If the manufacturer indicates that the serial output of the scale is CMOS it is very likely that the I/O at the scale end is implemented at logic levels that match the digital circuitry inside the scale. In todays technology that could very well be 1.8 volt or 3 volt logic. I notice that from the web page that the 321P model would be ordered with the 321PAX cable so that the scale can be connected to a PC.

If I were you I would look at also procuring this cable if you buy the 321P scale. With the cable you can then interface from your microcontroller via the same voltage levels that a PC would use. Do note that the cable may very well have level translation circuitry built into the cable. And it is also possible that the cable may bias such translation circuitry from the port handshake lines on the PC end of the cable.

Once you have the scale and cable in hand you can connect it up to your PC and use their PC Windows based software to excite the serial interface. While communication is in process you can peek in at the pinning at the scale end of the cable and the PC end of the cable with an oscilloscope. From this you should be able to determine the baud rate, voltage levels, and logic polarity of the serial signals used by the scale. That information should permit you to determine if you want to try to interface your microcontroller directly to the scale or if you simply want to equip your microcontroller hardware with a standard MAX232 type chip and use the 321PAX cable. And if you do use that cable make sure to check out that you may have to deal with the port powered level translator that I mentioned before. It is not uncommon for PC application software of this type to set the two RS232 output handshake lines from the PC to states of one at a high level (+6 to 12 volts) and the other to a low level (-6 to -12 volts). The voltage difference between these pins can make a find of "powersupply" to run an interface circuit. [Hint...port powered mice, RS485 converters, and other devices work this way]. If the 321PAX cable works like this then you will have to make sure to wire the serial port connection from your microcontroller to support this appropriately. Once you know how it works to would be as simple as connecting the appropriate pins of the D9 end of the 321PAX cable to the V++ and V-- pins of your MAX232 chip.

If the scale company does not pass over information on how the serial interface protocol works it should be possible for you to discover this using a comm protocol analyzer. It is possible for you, while running the scale via the standard cable from a PC, to connect the TxD and the RxD wires over to a second PC that has two COMM ports. The original TxD and RxD signals would be connected to the RxD inputs, one on each of the COMM ports, of the second PC. To look at the serial data flow between the scale and first PC, the second PC would be setup with software to receive the inputs from the two snooper COMM ports and display that data on the screen. There is some pretty nice software for doing this function in the form of a program called WinHexCom. You can read about this program and download a free testing copy of it from this web page.

Aerospace Software Ltd. - WinHexCom - Win32 Protocol Analyzer


Have Fun
MICHAEL KARAS