Saturday, February 8, 2014

Introduction to the PCjr., or How NOT to Destroy It!

IBM PCjr. Startup Screen - Maximum RAM Count (but you can have more)
Congratulations!  You have just won a PCjr off ebay, scored one off craigslist, took one from the recycling center or was given one by Uncle and Aunt Lots-o'-Stuff in the Attic.  Its just arrived, you open the box, unwrap all the pieces, now what do you do with it?

First Things First : To use a PCjr., you need, at a minimum the system unit, the keyboard and the power cable.  If you are missing any one of these three, the PCjr. is useless.  I am going to go through each in terms of simple to complex :

The External Power Adapter: This is a large black power converter.  It has wires on either end.  One the end of one wire is a three-pronged plug.  On the other end is a rectangular three pin connector that connects to the back of the system unit.  The port is in between the A and the C ports.  On early bricks, the cable with the three-pronged plug is detachable from the large brick.  A standard three-pronged computer power cable should fit into the brick.  The brick only supports 120v/60Hz AC power, so in Europe you will need a step-down converter.  It outputs AC power, the AC to DC conversion is done inside the system unit.  The power cable can fit into the system either way (polarity is irrelevant for AC).

The Cordless Keyboard : The original Cordless Keyboard, later known by everybody, even IBM as the "chicklet" keyboard is the less common of the two IBM PCjr. Cordless Keyboards, especially after IBM offered a free trade in to the non-chicklet keyboard.  IBM called the replacement keyboard the "Freeboard", presumably because it offered some freedom from the frustration of the chicklet and was a free replacement for PCjr. owners who hated the chicklet keyboard.  Internally, they function identically.

The PCjr. Cordless Keyboards use rubber-dome technology and can feel stiff yet squishy.  The keyboard communicates with the system via an infrared sensor or via an optional keyboard cable.  It takes 4 x AA batteries, and you should check the battery compartment to see if there are old corroded batteries.  If there are, you should clean the contacts with cotton swabs or Q-tips with apple cider vinegar or lemon juice and follow it up with baking soda.  You may need to unscrew the board to inspect for leakage on the PCB.  The infrared can work up to twenty feet away according to IBM, but line of sight must be maintained and the sensor on the keyboard should be as perpendicular as possible to the sensor on the computer.  Rechargeable batteries may or may not provide reliable power for the keyboard.

The System Unit : The computer is inside on a large system board.  Most PCjr's you will find these days are the Enhanced Model, which come with a 5.25" diskette (floppy disk) drive and a 64KB memory expansion.  These have 128KB of RAM total, not counting any extra memory added via sidecar.  The Basic Model did not come with a diskette drive or the 64KB memory expansion.  If you are getting a boxed unit, it should come with all the above and two books, the first being the PCjr. Guide to Operations and the second being the "Hands On BASIC for the IBM PCjr."  A complete unit should also come with a plastic plate to cover the sidecar expansion and a Basic Model also comes with a plastic cutout to cover the floppy bay hole.

Checking for damage : The expansion ports on the rear of the PCjr. use BERG-style connectors, which little more than pins bent at right angles sticking out from the system board.  These are more prone to damage than connectors set in a D-shell, but it was cheaper for IBM to implement them in this way.  Check for any bent, broken or pushed in pins.  Most of the connectors have a missing "key pin", so check below to discover pins that are supposed to be missing.  Gently straighten or pull out any pins with a small pair of pliers or tweezers.  Expansion cables are held in the port by friction alone, so do not try yanking cables out. Similarly, the sidecar expansion port also uses pins, but at least these are cased in a plastic connector like an IDE port.

Opening the case : The case top is held down by friction and tabs, there are no screws to deal with.  Using a small screwdriver, insert it into each of the three indentations at top rear edge and gently pop that portion of the case off.

Inside the case : There are up to four internal expansion boards.  Fortunately each can only fit one way.  There are no jumpers or dipswitches inside or on the internal expansion board.  IBM did not use jumpers (officially) on its sidecar expansion board either.  Everything is configured in software, which was rather progressive for the time and technology.  If you want a true definition of plug and play, here it is!

On the left side of the unit (with the cartridge ports facing you), is the power board, which is always present.  The power board comes in either a 33W short card or a 45W long card.  If the power card is about 2/3rds the length of the case, it is the short card.  If it extends nearly to the front of the case, it is the long card.  There is a 2-pin staking connector which the power board must be inserted into when it is put back in its slot.  There are grooves in the case to stabilize the card, make sure the card fits into them.

The second board is the 64KB Memory and Display Option.  As its name suggests, this adds 64KB to the 64KB on the motherboard for a total of 128KB inside the System Unit.  Early memory boards are covered in a metal shield, later boards are just a bare PCB.  If this board is not present, you will only have 64KB of RAM in the system and a big hole if any sidecar memory expansions exist.

The third board is the PCjr. Internal Modem Option, and as it was always an optional upgrade, it probably won't be present in your average PCjr. System Unit.  IBM's modem was not very popular since it only communicated at 300 baud and did not use the industry standard Hayes command set.  Third party Hayes-compatible modems were also made to fit in this slot.  Like the power board, this card connects to a 2-pin staking connector and grooves in addition to its slot.  This is the only internal upgrade not to come automatically bundled with the PCjr. System Unit.

The fourth board is the PCjr. Diskette Adapter.  This connects to the Diskette Drive via a 34-pin ribbon cable with a pin header on one end and a card edge connector on the other.  It only supports the one internal drive.  Note the side of the cable with the colored wire, that side must correspond to pin 1 on the board if the diskette drive is going to work properly.  Pin 5 is missing on the adapter pin header as a key pin to prevent wrongful insertion.

The diskette drive is mounted on a unique plastic drive support.  It is connected to the board by two standoffs that push through the bottom of the case.  To remove the drive support mechanism (and the drive itself), first unplug the cables for the diskette drive power connector and the fan power connector from the power board.  Next, disconnect the drive data cable.  Next, with a pair of pliers, squeeze the tabs in and push upwards on the standoffs until the constrict and pop up from the bottom of the case.  These are not screws, so do not twist a screwdriver in them.  Finally, pull up on the back of the diskette drive and it should be able to be lifted out of the system with no further difficulty.

If you have removed all the internal boards and the diskette drive support, you will have complete access to the interior of the PCjr.  Check for any signs of damage and be prepared to dust.  There is a raised metal box on a small PCB, that is the Infrared Receiver.  Make sure it is firmly seated on its slot.  The receiver is connected to the System Unit by its connector and two plastic standoffs, so you won't be able to remove it without removing the System Unit itself.  The System Unit can be removed from the case by unscrewing four screws at the rear of the system, at least one screw holding down the cartridge ports and removing whatever sidecars are attached.

Rear Expansion Ports :

Except for the RCA ports for video and audio and the modem port (if one is installed), there is nothing standard about the PCjr.'s expansion ports.  However, the hardware that the PCjr. is designed to connect to is standard with one exception.  Looking at the rear of the machine, the ports are lettered as follows :

J   J   L   K   LP    T             V      M   D         S        C      P    A

J = Joystick - There are two joystick ports in the PCjr., with the first port,  sometimes called the right joystick being closest to the edge of the System Unit.  The inner port is sometimes called the left joystick.  These connect to the PCjr. Joystick, which functions just like a standard PC joystick.  The PCjr. joystick just uses a different connector.  River Raid, Demon Attack and Microsurgeon will not work without a joystick connected.  8-pins, with Pin 1 missing as the key pin.

L = Spare - These pins are not connected to anything on the system board. 6-pins, with pin 4 missing as the key pin.

K = Keyboard - For the optional Keyboard Connector.  The keyboard connector has a phone jack on one end and a large RFI ferrite choke.  It plugs into the PCjr. keyboard and allows you to use the keyboard without IR and batteries.  Third party keyboard replacements also plug into this port, but must be specifically designed for the PCjr.  If there is anything plugged into this port, then the system unit will not receive the input from the IR receiver, even if its just an unconnected wire.  6-pins, with pin 1 missing as the key pin.

LP =  Light Pen - IBM never marketed a light pen with a PCjr. style connector, but PC Enterprises did.  6-pins with pin 6 missing as the key pin.

T = Television - For the TV Connector, an RF switchbox to connect to an antenna screw terminal on a TV.  You may need a 300 to 75 Ohm balun connector.  External Audio can be heard from this connector. 6-pins, with pin 2 missing as the key pin.

M = Modem - A standard RJ-11 phone jack connector will be found here if you have an internal modem installed, otherwise it is just a hole.

V = Video - RCA Composite Video jack, better than RF, but not as sharp as :

D = Direct Drive Video - This connector supplies a CGA-compatible video signal and a mono audio signal.  The standard IBM PC Color Display Model 5153 or IBM Enhanced Color Display Model 5154 or compatible monitors can connect to it via a 9-pin adapter.  IBM released one because its PCjr. Color Display was not available at launch.  The IBM PCjr. Color Display Model 4863 has a connector that plugs directly into this port.  The IBM PCjr. Color Display has a built-in speaker with a volume control wheel for the external audio output.  The IBM PC Convertible Color Display Model 5145 also uses the same connector as the PCjr. Color display, but may or may not have a speaker for the external audio.  18-pins.  An adapter exists to allow the PCjr. and Convertible displays to work with a 9-pin D-shell port.

S = Serial - Any serial device can use this port with a proper adapter, such as from IBM.  The only device I know of that has the connector for this port is the IBM PC Compact Printer.  An adapter exists to allow the Compact Printer to be used with a standard serial port.  16-pins.

C = Cassette - IBM made a cable to interface with cassette drives and cassettes for data storage and retrieval.  It has two mini-jacks and one micro-jack.  The cassette interface is functionally identical to the interface in the IBM PC Model 5150.  8-pins, with pin 5 missing as the key pin.

P = Power Connector - See above

A = Audio - RCA Mono Audio Jack, necessary to hear sounds from the external audio unless you are using the TV Connector or the 4863 PCjr. Color Display.

Cartridge Ports :

On the front of the machine, there are two cartridge slots.  These slots are functionally identical.  Like any cartridge slot, they can accumulate dust and should be cleaned with a Q-tip and 99% Isophropyl Alcohol.  The most common cartridge that can be found is the BASIC cartridge (IBM Cartridge BASIC).  This cartridge is intended to supplement the Cassette BASIC already in the system ROM and add support for the advanced sound and video capabilities of the PCjr. and diskette drive support.  Game cartridges are generally rare, especially the good ones from Activision and Imagic.  Lotus 1-2-3 is a rare example of a cartridge program with a cartridge for both slots.

Sidecar Port :

On the side of the PCjr., there is the expansion port of the system, the sidecar port.  If there are no sidecars installed, there may be a plastic faceplate, which snaps into two holes and can be easily pried off with a screw driver or fingers.  Sidecars are pushed onto the pins, then screwed into system unit or the next sidecar with a flat head screwdriver.  You can extend the length of your system quite considerably with expansion sidecars.

Most PCjr.s have at least an IBM Parallel Printer sidecar attached, as the PCjr. did not provide a parallel port in the system unit.  The Printer sidecar provides one unidirectional parallel port using the standard DB-25 connector.

Many PCjr.s also have a Memory Expansion Attachment sidecar, from IBM or a third party company.  These sidecars add at least 128KB of RAM to the system.  Few self-booting programs will recognize more than 128KB of RAM in a PCjr.  The memory expansion's device driver must be loaded in DOS for DOS to report more than 128KB available to programs.    A device driver called JRCONFIG will work with any memory expansion.

If you have a 33W power board in the system unit and use more than two sidecars, you will need the IBM PCjr. Power Attachment sidecar, which provides more power for sidecars.  This sidecar must be inserted before any sidecars it needs to power (closer to the system unit) and has a separate power supply identical to the PCjr. system unit power supply.  They use the same connector.  With a 45W power board, you can use three sidecars comfortably.

Turning the System On :

When you turn on the screen, you will immediately see, if the system is working at all, the IBM logo and the fifteen RGBI colors that a CGA monitor can display in addition to black.  The memory will be counted and tested.  Once the memory has been counted, the screen will change to a blinking cursor, the diskette drive will seek and try to boot a disk in the drive.  Any expansions with a bootable ROM, like a hard drive or SCSI expansion interface, will attempt to boot at this time.  If neither the diskette drive nor any bootable interface is available or functioning properly, then the PCjr. will boot a cartridge, and if no cartridge is inserted, then Cassette BASIC.

Fan and Heat :

The fan inside the system blows cool air on the floppy drive and diskette.  IBM felt this was necessary to prevent a disk from melting inside the drive after heavy usage.  The fan is not intended to cool the system, that is what all the vents are for.  You can disconnect the fan without immediate apparent issue if you do not often use the disk drive, but the fan improves airflow inside the case.  This should help the Power Module (the hottest element in the system by far) and the chips underneath the floppy drive (including the CPU).  Although you may be tempted by the prospect of silent operation, leave the fan on.

Keyboard Adventure and System Diagnostics :

The PCjr. has a built-in program called Keyboard Adventure.  This can be loaded by pressing the Esc key just after Cartridge or Cassette BASIC has loaded.  You cannot have pressed any other key before pressing the Esc key.  This program was made to help introduce first-time computer users to the keyboard, and can be used to test the keyboard.  A full description of the program can be found in the PCjr. Guide to Operations manual, which came with every system.  Press Ctrl + Alt + Del to exit and reboot the system.

To start system diagnostics, any time after the PC has booted, whether to BASIC or to a disk, press Ctrl + Alt + Ins.  This will appear to reboot the system, but the memory will be counted and you will be shown a screen with several ASCII graphics and numbers underneath them.   The tests are rather cryptic and are fully explained in the Guide to Operations with the exception of  Test Tags H, I, L, M, N, O & P.  Here are a list of the Test Tags and what they test :

Test Tag 1 - Diskette Drive (requires spare floppy disk)

Test Tag 4 - 40-Column Display modes and 16KB graphics modes

Test Tag 8 - 80-Column Display modes and 32KB graphics modes (requires IBM PCjr. 64KB Memory and Display Expansion Option)

Test Tag 5 - Light Pen

Test Tag 6 - Joysticks

Test Tag 9 - internal PC Speaker and 3-voice sound (external speaker, TV Connector or PCjr. Color Monitor required)

Test Tag J & K - Cordless Keyboard  (Test Tag J treats the keyboard as a 62-key keyboard, Test Tag K treats the keyboard as if it were an IBM PC 83-key keyboard and allows you to use key combinations for the missing keys)

Test Tag G - IBM PCjr. Internal Modem

Test Tag H - IBM PC Compact Printer

Test Tag I - RS-232 (with loopback Plug)

Test Tag L & M - IBM PCjr. Cluster Adapter Attachment

Test Tag N, O & P - IBM PCjr. Speech Adapter Attachment

To escape from any test, press Fn + Break.  Press Ctrl + Alt + Del to exit and reboot the system.

Error Messages :

If you are unfortunate, you may encounter an error message.  Error messages are assigned a letter, here are a list of error messages and possible solutions :

Error A - Memory Problem, remove memory expansion sidecars and internal memory option to try to isolate which one is causing the program.  Will cause the system to hang.

Error B - Keyboard Problem, move the keyboard away from any extremely bright light source, replace the batteries, move the keyboard closer to the system if using IR and do not press any keys during the bootup process.  May cause system to hang.

Error C - Cassette Problem, press Enter to continue booting without using the cassette.  Usually caused when the cassette motor relay goes bad.

Error D - Serial Port Problem, may hang system.  Unplug any device from the serial port.

Error E - Modem Problem, press enter to continue booting without using the Modem.  Make sure the modem is properly inserted.

Error G - Cartridge Problem, may hang system if cartridge is inserted, remove cartridge.

Error H - Diskette Problem, press enter to continue booting without using the Diskette Drive.  Make sure the diskette drive adapter card is properly plugged into the system, the diskette drive is properly connected via the cable to the diskette drive adapter, the diskette drive is plugged into the power connector.

Error X - Unknown, may hang system.  May not actually exist.

Video and Audio Connections :

The PCjr. is very video friendly in that it has a composite video output jack that can connect to almost any modern TV.  You do not need a hard to find CGA compatible monitor to get video output.  The colors may not be as pure, the pixels may not be as distinct, but graphics should be visible.  If you boot to DOS and you see 80-column color fringed text, use the DOS command MODE BW80 to switch the text to black and white.  If you want more legible text, then use MODE BW40 to use a 40-column text mode.

If you find that your PCjr.'s display is off center or partially cut off, press Alt Left Arrow  to shift the visible display to the left and Alt Right Arrow to shift the visible display to the right.  You can also enable or disable the keyboard audio click by pressing Ctrl + Alt + Caps Lock

The internal beeper inside the PCjr system unit will only generate PC Speaker sounds.  If you want to hear sounds from the 3-voice chip, the PCjr. Speech Adapter or from the Cassette Port, you will need to connect the PCjr. to an amplified speaker via the RCA audio output jack, to a TV using the TV Connector switchbox, or to a 4863 PCjr. Color Display.

COM and LPT assignment.

The built in serial port is assigned to COM1 and LPT1 in the BIOS if there is no Internal Modem or Parallel Printer Attachment installed.  However, the resources used by the serial port are I/O 278H and IRQ3, which are generally assigned to COM2.  The Internal Modem uses I/O 3F8 and IRQ4, the BIOS assigns the Modem to COM1 if it is installed and the serial port becomes COM2.  If the Parallel Printer Attachment is installed, it becomes LPT1 and you may need to redirect LPT1 to COM1 or COM2 to print to the serial-based Compact Printer.

Recommended Programs :

Most DOS programs will require a 256KB PCjr., so if you don't have one you are probably going to be limited to self-booting (PC Booter) programs.  King's Quest I & II, The Black Cauldron and Below the Root are excellent adventure games that take advantage of the enhanced graphics and sound features of the PCjr.  Super Boulderdash and Jumpman are solid ports of these classic games with PCjr. features.  Microsoft Flight Simulator 2.0 looks best on the PCjr. and can take advantage of a 256KB PCjr.

The Device Driver JRCONFIG is a must if you are running a 256KB or more PCjr. in DOS.  The PCjr. was released with PC-DOS 2.1, and was officially last supported in PC-DOS 4.0.  DOS 5.0 and above require patching via DEBUG to run on the PCjr.  While DOS 3.3 and below can run in a 128KB PCjr., DOS 4.0 and above are not likely to leave much memory left for programs unless 256KB or more is in the system.


  1. Always enjoy reading your blog. One note about the display alignment steps above....after testing this myself, it appears to be Ctrl-Alt-Left/Right arrow key.

    Keep up the good work! Cheers

  2. Even though it’s an afterthought for most builders, the power supply is actually one of the more important parts of a build. Picking a quality power supply can mean the difference between a well running system and one that suffers from crashes and boot failures. Worse yet, cheap generic models can literally explode into flames, taking the rest of your computer with it. t164m