Extended RS232 Information and Trouble Shooting

The serial RS232 interface

The RS232 is an quite old but reliable interface for data exchange with relatively slowly external devices. The connection needs a special cable with so called DB9 or even DB25 connectors. There are one or two connectors on a common standard PCs with the following pin layout:

A little Theory: The data exchange takes place in a asynchronical and voltage driven way on a transmission line (TX Transmit data) and a receiver line (RX Receive data). All other signals are be used for setting and testing of status information. Asynchronical means that there is no clock line. Voltage driven means that you can measure the voltage at the signal lines with common measurement devices or oscilloscopes. According to then RS232 standard, a logical 1 (Mark) should be found between -3 and -15V, a logical 0 (Space) between +3 and +15V. Voltages between -3 and +3V are undefined. The voltages are measured relative to the signal ground (GND) and are placed between 9 and 12V in general. The transmission rates are generally between 1200 and 115200 bit per second (Baud). It is important, that sender and receiver have the same transmission rate.

Attention: For a working data exchange the TX line of the sender must be connected to the RX line of the receiver. Therefore you need the right cable. Unfortunately there are several possibilities and you can't use every RS232 cable, even though the connectors fit. In the worst case you short circuit two sender lines and that may lead to hardware defects.

The maximum length of a RS232 cable depends on the quality of the cable (capacity and protection) and the transmission rate. At 9600Baud and less the cable can be up to 100m long. At 19200Baud and more the cable should not be longer than 15m.

Trouble Shooting

Defects in the RS232 hardware can lead to very different errors, from total crash to occasionally  transmission errors.
You can easily test the hardware with a temporary bridge between sender line TX and receiver line RX. Than all sent data will be received and should be equal when the hardware works correctly. You can use test connectors as temporary bridge between RX and TX which you can simply create by yourself. You need a common DB9 female connector and solder a wire between pin 2 and pin 3.  Complicated test connectors have additionally bridges between the status lines for testing uses (View from behind, Solder side):

RS232 DB9 Simple Bridge

RS232 DB9 Profi Bridge

Now you need a corresponding software. In the simple case you can use the Windows HyperTerminal to test with the Simple Bridge connector if all sent signs will be received. Additionally you can use our own software for special diagnostic test on Tanita scales:

Download RS232 test software

Test connection to scale: Choose the COMx interface from the field "RS232-Port", on which the scale is connected. The scale type from the field "Scale pre settings" defines automatically the right baud rate and data type. Not every scale has a bidirectional interface. Some can only send data for instance when the scaling process is finished. In this case the "Output" field is disabled. Otherwise you can enter for instance the command W? and commit with the enter key. The scale should answer with a type number. If you can repeat this without any errors, the connection is basically all right.

Hardware check with test connector: With the button "Hardware Check..." you can run a transmission test with the Simple or Profi Bridge connector. If you use an USB RS232 converter you should only use the Simple RX-TX test, because some converter produce strange status signals.

You can switch the status lines with a mouse click on the DTR and RTS check boxes. The status signals DSR, CTS, DCD and RI change accordingly to the connected device. For instance when using the Profi Bridge connector the DTR is connected with DSR/DCD and RTS with CTS/RI.

Trouble Shooting with Oscillographe

A good electrical engineer will not give up at a failed software test. You can very well view an analyze the signals with a memory oscilloscope. The professional can even recognize the content of the data.

Correct Signal (TX/19200Baud):

Example for a defective USB-RS232 converter:

At a transmission rate of 19200 baud every data bit is 52µs long. In the left picture the voltages are in the correct range of -10 and +10V and the edges are steep enough.
In the right pictures you see a data signal mixed with a trouble signal, which can produce occasionally transmission errors. The reason can be a defective charge pump, which produces the +/-9V from 5V in the USB converter. You can throw the USB-RS232 converter into the trash can.

Useful Tools

The service technician, which has to deal with these problems a lot, might use a so called "RS232 Tester". Several distributors sell these devices and are easily found in the Internet. 
You can optically control the status of single signals with these boxes and make out very fast, if there are broken lines or wrong cables.

---

Back to FAQ page