin (usually called TD or TXD) to the receive data pin of your computer (RD or RXD) and vice versa. Additionally, it’s essential to connect the ground pins (GND) for reference3.

Although this seems simple, attempts to establish communication with a scale using RS-232 fail often fail at this basic stage.

To understand why this is such a common problem, take a look at the photos of the RS-232 interface of A&D’s popular FX-120i balance and Radwag’s PUE C32 weighing indicator:

Both weighing instruments have a male D-sub port with 9 pins (DE-9M, frequently but incorrectly called DB-9M)4. You might therefore think that they can use the same cable. You would be wrong, as their pinout is not identical:

Radwag’s PUE C32 receives data (RD) on pin 2 and transmits it on pin 3 (TD), requiring a crossed cable when connected to the standard DE-9M port of a PC. A&D’s FX-120i receives data on pin 3 (RD) and transmits it on pin 2 (TD). It requires a straight cable.

RS-232 cable terminology

Null modem cables are also known as crossover or crossed cables. They cross-connect certain pins, such as RD and TD.

Straight cables run straight through and are also called straight-through cables or 1:1 cables.

There are other types of serial cables, but these two are most common.

If you use the wrong type of cable, you connect TD to TD and RD to RD. This would lead to a situation like the one illustrated below, where both kids are holding the cans to their ears and no communication is possible:

While we can immediately recognize that something is wrong in this picture, no one can tell a straight cable from a null modem cable just by looking at it:

Key takeaways: The fact that you can plug in a cable means nothing – it’s the pinout of your weighing instrument and the wiring of your cable that matters.

If the electrical connection between your scale and computer is not established correctly, it does not matter which software you use. It will not receive any data.

There are many more aspects of RS-232 communication to explore – we’ll do that in future articles. In the meantime, don’t hesitate to leave a comment, but please understand that I can’t remotely diagnose communication problems for free.


1Bidirectional communication on a single string is technically possible but doesn’t seem very practical.

2Did you notice that this schematic doesn’t tell you the gender of the port (DE-9M or DE-9F)? While it was just a simplified example, many actual user manuals omit this information as well, adding to the general confusion surrounding RS-232.

3Some scales and balances require connections between other pins. This is an advanced topic for a future article.

4RS-232 ports on scales come in many different forms and are not limited to DE-9M. You’ll frequently encounter DE-9F, DB25 (especially on older balances), RJ11, RJ45, and many others.

Using verified Ohaus SJX scales with our software

Most class I and II weighing instruments use what is called an “auxiliary indicating device” with a “differentiated scale division”1: On the display, the final digit or digits are visually distinct from the others. This is also the case with Ohaus SJX scales that have been verified (e.g. the SJX6201M):

Note how the scale division (d) is one tenth of the verification scale division (e) when weighing in g

This digit is enclosed in square brackets when the weight is sent over one of the optional interfaces (such as RS-232, USB, or Ethernet). Example:

   1234.[5]     g

By default, our software does not capture a digit that comes after a non-numeric character. This can be easily fixed in two steps. We’ll use screenshots from Simple Data Logger as an example, but the procedure is basically the same for our 232key virtual keyboard wedge software2.

1. Change the regular expression used to capture the weight

In the Input tab, click the Customize button and replace the regular expression with the following one:

(-*[0-9]*\.?[0-9]*\[[0-9]+)

This will ensure that the opening square bracket and the last digit are also captured (shown in blue in the Event Log):

Remove the bracket

The string we’ve now captured is not a number because it contains a bracket. Writing it to a file like this would make things complicated when processing the data later.

Fortunately, you can easily remove the bracket: Go to the Process tab, check Enable Editing, and enter the opening square bracket “[” in the Remove characters field:

SDL will now properly write the complete weight as a number to the CSV file, which can be opened in Excel or other programs:

Important: If you are utilizing our software in connection with a verified scale, please ensure that such usage is compliant with the regulations and requirements of your local jurisdiction.


1: See OIML R76 (PDF) for further information
2: For an example that uses 232key, see this article

How to connect your Ohaus Valor 7000 scale to a PC and transfer the weight into any application

I’ve made a short, but comprehensive walkthrough video on connecting an Ohaus Valor 7000 scale to a PC. It uses the standard RS-232 port, a suitable data cable, an RS-232-to-USB converter and our free 232key virtual keyboard wedge software.

This will enable you to transfer the weight directly into a variety of applications as simulated keystrokes, such as Excel, a LIMS, or any other application of your choice. It even works with browser-based applications!

This guide can also be used for the very similar scales from the popular Ohaus Ranger 3000 series.

Ohaus Valor 7000 with 232key software

Video transcript

Introduction

This video will show you how to connect your Ohaus Valor 7000 scale to a PC and configure the 232key Virtual Keyboard Wedge software.

Connect the RS-232 cable to the scale

To prevent accidental overloading of the scale, remove the weighing pan and pan support. Then turn the scale upside down.

Open the compartment on the bottom. Connect the serial cable and secure it with the thumb screws.

Remove the cable breakaway tab

Before closing the compartment, break the middle tab on the cover. Turn the balance back over and put the weighing pan back in its place.

Connect the USB converter to the RS232 cable

Connect the USB converter to the other end of the cable and secure it with the thumb screws.

Connect the USB converter to your PC

Plug the converter into a USB port on your PC. The driver should install automatically. If not, see the video description for download links.

After the device is recognized by Windows, a new COM port will appear. You can find this port in the Windows Device Manager under “Ports (COM & LPT)”. In our case it is called COM9, but the number will probably be different on your system.

Configuring 232key

Download and install 232key, then launch the application.

In the “Input” tab, select the new COM port.

Set the “Device” to Ohaus and press the “Set defaults” button to set the serial interface parameters.

In the “Output” tab, select your keyboard type and, if necessary, a key that should be “pressed” after the weight is entered.

Go to the “Start” tab and press the “Start” button. 232key will now start listening for incoming data on the specified COM port and output it as simulated keystrokes.

Switch to the target application, e.g. Excel or a LIMS. For testing purposes we’ll just use Notepad.

Transfer the weight

Turn the scale on. Place an object on the weighing pan and press the PRINT button.

The weight is entered into the target application.

For more information, please visit our website at 232key.com.