
How to add external disk drives to your TRS-80 Model 4P
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These modifications are not approved by Tandy, if you choose to make these modifications, you are doing them on your own.  If you have any warranty left on your 4P at all I would suggest that you wait until it has expired before you attempt to do anything to your computer that Tandy does not approve.  To make these modifications a good knowledge of soldering printed circuit boards is recommended.  These modifications are spelled out in what I hope is a clear and concise manner.  If you have any questions about them, then before you even start, give me a call on my BBS at 317 882 9330.  Leave me your name and a brief explanation of what needs clarified.  Also your phone number if you would like me to call you (collect).  This text file is also posted on my system in the TRS-80 conference.  The following texts were used as reference materials in compiling this file.

  Model 4/4P Technical Reference Manual   Cat # 26-2119
  Model 4P,4P Gate Array Service Manual   Cat # 26-1080
  The January 1986 80Micro Magazine

I have made these modifications to my own 4P and they worked fine, I will not assume any damages caused to your computer if you choose to follow the instructions in this text.

First, remove the cover of your 4P, it is held on by a total of 6 screws.  There are two painted screws by the cover latch and two screws under the handle.  Set the unit screen side down on a protective material and then gently lift the housing straight up.  Do not force it off, you may need to wiggle it a bit back and forth.

Once you have the cover off and are looking straight down into the machine you will see the disk drives.  Remove the shield housing from around the drives, there should be 7 screws holding it on.

Once you have that off then you need to remove the drive cable. Here is where things get a little more difficult.  First you now need to take a piece of 34 conductor wire that is about 36" long and add the connectors to it.  I suggest buying all new connectors but you can use the old ones if you want to.  First attach a 34 pin female card connector to one end of the cable, this will attach to the FDC board.  MAKE A NOTE OF HOW THE OLD CABLE IS CONSTRUCTED....WHAT YOU ARE ABOUT TO DO IS TO RE-ATTACH CONNECTORS TO YOUR NEW RIBBON IN THE SAME WAY THAT THE OLD ONES ARE WITH THE EXCEPTION OF THE CABLE PROTRUDING OUT ABOUT TWO FEET PAST WHERE YOUR DRIVE 1 CONNECTOR IS.  IF YOU ARE USING NEW CONNECTORS YOU NEED TO DO THE FOLLOWING TO THEM FIRST!!  TO ACHIEVE PROPER DRIVE SELECT, GET THE CONNECTOR YOU ARE GOING TO USE FOR DRIVE 0 (ZERO) AND PULL OUT PIN NUMBER 10 (TEN), THE CONNECTOR FOR DRIVE 1 (ONE) NEEDS TO HAVE PIN 12 REMOVED.  ALSO ON THE CONNECTOR FOR DRIVE 2 (TWO) YOU HAVE TO PULL PIN 14 AND FOR DRIVE 3 (THREE) YOU NEED TO PULL PIN 6 (SIX).

Once you have placed connectors for drives 0, 1 and the FDC on the cable it should look JUST like you old cable with the exception of about 2 feet of wire left past the drive 1 connector.

Like this:

  Old Cable: H======H======H <----Drive 1 connector
             H======H======H      Pin 12 pulled
      ------>H======H======H
     |               <---
     FDC connector      |--Drive 0 connector
                           Pin 10 pulled


                         <--Drive 1 - Pin 12 pulled
  New Cable: H====H=====H===================
             H====H=====H===================
      FDC -->H====H=====H===================
                   <--Drive 0
                      Pin 10 pulled

After you have the cable made, attach it back on the drives and the FDC, now down somewhere toward the end put the Drive 2 and Drive 3 connectors on the cable.  BE SURE AND PULL PIN 14 FOR DRIVE 2 AND PIN 6 FOR DRIVE 3!  Place them about 14" apart.

Now for the fun part...modifying the Mother Board.

If you have an internal modem in place..remove it now.

In order to access the external drives you must now make a few modifications to the motherboard.  If you have a Gate Array version you have it made.  If you have the older version then pay close attention to your section.

If you have clustered arrow keys and a factory green screen monitor, odds are that you have a Gate Array version of the Model 4P.  If you have separated arrow keys and a B & W monitor then you most likely have a older version.

The motherboard is enclosed in a protective case on the bottom of the computer.  The case needs to opened up by removing the eight screws found four per side.  Once you have the screws removed then pry the casing apart. There are four power connectors attached to the motherboard and they are notched so that they will only go on one way.  Look at them and remember how they are plugged in.  Remove the connectors.  Now remove the entire motherboard and its casing.  MAKE SURE YOU ARE GROUNDED WITH GROUNDING STRAPS OR JUST PLAIN BE CAREFUL.  YOU DON'T WANT TO BLOW ANY CHIPS.  Remove the keyboard cable and make note of how it is attached to the motherboard.  Then remove the motherboard by removing the supporting screws, they are chrome plated.

Now place the removed motherboard on a soft surface and go get something to drink.  At this point my nerves were a little worn to say the least.

Here are the following modifications you will need to make.

ORIGINAL MODEL 4 VERSION..B & W MONITOR AND SPLIT ARROW KEYS..

First locate U75  (74LS02)

  Now clip pins 1 2 and 3 as close to the IC body as possible. Take your NEW 74LS260 (you need to buy this) and bend all pins straight out except for pins 7 and 14.  You can cut off pins 4,6,8,9,10, and 11.

  Now locate U53 and piggyback your 74LS260.  Make sure that pin 1 aligns with pin 1.  Solder very carefully pin 7 to 7 and 14 to 14.

  Now for the jumpers:

    Component side jumps are:

     FROM                      TO
     U32 PIN 5                 74LS260, PIN 2
     U32 PIN 2                 74LS260, PIN 1
     U32 PIN 10                74LS260, PIN 12
     U32 PIN 7                 74LS260, PIN 3
     74LS260, PIN 7            74LS260, PIN 13
     U74 PIN 13                74LS260, PIN 5
     U32 PIN 7                 U20 PIN 9
     U32 PIN 10                U20 PIN 11

   Solder side jumps:

     FROM                      TO
     U20, PIN 8                J5, PIN 14
     U20, PIN 10               J5, PIN 6

Now button things back up a little bit, attach your external single sided drives and try it out.

MODEL 4P GATE ARRAY VERSION - BY FAR THE EASIEST

    No pin clips at all, just do the following:

    COMPONENT SIDE JUMPS:

     U14 PIN 9        TO       U34 PIN 7
     U14 PIN 13       TO       U34 PIN 10

    SOLDER SIDE JUMPS:

     U14 PIN 8        TO       J5 PIN 14
     U14 PIN 12       TO       J5 PIN 6

That should just about do it.  Again, I will assume no liability for any damages that you may cause your computer.  I have made these modifications to my own 4P and they worked fine.  I only had one external drive though.  If all you want to add is one then just don't put the connector on for drive number 3.  If you have any problems or questions before you try and make these changes, please call my BBS at 317 882 9330, 24 hours a day at either 300 or 1200 baud.

Brian Murrey for TRSLINK

