Sharp PC-G850 Gadget

Sharp PC-G850 Gadget

The PC-G850 is one of the last pocket computers. It is an impressive machine and has a lot of interesting features.

And a nice keyboard. The features include:

• BASIC programming
• C programming
• Z80 assembler
• CASL (Japanese assembler)
• PIC assembly
• 11 pin interface with PIO mode
• Support for PIC 16F84 programmer attached to 11 pin interface
• Serial I/O on 11 pin interface

The PIC programming interested me, and the 11 pin interface means the machine can interface to all sorts of things. The first problem, though, is that I don't want to type in programs and lose them when the batteries drain. So, I decided to make a gadget that attaches to the 11 pin interface and gives me some storage. I've done a couple of these gadgets in the past (one for the fx502p cassette interface and one for the Microtan computer using TTL levels) so I based it on those.

It's a 'blue pill' ARM processor board together with an SD card module and an OLED display. Three buttons allow the gadget to be operated with no computer attached. If a computer is attached then the Arduino IDE serial monitor can be used to issue commands.

The board is plugged in to a series resistor interposer that provides a degree of protection to the PC-G850 when the board is attached. If a PIO line is set as an output and drives an output on the gadget then the resistors limit the current and hopefully the damage.

The OLED display has a menu system that allows files to be sent from an onboard RAM buffer, received from the Sharp into a RAM buffer and read and written to and from SD card.

The code uses interrupt routines to receive data from the PC-G850, the USART on the ARM is also attached but I haven't written any code to use it yet. There is also the possibility of using handshaking while receiving data which should allow the gadget to pause the Sharp while writing directly to SD card. That would allow large downloads. At the moment everything sent or received has to fit in an onboard 10K RAM buffer.

I used the ST LINK programmer for the first time on this board and it is much faster than the serial programming method I used before.

I'll probably create a PCB for the gadget, it'll be a bit more mechanically sound.