TRS-80 Model 1 FreHD Expansion Interface

Fred Vecoven's FreHD Hard Disk emulator has been a very popular upgrade for TRS-80 Model 3 and 4 users since its launch in 2014.
 
If you have a Model 1 with a Radio Shack expansion interface then you can use the FreHD using the Model 1 adaptor board.  This includes autoboot support if you are happy disassembling your keyboard to update the ROM... but stock 16K Model 1 owners don't have this option unless they could find a Radio Shack Expansion Interface.
 
The Model 1 FreHD Expansion Interface removes the requirement for the Radio Shack Expansion Interface and FreHD Autoboot ROM replacement on a stock 16K Model 1.
 
The small board adds 32K RAM (bringing the total Model 1 RAM to 48K), an optional EPROM mapped to the empty address block at 0x3000 (for hackers who want to put something into this address block... such as the Z80 Monitor from the Dick Smith System/80 Blue Label) and a PIC microcontroller.
 
The PIC emulates just enough of a WD1771 Floppy Disk controller so the standard Level II ROM sees a controller and requests the 256 byte boot sector.  The boot sector checks a FreHD is connected and if found and running firmware with autoboot support (2.13 or later) will load the FreHD autoboot menu.
 
The prototype was built on prototype board using point to point wiring.  It proved that the design worked but was only reproducable by the most dedicated hardware hackers.
 

 
This short Youtube video describes the prototype.

 
The production board was created using the Eagle Light Edition with the board just fitting within the 100 x 80mm size limit.

What parts to do I need to build one?
 
  • 1 x PCB
  • 4 x 74LS245 Octal Bus Transceiver (IC1,IC2,IC3,IC4)
  • 1 x 62256 32K Static RAM (IC5)
  • 1 x 28C16 EEPROM (IC6 - Optional)
  • 1 x 22V10 GAL (IC7)
  • 1 x 18F13K22 (IC8)
  • 8 x 0.1uf Ceramic Caps (C1,C2,C3,C4,C5,C6,C7,C8)
  • 1 x 100uf Electrolytic Cap (C9)
  • 2 x 2N3904 or PN2222A Transistors (Q1,Q2)
  • 2 x 1K Resistors (R1,R2)
  • 2 x 4K7 Resistors (R3,R4) - Version 1.1 PCB only.
  • 1 x 2x20 Pin Header (JP1)
  • 1 x 2x25 Pin Header (JP2)
  • 2 x 2 Pin Header (JP3, JP4)
 
How does it work?

  • IC1, IC2, IC3 and IC4 buffer the signals between the TRS-80 and the expansion board and FreHD.  
  • IC1 and IC2 buffer the address bus.  These signals are always one direction... from the TRS-80.
  • IC3 buffers the TRS-80 /IN, /OUT, /WR and /RD signals.  These signals are always one direction... from the TRS-80.
  • IC4 buffers the data bus.  This is bi-directional.
  • IC5 adds 32K of RAM.
  • IC6 adds 1.5K of ROM starting at 0x3000.
  • IC7 selects IC5 and IC6 when the TRS-80 reads and writes memory above 0x8000 (IC5) or in the block starting at 0x3000 (IC6).  It also selects IC8 when the TRS-80 reads and writes to the floppy disk controller (0x37EC, 0x37ED, 0x37EE, 0x37EF).  When reading from the onboard devices (IC5,IC6, IC8) or the FreHD, IC7 will control the direction of IC4 to push data back onto the TRS-80 bus.  It also generates the /IORQ signal for the FreHD from the TRS-80 /IN and /OUT signals.
  • IC8 runs code that emulates a WD1771 floppy disk controller and provides the 256 boot sector.
  • Both the FreHD and IC8 rely on asserting /WAIT to the TRS-80.  Q1 and Q2 operate as an AND gate to allow either device to asset /WAIT by pulling their own /WAIT lines to ground.
 
Are assembled boards available?
Yes.  Bas Gialopsos (UK and Europe) Ian Mavric (the rest of the world) and make these available.  Contact Bas - This e-mail address is being protected from spambots. You need JavaScript enabled to view it and Ian -  This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 
Ian created a great video showing the interface in action.
 

 
 
Design Files.... if you want to build it yourself!
 
Version 1.1 contains the following minor changes:
  • Adds 2 x 4K7 pull up resistors (R3 and R4) on the FreHD /WAIT and /EXTIOSEL lines to prevent these floating when the FreHD is not connected.
  • Minor change to the board overlay symbol for the three links.
  • Minor change to the board overlay to put the JP1 symbols on the top of the board.
 
Version 1.2 contains the following minor changes:
  • Corrects connection of IC5 62256 /CE and /OE pins
  • 18F13K22 Configuration included in .HEX
 
Version 1.3 contains the following minor changes:
  • Added 2.1mm DC power plug.
  • Relocated mounting hole to fit 2.1mm DC power plug.
  • Relocated C9 to fit 2.1mm DC power plug. Might be tight for an electrolytic but a tantulum will do the job.
  • Mounting holes reduced to 3mm.

 

 

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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn