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--------------------------------------------------------------------------- 

                    ONE FOR THE MONEY, TWO FOR THE SLOW:

---------------------------------------------------------------------------     

                      ADDING A SECOND DRIVE TO THE PCjr

                           by David G. Brader

                                HCM Staff     

         ------------------------------------------------------------

         Much software designed for the IBM PC would run faster and

         more conveniently on the PCjr if the smaller machine had a

         second disk drive.  Now you can add your own...

         ------------------------------------------------------------

[Caveat:  What follows is a hardware      and add a second drive.  Now here was

modification procedure for a PCjr         a challenge!

with 128K of memory and the IBM             Our answer to this challenge? 

single-disk system.  The successful       Modification of the disk-drive con-

implementation of this procedure will     troller board, requiring only the

result in a PCjr system capable of        addition of two IC's (integrated

accessing two disk drives.  This          circuits), the opening of two circuit

project assumes that you have the         paths on the board, and the construc-

requisite technical skills and            tion of a new flat cable.

knowledge to complete it.  We have          Initial tests of the hardware un-

done our best to include all the          covered a roadblock--The IBM PCjr

necessary data and procedural steps       BIOS program (Basic Input/Output

to guide a knowledgeable technician.      System stored in ROM memory) was the

Home Computer Magazine, its publish-      culprit.  When Junior is reset or

er, and the author assume no liabil-      powered up, BIOS sets what are called

ity for unsuccessful project comple-      "equipment status software flags" in

tion or damage to any of your             RAM memory which always report that

equipment.  Modification of the IBM       only one disk drive (device name A:)

PCjr will probably void any remaining     is connected.

warranty.  These instructions are           I therefore had to design a

offered as is, and readers should         software fix to fool the system. 

proceed at their own risk--Ed.]           With some special batch files

                                          installed on the PC-DOS 2.1 disk, I

The Drive for Success                     retested the system.  DIR B: was

                                          entered at the keyboard, the red

  Tired of swapping diskettes in and      light on the second drive came on,

out of your single-drive PCjr?  What      and a moment later the directory

if you were told, "For less than $300     listing of the data disk appeared on

(depending on how much you pay for        the PCjr screen--it worked!

the drive) and a few hours work you

can have a second disk drive on           Three To Get Ready

Junior" Well, now you can.

  When the PCjr was first announced         Before attempting this modification

as a single-drive system, we said,        you need to accomplish three tasks:

"Sure, bet all you have to do is          1) Read this entire article and study

stick an extension cable out the back     the illustrations carefully, (2)

of the box and daisy-chain a second       purchase the parts needed for the

drive to it."   We were wrong.  IBM       modification, and (3) gather the

very carefully de-engineered their        necessary tools.  Use the parts list

disk-drive controller  design making      in Figure 1 and the tool list in

it impossible to simply change cables     Figure 2 as checklists to make sure


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you have everything you need.  When       board.  This is easier said than

shopping for a second disk drive,         done--use care not to damage adjacent

make sure the drive is an IBM-com-        pins.  Bend the pin up and out from

patible, double-sided, double-density     the board (so that a wire can be

model, and that it comes with a case      soldered to it).  Move the capacitor

and power supply.  Don't order a          back in place and resolder its leads

cable with the drive because a            in the hole from which it was

special one must be built for this        removed.

application.

                                          Congrats, 10% Of The Job Is Done!

Voiding The Warranty

                                            Now the two new IC's must be

  It is important that you have good      prepared for mounting on the printed

lighting for this work.  Disconnect       circuit board.  Because there is

all cables from the rear of the PCjr      insufficient room to place these IC's

including the power cord.  Using a        on the main board area, they must be

wide-blade screwdriver, pry up the        "piggy-backed" onto other IC's that

top cover at the rear of the unit.        are already mounted on the board. 

Slide the cover back and remove it.       Bend the "parasite" IC's pins

Locate the disk-drive controller          straight out, leave them alone, or

board with the wide flat cable            cut them off, depending on the use of

connecting it to the disk drive.          each one (as described below and

Disconnect the cable at the disk          shown in the accompanying photos). 

drive and carefully remove the            The pins are counted counterclockwise

controller board.  Remove the flat        as viewed from the top of the IC. 

cable from the controller board and       Pin 1 is always located to the left

lay it aside.  Place the controller       of the dimple that is in one end of

board, component side up, on a clean      an IC.

surface.                                    The first one of our IC's is piggy-

  Referring to the board diagram in       backed to the existing IC at location

Figure 3, use a small, sharp pair of      ZM1 (see figure 3) which is of the

diagonal cutters to cut pin 1 of          same type as the new IC (74LS175). 

integrated circuit ZM21 free of from      Investigation of the design shows

the printed circuit board (snip it as     that both IC's use or need to be

close to the board as possible).          connected to the same CLEAR, CLOCK,

Carefully bend the lead up and out        and DO inputs (pins 9, 1, and 4

from the board.  We will be soldering     respectively).  They also share the

a wire to it later.                       +5 volt power and ground pins (pins

  One other integrated circuit (IC)       16 and 8).  By bending all the other

pin also requires this procedure, but     pins (numbers 2, 3, 5, 6, 7, 10, 11,

the task will be more difficult           12, 13, 14 and 15) up and straight

because pin 13 of ZX4 is partially        out from the IC body, the IC is ready

hidden by a capacitor mounted on the      for mounting (actually, pins 10

board.  Using the soldering iron,         through 15 can be clipped off because

melt the joint at the left lead of        they are not used).  The prepared IC

the capacitor while gently pulling up     is shown in the accompanying photo. 

on the lead with the long-nosed           To actually mount this parasite IC,

pliers until the capacitor lead comes     it must be carefully aligned and

free from the board.  Carefully bend      seated on the host IC (ZM1), and all

the capacitor away from the area of       shared pins joined with solder (pin 1

pin 13 on IC ZX4.  Using the small        to pin 1, pin 4 to pin 4, etc.).  Be

cutters, snip the pin free from the       careful not to mount it backwards.


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  On the other parasite IC (74LS10)       wire is installed, try pressing the

to be mounted there are only two          old ribbon cable connector onto U2 to

shared pins.  The +5 volt power and       make sure it fits tightly over all

ground pins (pins 14 and 7) are the       pins.  After checking the connector

ones used to piggy-back this IC.  All     fit, carefully remove the flat cable

other pins are bent up and out from       from U2 and set it aside.  The disk-

the IC body.  The unused pins, 1, 2,      drive controller board is now ready

12, and 13, may be clipped off.  The      for installation in the PCjr.  Place

host IC for this parasite is in           it aside for the moment.

location ZM9, as shown in Figure 3

(the host IC is a type 74LS08).           Making The New Flat Cable

Carefully align and seat the new IC

on the ZM9 host and solder pin 14 to        A new cable must be made that

pin 14 and pin 7 to pin 7.                replaces the old flat cable and that

                                          extends out the side of the PCjr. 

Time To Get Wired, Folks!                 This requires the installation of

                                          three specially-matched, ribbon cable

  There are ten wires that must now       connectors at the right points along

be connected to the printed circuit       the length of a flat ribbon cable. 

board.  Using a low-wattage soldering     These connectors are usually in-

iron, attach the wires according to       stalled using a special arbor press

the chart in Figure 4.  Refer to the      tool and custom dies, but it is

partial schematic in Figure 5 showing     possible to install them yourself. 

the affected portion of the disk          Slide the ribbon cable through the

controller logic, the pictorial           gap formed by the main body of the

illustration in Figure 3 for parts        connector and its plastic backing

location, and the photograph of the       plate until the connector is located

wired board.  Double-check each           at the right spot on the cable. 

solder connection that you make for       Using your hands, press the plastic

correct location and a solid joint.       backing plate into the main connector

In each case, inspect with a              body as hard as you can to keep it

magnifying class to see if any of the     from moving during the final step. 

solder flow caused a short to an          Using a wood-working vise (don't use

adjacent connection.                      a vise with metal jaws), clamp the

  The wire that connects ZM9 (piggy-      new connector assembly in such a

back) pin 8 to connector U2 pin 10        fashion as to apply further pressure

must be installed with additional         o seat the plastic backing plate into

precautions.  First, identify the         the connector's main body.  Tighten

correct U2 connector pin by holding       the vise slowly while watching the

the board in front of you and             gap between the flat cable and the

counting 5 over from the right side       connector body.  Stop tightening when

on the bottom row as shown in Figure      the cable and connector appear to

3.  The wire must be wrapped tightly      have formed an airtight seal. 

at the very bottom of the pin and         WARNING: OVER-TIGHTENING THE VISE MAY

soldered with a minimum of solder.        DAMAGE THE CONNECTOR CABLE ASSEMBLY.

If the length of the pin is obstruc-      |  Starting with one end of the

ted by the wire being placed too high     |ribbon cable, install the connector

or excess buildup of solder, the flat     |that will mate with the controller

ribbon cable connector that mates         |board match the distance between the 

with U2 will not seat properly,           |connectors on the old IBM flat

causing possible intermittent opera-      |cable.  Install this  connector on

tion of the disk system.  After the       the same side of the cable as the old


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IBM cable. The last connector, which      configuration of the new drive that

will mate with the new external disk      you purchased.

drive, is mounted on the other end of     ------------------------------------

the cable (facing the same side as            "Initial tests of the hardware

the other disk drive connector).               uncovered a roadblock--the

  Notice how the old IBM cable was                  IBM PCjr BIOS.....

folded, and duplicate folds in the        -------------------------------------

new cable.  [Home Computer Magazine         The new disk drive must be set up

has a kit available for this project.     as "Device Name B:" for the Disk

It includes the two integrated            Operating System.  Locate the drive-

circuits and the new finished flat        select jumper area on the drive's

cable.  The cost of the kit is            circuit board.  Usually, it will have

$49.95.--Ed.]                             letters printed on the circuit board

                                          next to it such as DS0, DS1, DS2, and

Cable Installation                        DS3 (sometimes they are numbered 1,

                                          2, 3, and 4).  Make sure that all of

  Install the modified controller         these jumper options are "open"

board in the PCjr, connect the new        except the second from the lowest,

cable to the controller board, and        which should be jumpered.  This will

connect the internal disk drive.          select this drive as the second one

Fold the free end of the cable so         ('B:').  Now locate the "load

that it passes over the right side        resistor" IC pack socket near the

where the slot is molded into the         drive-select jumper area.  If the

case lip.  Crease the cable so it         socket is not empty, remove the

will pass up and over any attachment      resistor pack and discard it.  Locate

when the PCjr lid is replaced.            the edge-type connector of the new

                                          drive.  Press the last connector of

Disk Drive Address Selection              the new flat cable (coming out of the

                                          PCjr) onto this edge-type connector. 

  You've got one last item to take        Plug in the power on the new drive

care  of before replacing the PCjr        and turn on its power switch.  If the

lid--the internal disk-drive selector     red light of the new drive comes on

jumper.  Notice that there are four       and says on, the flat cable connector

sets of pins on the disk-drive            is upside down.  Turn off the power

circuit board next to the edge-type       of the second drive, reverse the flat

connector with the new flat cable.  A     cable connector  the new drive end,

plastic and metal jumper device is        and try again.  The red light should

mounted on the second set of pins         not come on.  Finally, replace the

from the left.  Pull this jumper off      external disk drive's cover while

that set and press into the far left      guiding the new cable out the unit's

set of pins.  This will ensure that       cable slot.

the system recognizes this disk drive

as "Device Name A:" which is the          Fooling PCjr BIOS

default drive at start-up time. 

Replace the PCjr lid, being careful         Any diskette that you use to "boot"

to guide the cable out the slotted        from will need to have at least the

cover lip.                                following files on it if it is to

  Place your new external disk drive      utilize the new disk drive:

to the right of the PCjr and remove          File Name          Origin

its cover.  The instructions that            COMMAND.COM        IBM DOS DISK

follow are general in nature because         DISKCOPY.COM       IBM DOS DISK

we have no way of knowing the precise        DEBUG.COM          IBM DOS DISK


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   AUTOEXEC.BAT       SOURCE BELOW        from the start.  This time the

   BOOT.BAT           SOURCE BELOW        SWITCH.BAT file doesn't exist, so the

   MODBOOT.BAT        SOURCE BELOW        program logic will GOTO the label

   SWITCH.BAT         SOURCE BELOW        :last.  At this point, the OFF.BAT

  Make certain that the diskette has      file is renamed to SWITCH.BAT and the

been formatted with the /S option so      DATE and TIME prompts appear.

that the IBM PC-DOS 2.1 and the             The BOOT.BAT file simply starts up 

COMMAND.COM file may be installed          the IBM DEBUG.COM utility with

before installing the rest of the         directions to take debug commands

files.  Then copy the DEBUG.COM and       from the MODBOOT.BAT file instead of

DISKCOPY.COM files from the master        the keyboard:

IBM PC-DOS 2.1 diskette onto the new

boot disk.  You can use a text                 DEBUG<MODBOOT.BAT

processor, such as the IBM EDLIN that

comes on the master PC-DOS 2.1 disk,        The MODBOOT.BAT file causes four

to create the four BATch files            things to happen:  It (1)assembles a

described here.                           short assembly language routine

-------------------------------------     starting at location Hexadecimal 9080

     "...BOOT will indirectly cause       in memory; (2)executes this same

     changes in the information that      routine causing the bit in the

        BIOS stored in RAM..."            system's Equipment Status byte (which

-------------------------------------     signals that a second disk drive is

  The AUTOEXEC.BAT file must contain      on line) to be set; (3)loads the

the following lines exactly as they       "boot track" from the diskette into

appear here, in order that the            memory starting at Hexadecimal 7C00;

appear:                                   and (4)executes this boot.  MOD-

                                          BOOT.BAT looks like this:

     IF EXIST switch.bat GOTO first

     GOTO last                                 A0:9080

     :first                                    XOR AX,AX

     RENAME switch.bat off.bat                 MOV DS,AX

     BOOT                                      OR BY [410],40

     :last                                     NOP

     RENAME off.bat switch.bat                        (insert blank line here)

     DATE                                      G=0:9080 9089

     TIME                                      L 0:7C00 0 0 1

                                               G=0:7C00

  AUTOEXEC.BAT is automatically

executed each time the PCjr system is       The last file to be created allows

powered up or reset.  As it executes,     AUTOEXEC.BAT to detect where it is in

it first checks to see if the file        its sequence.  The SWITCH.BAT file is

SWITCH.BAT exists on the disk in          only a dummy which must exist on the

drive A:.  If it does (and it should)     disk, but may contain anything at

the program logic will GOTO the line      all.  We simply entered a comment

labeled :last.  Here the program          line describing its function as

renames the SWITCH.BAT file to            follows:

OFF.BAT and executes the file called

BOOT.BAT.  BOOT will indirectly cause          REM This is a dummy file

changes in the information that BIOS

stored in RAM and will affect a             Once these four files are on your

restart of the system.  This means        new boot disk, label that disk as the

that AUTOEXEC.BAT is executed again       PCjr Dual Disk Drive System Disk.


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                                          [The above was taken from the Septem-

Now, Go Man Go!                           ber 1984 issue of Home Computer

                                          Magazine, pages 82-86.  All schemat-

  This is it.  Make sure the new Dual     ics included below are simplified

Disk Drive System Disk is in the A:       sketches of the original, not full

drive (the one in the PCjr case).         copies.  This reproduces the full

Place any other good IBM formatted        text of the article, (including the

diskette in the new external drive.       obvious typo marked with | in margin)

Turn on the power to all system units     along with as many of the Diagrams

(the PCjr itself should always be         and Figures (see below) as possible. 

last), and watch the screen.  After       Obviously none of the 7 photographs

about 45 seconds of clicks, flashes,      are included.  This magazine is no

and text scrolling on the screen, you     longer available because it is no

should see the prompt for the date.       longer in business so the "kit"

Enter the date and the time.              mentioned in the article is not

  The big test is to now try access-      available from them either, although

ing the second drive.  Enter DIR B:       it is available from other vendors.  

and press the [ENTER] key.  You            Also, the procedure for getting the

should see the second disk-drive          PCjr BIOS to recognize additional

light come on, and the directory of       disk drives may not be necessary if

the diskette in ;that drive should        you use the Public Domain JRCONFIG

appear on the screen.  If it doesn't,     program by Larry Newcomb; his program

recheck each of the new files that        seems to do the same thing (and much

you placed on the boot disk for           more). 

errors.  Also, have your modification     ------------------------------------

work checked by someone else for          Figure 1.              TOOL LIST     

errors. Once you are successful,           Description               Use

place a blank diskette into the new

drive (B:), enter DISKCOPY A: B: and      Small long-nosed    Bending leads on 

press [ENTER].  Press any key to          pliers              IC's

start the process.  Watch how fast

the system now copies a disk!  Isn't      Very small          Cutting IC leads

that worth all this work?  Label the      diagonal-side 

new disk as the working backup to the     cutters

Dual Disk Drive System Disk and copy

any other files that you usually use      "No nick" wire      Removing wire

(such as the Easy Writer II system        strippers           insulation

files).

  OK, that's it.  Have fun exploring      Low-wattage         Soldering  

the wonderful world of IBM PC             soldering iron      connections 

software!

NOTE: Listed above are the manufac-       Fine-wire rosin-    Soldering

turers of the specific products what      core solder         connections

were used in our project.  There are

other manufacturers of similar            Spool of #28       Additional

products that will work as well.          tinned-copper      circuitry  

Sources for these items are major         wire with                    

electronics component distributors        insulation 

and mail order houses.  Sorry, you

are on your own when locating the         Wooden-jaw vise    Making flat cable

disk drive.
                               Screwdriver set    Various uses   


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Figure 2.             PARTS LIST For PCjr Disk Drive Controller       


QTY  DESCRIPTION                   MANUFACTURER   

 1   SN74LS175 IC                Texas Instrument   

 1   SN74LS10  IC                Texas Instrument

 2   RF16-2852-0                 Texas Instrument

       Ribbon Cable Receptacle                

 1   SMF34-2841-0                Kel-Am Inc.   

       Ribbon Cable Socket                

 4   Feet of GEX28-34            Kel-Am Inc.   

       Ribbon Cable                 

 1   IBM compatible  dual-       MPI         

       sided  double-density 

       Floppy disk  drive with power 

       supply and case.   We used an 

       MPI model 52 with a case and 

       supply from a broken drive.


Figure 3.[Simplified sketch only; check Figure 4 and text for exact locations.]

_____________________________________________________________________________

|                             ,--------------------------------.             |

| U2 ____________________     |       XXXXXXXXXXXXXXXXXXXXX    |             |

|   [______________._____]  ***       ]XXXXXXX ZM5 XXXXXXXX    |             |  

|                  `........|||.      XXXXXXXXXXXXXXXXXXXXX    |             |

| ]XXX   ]XXX ]XXX ]XXX ]XXXXXXX ZM9   ,,,,,,|           __    |  ]XXXX      |

|                      ::::.**|        ,             ---{__}-- |             |

|  ZM21                ::     `--------,----------------,   ZM4|        ZM1  |

| ]XXXX ]XXX ]XXX ]XXX :: ]XXXX ]XXXX  ,  ]XXXXXXX    ]XXXXXXX |_]XXXXXX     |

| :                    -:--------------;;;;;;;;;;;;;;;;;;;;;;;;;;;;,;::..... |

| :                     :                                            :.... : |

| : ]XXX ]XXX ]XXX ]XXX : [[]]  ]XXXX                             ]XXXXX : : |

| :                     :................................................: : |

| :........................................................................: |

|____________________________________________________________________________|


Figure 4.

             Wire Connection Chart for Controller Board 


   From Location                         To Location


IC                 Pin #                 IC                   Pin #

ZM1(piggyback)         2                 ZM9(piggyback)           9

ZM9(piggyback)         9                 ZM9(piggyback)          10

ZM9(piggyback)        10                 ZM9(piggyback)          11

ZM1(piggyback)         3                 ZM9(piggyback)           3

ZM1(piggyback)         5                 ZM5                      7

ZM1(piggyback)         6                 ZM9(piggyback)           4

ZM9(piggyback)         4                 ZM9(piggyback)           5

ZM1(piggyback)         7                 ZM21                     1

ZM9(piggyback)         6                 ZM4                     13

ZM9(piggyback)         8                 U2(connector)           10


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Figure 5. [Simplified]


             ..................*.....................

                               :                    =<cut    ZM4

                               :                    :   13|\ 12 

                               :                    `--*--| >o------------

                               :                       |  |/  Motor Enable

                               :     3 _ZM9            |              

                               : .----*\   \ 6         |

      ,------------------------:-|--*--*>4  }o---------'                       

      |                        : |  `-*/___/                       

      |                        : |   5    74LS10

      | .----------------------:-'               

      | |                      :    9  _ZM9         

      | |                      :   *--*\   \ 8  Drive Select 0

      | | .--------------------:---*--* >10 }o------------------------------

      | | |                    :   *--*/___/                      U2 PIN 10

      | | |                    :    11   74LS10     

      | | |                    :         

      | | |                    =<cut   |\  3    Drive Select 1

      | | |                    `.*----*| \o--------------------------------

      | | |                      |   1 | /      

      | | |                      |     |/ ZM21

      | | |   _____ ZM1(74LS175) |    

      | | |-o|1  15|-- ,---------'         

      | | `--|2  14|o- |

      | `---o|3  13|-- |

      |     -|4  12|-- |

      |  ....|5  11|o- |                

      `--|--o|6  10|-- |                             

         | ..|7   9|-- |                      ZM5

         | | |_____|   |                 ___________

         | |           |                 |1         | UPD765

         | `-----------'                 |6         | 

         |                      ----*----|7         |

         `--------------------------'    |8         |

                                         |9         |

                                         |10        |

                                         |11        |

                                         |12        |

                                         |13        |

                                         |        14|--

                                         |          |

                                   ,-----|16        |

                                   |     |          |

                                  ====   |          |

                                   --    |          |

                                       -o|3         |

                                       -o|2         |

                                       -o|4         |

                                       --|5         |

                                         :__________|


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