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Cheapo IDE interface for Model 4p running MM CP/M

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    Try a different drive maybe? Looking at the model 4 tech ref, it seems to me there are two potential difficulties with the circuit as-is:
    • CS0 is raised coincident with IORD, but it should be selected first
    • No buffer on data bus means the IDE 3v3 logic levels could be ambiguous after travelling down two ribbon cables to get to the TRS80's internal buffer

    I'm going to rework the logic a little bit. Also we can add access to CS1 (alternate status register) via A4 if that would be desirable?


      I had an idea about the data bus. Thought about adding a 74LS245 Octal bus transceiver to the 8 data lines. Boosting the data bus. here is a data sheet for what to do with pins 1 and 19.
      Last edited by Chromedome45; June 16, 2013, 05:43 AM.


        I am now able to get the 5.25 WD, the 2.5 Fujitsu and the Sanddisk 1 gig CF all working. WooHoo!

        I changed the 3 paralleled inverters back to one inverter. I then added an LS244 line driver and passed the output of U3B into two of the line drivers. One of the outputs of the line driver I connected to /IOR on the ide connector and the other to pin 43 (/EXTIOSEL) of the TRS80 expansion interface. The 220 ohm pull-down resistor is no longer needed.

        I made this change after looking at the actual schematic for the RS 5 meg HD and noticed they were using line drivers for the /EXTIOSEL among other lines.

        If I had an inverting line driver laying around I would replace the simple inverter with it negating the need for an LS04 and LS244.

        A comment on the data lines...I haven't really witnessed any issue with the data, only the control lines. I guess it wouldn't hurt to buffer them.



          Thanks, I'm nearly done with revisions taking all of this into account. Will post back tomorrow as the routing needs completely re-doing.


            I'm trying to keep a copy of the Original Drawing updated with the changes from this thread. I still have some questions:

            1. Do the Address Lines still need Pullup Resistors even though the signals are generated by the 74LS244 for XA0 thru XA7?
            Ref: Attached 74LS244.png

            2. Also I'd need to add Pullup resistors for:

            3. What gives you the ENABLE (LOW) for the 74LS244? You've skipped that enable information.
            (I only show 1/2 of the 74LS244 on the updated drawing.)

            Is this what you currently have wired on your Breadboard, with the exception of the Address Lines Pullup Resistors and the other signals referenced in Question 2?

            I'm wondering if you can easily remove the Address Line Pullups and see if the unit still functions. Is that possible?


            Attached Files
            Last edited by ldkraemer; June 17, 2013, 03:09 AM.


              What did you do with pins 1 & 19? are the outputs coming from the LS244 High or Low? If you want the outputs low tie pins 1 & 19 to ground.

              LS240 is an inverting line driver. See data sheet.

              I tried buffering the Data lines using an LS245 could not get 80 on port 47H actually all pins were 120. So just be advised.

              LS244 data sheet:
              Last edited by Chromedome45; June 16, 2013, 03:57 PM.


                Larry, for the tri-state control line I just tied it low. I tied the unused one high and grounded all the unused inputs. For the address and control lines, given they are driven with tri-state logic, I am going to leave the pull-up resistors in for the moment.

                Frank, for the half of line driver you are using, tie the control pin low.

                If you have an ls240 laying around its worth trying that in place of the ls04 and ls244.

                Last edited by FireBox; June 16, 2013, 04:16 PM.


                  Matt, So pins 1 & 19 are low? Getting 255 on 40H and 80 on 47H. Oh your still using CP/M still trying LDOS
                  Last edited by Chromedome45; June 16, 2013, 04:55 PM.


                    Frank, do still have the ls245 in your circuit?



                      No I took it out. Didn't work anyway. All's I got on the ports 40-47h were 127 on each of the 8 ports. I am trying your latest mods and still that software for LDOS doesn't work. Put in 1 head and 32 tracks and it's still sitting there! Port 47h is giving 80 with this mod.


                        We need to get an LDOS expert to look at the hd source code (from that link) to see if they can figure it out. I looked at it yesterday and it looked ok but again I am no expert.



                          Also tried the RS version non modified and same thing but I kind of expected that. If you have any more hardware ideas let me know.


                            I think you have the interface working given you are getting 80 decimal back from port 47h. We need to find someone with the LDOS assembler to recreate the hd executables you are using ensuring the IDE switch is set to true in the code.



                              I searched until I located the schematics for the Radio Shack 5 Meg Hard Drive. It appears that they are using 74LS244's for the Address Bus and also the other Control Signals.
                              REF: 5-MEG-HD.png

                              Would it be wise to follow their lead, and use 74LS244's for the Control Signals and Address Lines versus the Pullup Resistors for both? Just asking.........


                              Attached Files
                              Last edited by ldkraemer; June 16, 2013, 05:55 PM.


                                Larry, I think it would be. What this thing is really doing is acting as a line receiver for the TRS-80 side and a line driver for the IDE side.

                                That is why I recommend replacing the LS04 and LS244 design with a single LS240. This will at least pickup the control lines to the IDE drive and the /EXTIOSEL line back to the TRS80 IO interface. It would be nice to buffer the address lines as well.

                                The one part about the pull-up resistors the buffers will not provide is creating a known state if the interface is disconnected and powered on -or- when the interface is connected and the Model 4 IO bus is in tri-state (like it is when at power it up and then idle). I probably wont buffer the address lines. They seem to be doing fine with the resistors + there is accidental write protection with the resistors in place.

                                I need to order some parts from Digikey for another project and at that time I will order some LS240s to mod the interface.

                                It would be great to keep it at 3 ICs