Back in the ’80s, Casio produced a line of calculators that was sold and advertised as “pocket personal computers”. This branding started with the PB-100 I believe, but the device concept started with a previous model, the FX-702P, produced in 1981.
While by today standards that claim seems a bit of an stretch, it held quite a bit of ground back then. These calculators featured a BASIC interpreter built-in, and internal memory to store user programs. They also featured a proprietary port/interface that allowed some Casio peripherals to be connected to the device, like a printer or a cassette drive. Personal computers back then were not much more than that.
The FX-850P model released in 1987 -and other calculators that followed- were a significantly step forward since their peripheral options (via the Casio FA-6 Casio interface) included standard Centronics parallel port and RS-232 serial connectivity, both ports widely used in computer hardware.
The FX-880P, released in 1990, was (I think) the last device in this line of Casio personal computers / calculators, and was basically a FX-850P with 32KB of RAM (instead of just 8KB).
When I was a kid I used to see this calculator in store catalogs and magazines and always wanted to have one. Many years later, when I was old enough to purchase my own stuff, I was finally able to get one (second hand) in pretty good condition, and I still use to this day.
Despite providing fairly standard ports in their FA-6 interface, the connector used by these calculators was still proprietary, and included all the signals to communicate with the supported peripherals (although some voltage/level conversion and support circuitry was still required, and that was exactly the function of the FA-6 interface).
What was relevant to me was the RS232 port offered by the interface. This allows you to connect the calculator to computers, GPS loggers, modems, serial terminals, microcontrollers and an incredibly big number of other devices.
Years ago I found the schematics for a serial adapter for the computer, but since the connector was so weird I ended up with a bunch of components in a breadboard and some cables connected straight into the port in a very unprofessional and most-likely unsafe fashion. It worked, but it wasn’t something I could call a permanent solution.
I always wanted to make a serial interface for the calculator but the stupid proprietary connector was always the problem… until now.
A couple of months ago I discovered that while the general layout and shape of the Casio connector was proprietary, the space between the pins (1.27mm) was actually standard (a bit of an obscure standard, but standard nonetheless).
I got a couple of 2-row right-angle 1.27mm pitch headers from user tubemen88 on eBay. You just need to cut one to size (15 rows. 30 pins) and “move” the plastic stopper a bit to make the pins longer where the calculator goes.
So at least the connector part was solved now and I could worry about the circuit.
RS232 vs USB
I no longer had the schematic I once used, but I remember it was basically a simple RS232-TTL converter. The pinout for this calculator is widely available online, but hey! here’s a copy in case the rest of the internet explodes and only my blog survives:
Converting a RS232 signal to TTL levels is fairly common practice if you are into electronics and microcontrollers, and it’s ridiculously simple thanks to ICs like the MAX232 from Maxim (or Texas Instruments).
But nowadays it’s also possible to build a USB-TTL interface using a FTDI FT232RL or similar, which would be actually simpler and would allow you to connect the calculator straight to the USB port of your computer.
Unlike USB (which has a 5V line), RS232 does not include a “power” line in the port, just signals and GND. Hacks and non-standard solutions aside, this is why every RS232 device needs to get power from somewhere else.
NOTE: The Casio port does have a 5V line though, but it’s too weak to power anything (or at least not the MAX232 and a LED according to my tests).
So I had to decide between USB and RS232 for my adapter: A USB board would draw power directly from the USB port and wouldn’t need a external power supply nor power-related circuitry. It would -however- be very limited in use, as it would only allow me to connect the calculator to a relatively modern computer and nothing else. Going RS232 would allow me to connect the calculator to old and new computers alike as well as standard RS232 devices, but it would be more bulky and would require not only a external power supply but also extra circuitry for that.
Despite my best efforts, I was unable to pick, so I decided to go with both… kinda.
Since I needed to build a RS232-TTL converter + power supply combo why not make it flexible enough to be used in other applications and not only with these calculators? I could also expose the Casio port via a header in a way that would bypass the RS232 circuitry, allowing me to connect a standard USB-TTL adapter (if I wanted to directly connect the calculator to my computer via USB), or directly interface the calculator to TTL logic (like microcontrollers!).
NOTE: For the Casio connector I modified a part from Piotr Esden-Tempski’s Eagle Libraries. I built a 30-pin part from his 1.27mm headers, and removed one pin from the resulting component so I could reach the pin behind easily in the PCB.
The board has 2 headers: JP2, which is a 2×4 header that exposes both the TTL side of the MAX232 and the Casio port, and JP1, which is a simple 2 pin selector that can be used to disconnect the power stage from the board in situations where you already have a 5V line in your circuit.
If you short the GND, TX and RX lines of JP2 with standard jumpers, and plug a 7-15V wall adapter (or a 9V battery) into the power jack, it becomes a fully functional RS232-Casio interface that will connect your calculator with any computer and RS232 device.
If you remove the jumpers from JP2 you can use the top row of this header to directly connect TTL signals to the calculator port, which would now be completely isolated from the RS232 circuitry. This means that you can connect stuff like USB-TTL adapters, microcontrollers, etc and no power adapter would be required as the RS232 circuitry is not used.
Similarly, you can use the bottom row of JP2 (which without the jumpers is disconnected from the Casio side) to use this board as a standard general purpose RS232-TTL converter for your circuits. External power is optional in this configuration; If the target circuit can provide 5V, no wall adapter is required, just connect the 5V from the target board to the VCC pin of the header and remove the jumper from JP1 to disconnect the power supply circuitry. If that’s not the case you can plug the wall adapter back to this board (and leave JP1 on) and now you have a 5V output that can also power your target device.
So in the end this is a sort of RS232-TTL + 5V Power supply combo, with an optional Casio port that can be disconnected from the rest of the board, allowing the use of each part of the circuit individually.
I decided to use a female RS232 connector so no exotic cables or adapters would be needed to connect this board to a computer (remember that this is also a RS232-TTL adapter). The tradeoff of this decision is that if I want to connect a device like a modem to the calculator, I would need a male-to-male null modem adapter plus the standard computer-to-device cable.
Connecting the calculator to your computer
Despite the additional uses for this board, the main thing you will probably want to do with this circuit is connecting your calculator to your computer. I could just connect a standard USB-TTL adapter to the top row of JP2 as explained before, but I’ll be going full-retro with a RS232 configuration, using almost everything this board has to offer:
To transfer a program stored in the calculator to your computer you need to go into programming mode (MODE + 1 in the FX880P), select the program slot you want to work with (in my case I’ll be using P1, so I press SHIFT +1) and write the following:
SAVE "COM0:5, N, 8, 1"
This will send the file through the RS232 connection at 2400 bps, with no parity, 8 data bits, and 1 stop bit. Make sure you have the same settings in your terminal software. When you are ready press ENTER on the calculator and the program should be sent to your terminal (from which you can save it to file or copy/paste it somewhere else).
To transfer a program from your computer to your calculator, go to programming mode, select the program you want to overwrite, and write:
LOAD "COM0:5, N, 8, 1"
In Termie now click Send File and select a text file containing the code. Make sure that the last line of your file also has a line break at the end, because that’s what the calculator uses to mark the “end” of a line. You could optionally start writing the code yourself in the terminal (making sure you send a line break after each line of code). When you are done sending the code press BRK on the calculator. The program should be stored in your Casio now.
You can change the communication speed but I’ve found that 2400 works pretty well. In the User Manual of the FX850/880P you can find the other speeds available for the LOAD and SAVE commands.
In the same text you’ll also find that you can actually create programs that open the serial port and talk to your computer or other peripherals, so in theory with this adapter you could make the calculator interact with pretty much any serial device including Arduino boards, sensors, modems, other microcontrollers, etc, etc.
I’ll try to create a git repo with my circuits and stuff in Eagle format, but I haven’t gotten around to it yet. For the time being refer to the pictures below, they should be all you need to build this adapter.