Casio CFX-9850 serial link to PC.

A data-point to place in public domain. This has been explored by others, so there’s nothing novel in this post. The serial RS-232 port has almost disappeared from modern desktop computers. This is a problem if you are looking to connect your Casio 9850 series calculator to your computer. The official FA-122 and FA-123 cables are difficult to source (for a sensible budget), and most (all?) of this series do not have USB.

I have successfully used the Casio FA-124 software, under Windows 7 (64 bit) to communicate with the calculator (CFX-9850G). I used:

  1. An off-the-shelf USB to TTL cable. It must be a 3.3V TTL type, and I’d recommend buying a good quality cable from a reputable seller. The ebay seller usb2ttl is a good place to start.
  2. A bit of 2.54mm single row PCB header. Looks like this.
  3. The Casio SB-62 cable. You may be able to use a 2.5mm stereo extension cable, but the calculator’s shape can make it difficult to use non-Casio cables.
  4. A 2.5mm stereo jack socket (not necessary if you’re cutting up a non-Casio cable).

You only need to make three connections. At the calculator, the tip of the plug is Tx, the ring is Rx and the sleeve is Ground. Connect the Ground to that of the USB to TTL cable, connect calculator Tx to the TTL Rx, and the opposite pair. That’s complete, a set of test leads will work if you’re in a hurry.

Check your computer has not assigned the virtual serial device to something greater than COM9 (if it has, you can change that port number to a single digit). Then the FA-124 software will be able to access it.


I acknowledge that my calculator tinkering is not entirely serious:



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