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.

Epson HX20 Interface

One of the nice things about older hardware is that a lot of it comes with expansion connectors or other ports that allow you to attach arbitrary hardware to it. The HX-20 is no exception and comes with an expansion port which allows access to the bus signals of the main processor (yes it has two - almost dual core). I wanted a simple IO port so breadboarded a simple 8 bit output latch and 8 bit input circuit using old fashioned 74 series logic.

As a more permanent solution I routed a PCB.

I then used an old PC HDD cable to attach to the HX20 which helpfully uses a standard 0.1" pitch IDC connector. Unhelpfully they have used a different numbering scheme to the rest of the world which means the pin numbers are all jumbled, but that's not and unsolvable problem, just an irritation.

The output port can be used to drive other hardware, I'm going to use this one to drive a relay module:

The bottom two bits of the data bus are wired up on this PCB, so you write to address &H5000 and D0 and D1 will end up latched in the output port. To read inputs you read the same memory location, again D0 and D1 are the data bits that are wired up.