NOTE  I am now shipping a newer version of the PCB (ver 2.3A). Circuitry is identical to previous version except all Address, Data and Control signals are accessible at edge pins to match my 6502SBC PCB. See Layout here.

What is it?

This is a Printed Circuit board for a Floppy disk controller. It will interface 5.25 inch Single Side shugart type floppy disk drives to generic 6502 computers. It also supports Gotek Floppy emulators. It supports up to 4 disk drives and requires OSI65D3.1 or OSI65D3.3 operating system on floppy or as an .HFE USB file for a Gotek. They can be downloaded at or below.

Why did you make it?

I have a love of 6502 based systems from around 1979 when I built my first kit computer (UK101) and I expanded it by adding a floppy disk drive. I wanted to take a nostalgia trip and recreate my system using purpose built PCBs. I still had the original OS on disk which booted perfectly after approx 40years!!

What makes it special?

You cannot buy this interface elsewhere so I had to research and design a PCB I could use.

cct diagram
It is based around the Elektor Junior FDC and is designed for OSI-65D and Shugart 5.25 Floppy Disks.   
They are also compatible with Gotek's running FlashFloppy and .HFE disk Images.   
In line with OSI's DOS, a PIA (6821,6521) is addressed @ $C000 and an ACIA(6850) is addressed @ $C010.  
OSI Floppy Disk Interface BOM
IC13 1 x 74LS00 Quad Two Input NAND Gate  
IC8 1 x 74LS02 Quad Two Input NOR Gate  
IC10 1 x 74LS04 Quad Two Input NOT Gate  
IC11/IC12 2 x 74LS07 Hex Buffer/Driver OC output  
IC7 1 x 74LS74 Dual D-type flip-flops  
IC6 1 x 74LS123 Dual retriggerable monostable multivibrators  
IC4/IC5 2 x 74LS138 3-line to 8-line decoder / demultiplexer  
IC15 1 x 74LS151 8-Line To 1-Line Data Selectors/Multiplexers  
IC14 1 x 74LS163 Synchronous 4-Bit Binary Counters  
IC3 1 x 74LS245 Octal bus transceivers  
IC2 1 x 6821 or 6521 PERIPHERAL INTERFACE ADAPTER Make sure it is a 6821 or 6521 and not a fake chinese R65C21P2  
IC1 1 x 6850 ACIA  
  5 x 150Ω Signal Pull-ups  
  1 x 1K    
  3 x 4K7Ω    
  1 x 6K8Ω    
  13 x 0.1uF Decoupling Capacitors (Thanks to Stephan M´┐Żhlstrasser for the correction)  
  2 x 1nF Monostable timing  
  2 x 47uF Reset on Power up  
  2 x 10KΩ VR Monostable trimmers. Leave mid-position  
* 4 x 16PIN HEADER For Dupont cables. and Floppy Ribbon Version 1 board only.  
* 2 x 17 PIN HEADER For Floppy Ribbon Version 2.3A Board Only  
* 1 x 36 PIN HEADER (right angled) For Version 2.3A Board Only  
  1 x 1N4148 Silicon Switching Diode  
  7 x 14Pin sckt    
  5 x 16Pin sckt    
  1 x 20Pin sckt    
  1 x 24Pin sckt    
  1 x 40Pin sckt    
  NOTE!!!  5V and Gnd can be connected to base of the 47uF next to address bus header.  
      [email protected]  


Version 1 board layout.
Version 2.3A layout
Connector Pins are:
NOTE: On Early 2.3A boards a strap must be fitted between the cathode of D1 to pin 16 of the 74123 next to it.
Later 2.3A Boards do not require the strap. You can test by checking that D1 cathode to Pin 16 of 74123 is 0 ohms resistance.
Because my SBC uses a serial terminal for input/output, the OSI DOS versions are slightly modified to use a relocated port. (The UK101/Superboard 2 have their ACIAs at $F000, I chose to have mine at $8000 to make it easier for address decoding.) In order for the Floppy Interface to work with my 6502 SBC, the 'bootloader' needs to be modified. It is easy to do if you wish to do it yourself:

With a genuine OS65D3.1 or 3.3 disk or image loaded, reset SBC and select 'D'.

After disk activity has finished reset SBC and select 'M'

Copy the address and data text from below:


and paste to host or your comms programme's equivalent.

You should then have A* as a prompt.

Original OS65D3.1 Image for 5.25 Floppy can be downloaded here.
Original OS65D3.3 Image for 5.25 Floppy can be downloaded here.
Modified OS65D3.1 Image for Gotek can be downloaded here
Modified OS65D3.3 Image for Gotek can be downloaded here
OS65D3.3 Tutorial Disks for Gotek can be downloaded here 1,2,3,4
I sell on Tindie
(c) 2021,2022,2023 Paul Bartlett