TRS-80 Model 4 IDE Drive Interface

The following document describes an IDE Drive Interface for the TRS-80 Model 4 and Larry Campanell's modifications for the LNW-80.

Features of the Model 4 interface are:

  1. Uses easily available off the shelf parts.  Component part numbers are shown for DigiKey (US) and Jaycar Electronics (New Zealand/Australia).
  2. Uses easily available IDE 3.5" hard disks (note that this interface is not compatible at a wiring level for 2.5" IDE drives found in notebook computers).  Drives that have been used with the interface are listed below.
  3. Modified versions (including source) of the standard LS-DOS hard disk driver (HDDVR6/DCT) and formatter (HDFORM6/CMD) programs are required and are available from this site.
  4. For compatibility with LS-DOS the interface uses 256 byte sectors so a drive will only store half it's rated capacity (standard IDE drives use 512 byte sectors).  Given the cost of the interface and the drives I felt this was an acceptable compromise.  It also made the driver modifications a lot simpler!

Things still to be done:

  1. I am currently working on modifications to the LDOS 5.x drivers and formatter.  This may allow the interface to be used on a Model 3 also but I do not have a Model 3 for testing.  The interface uses ports &H40..&H47 which I believe do not clash with any other devices likely to be connected.

Building the Interface:

  1. The following GIF shows the schematic for the interface.  Click on the image for a more detailed version.  Save it to your machine and print it from your favorite graphics program to get a workable schematic.

  1. The drive can be built on prototype board if you are fairly confident with point to point construction (I build the prototype this way and it has performed well) . 
  2. The interface can be mounted internally within the Model 4 and both the interface and drive powered from the Model 4 power supply (watch you don't exceed the max current limits on the supplies though) or externally in a suitable box. 

Interface Parts List 

DigiKey Jaycar Description Qty
DM74LS85N-ND ZS-5085 U1 74LS85 4 Bit Magnitude Comparator 1
DM74LS04M-ND ZS-5004 U3 74LS04 Hex Inverter 1
DM74LS27N-ND ZS-5027 U2 74LS27 Triple 3 Input NOR Gate 1
P4849-ND RC-5490 0.1uf Monolithic Capacitors 3
1.0XBK-ND RR-0572 1K0 1/2W Metal Film Resistors 2
MHB40K-ND PP-1114 40 Pin IDC Vertical Header 1
MHB50K-ND PP-1115 50 Pin IDC Vertical Header 1

Power Supply Parts List (not shown in schematic)

DigiKey  Jaycar Description Qty
  ZV-1505 7805 +5v Fixed Regulator 1
P5183-ND RE-6180 220uf 50v Electrolytic Capacitor 1
P5134-ND RE-6066 10uf 16v Electrolytic Capacitor 1

TRS-80 Bus Attachment Cable (not shown in schematic)

DigiKey Jaycar Description Qty
C3AEG5018M-ND PS-0990 50 Pin IDC Line Socket 1
    50 Pin IDC Edge Connector 1
  WM-4508 50 Way IDC Ribbon Cable 1


  1. Schematic does not show the power supply circuit of the 0.1uf Monolithic Capacitors.
  2. DigiKey part number for the attachment cable refers to complete cable.

Installing the Software:

  1. Download IDE.DSK and use your TRS-80 emulator to copy HDDVR6/DCT and HDFORM6/CMD to TRS-80 disks.
  2. With your IDE interface and drive connected and powered up start Basic, type ? INP(&H40) and press ENTER.  If the drive is communicating properly you should see either 80 or 88 printed on the screen.  If you see any other values then recheck your interface wiring.
  3. At the LS-DOS prompt type SYSTEM(drive=4,disable,driver="hddvr6/dct") and follow the prompts to define the logical drive parameters.  Refer to the Radio Shack Hard Disk documentation for notes on this procedure.  It is important to keep note of the parameters you use for each drive because unless you SYSGEN your system disk once the hard disk partitions have been configured you will need to repeat the process after the machine is restarted.
  4. When you have defined the logical drive parameters run HDFORM6/CMD to format the drive.  This program is also described in the Radio Shack Hard Disk documentation.

Thanks to....

Tim Mann for his great collection of Logical Systems Inc software.

Frank Durda IV for his LDOS Source Reconstruction Project and especially the hard disk driver source.

TRS-80 Model 1/LNW-80 IDE Drive Interface

The following describes Larry Campanell's modifications to the IDE Drive Interface for the TRS-80 Model 4 for use on the TRS-80 Model 1 and LWN-80.  Larry has also provided drivers for DOSPLUS 3.5.

Some comment from Larry regarding the new interface....

"I did have to make a few changes to your hardware design because the Model I's expansion bus does not provide the /IORQ signal. I replaced U1 & U3 with a 74LS00 and 74LS32. The only other addition was to hang an LED on the IDE interface's /DASP signal to provide a "HDD Active" indicator."

"As for the software: I used your LDOS drivers as a guide but I created my driver and formatter for DOSPLUS ver 3.5 because I am more familiar with that OS. Refer to the attached zip file containing the driver (TRSIDE1D.SRC) and formatter (TRSIDE1F.SRC) source files, both in EDTASM format."

"I did have one problem, though -- not all IDE drives would work. Maybe I have a subtle hardware or software problem but, only one of the four IDE drives I tried would work. The working drive performs flawlessly leading me to believe the basic hardware & software design is good, however, the other three drives won't even pass the simple buffer read/write test I added to the driver's loader module.

The drive that works for me is:

Quantum ELS85AT 3.5" 80 Meg

The drives that wouldn't work are:

Western Digital WD2340 3.5" 340 Meg

Conner CP2081 2.5" 80 Meg (using a 2.5" to 3.5" adapter card)

Conner CP30204 3.5" 212 Meg

Anyway, Quantum the 80 Meg drive is more than enough for DOSPLUS 3.5 since it can only handle 8 drives (4 floppy, 4 hard) and each is limited to 200 tracks. Using a track offset of 200 for each DOSPLUS drive, I partitioned four 8 Meg drives and still have a 150+ tracks left over on the HDD.

I then created a boot floppy that will load the drivers and substitute the first HDD partition for drive 0 allowing the DOSPLUS system to run directly off the HDD. Wow, the computer really jumps. Type DIR and the files fly by on the screen. Programs seem to load instantly -- no more waiting for the floppy to chug along."

Download TRSIDE1Z.ZIP for Larry's driver and formatter source.

Whats next....

If you have any questions about the interface feel free to send me an email.  My email address is  I also keep a close watch on the comp.sys.tandy newsgroup under the JAQAS alias.

The LDOS 5.x drivers are on their way........ send me an email so I know you are hanging out for them and I can notify you when this site changes.

I am looking at the possibility of having some PCB's for the interface made if there is sufficient demand.  If you are interested in a hard disk for your Model 4 and don't feel confident building the interface without a PCB or would like a built up PCB please send me an email and I will see how the numbers work out.


(C)opyright 2000, Andrew Quinn Last Updated 08 Feb 2001

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