Saturday, March 20, 2021

IBM JX - The IBM PCjr. 2.0 (Revised Article)

I wrote an article about the IBM JX (note no "PC" in the computer's name) five years ago.  Much of that article was subject to speculation due to the lack of hard information about the IBM JX.  A significant portion of that article had out-of-date information, so with the help of a site dedicated to the IBM JX and two Youtube videos really showing the system off for the first time, I have rewritten that old article (except for the following paragraph) entirely.  

In 1985, IBM released a computer in Japan, Australia and New Zealand called the IBM JX, Model 5511.  Essentially it was an upgraded PCjr., and in some respects what the PCjr. should have been.  In fact, upon rumors of the JX making it to the United States, at least one commentator dubbed it the PCjr. 2. Released in 1985, just as the PCjr. was being discontinued, it proved to be the last machine by IBM with any substantial PCjr. compatibility.  However, it was not successful anywhere it was released and consequently is extremely obscure today.

The Japanese JX should be considered separately from the Australian JX because the Japanese JX has much more expanded capabilities compared to the Australian JX and the PCjr.  Let's start with describing the Australian (and New Zealand) JX first, then proceed to the Japanese JX.

The Australian JX

The Australian JX, Model 5511, is very, very close to the original IBM PCjr. in many respects. It boots up just like an IBM PCjr. does with a memory count, the large IBM logo and the 16 color strips.  The graphic and sound capabilities should be fully PCjr. compatible.  It has ports on the back for light pen, cassette, audio, printer, joysticks 1 & 2, display and keyboard and two front cartridge slots.  The printer port is a standard DB-25 and the audio jack is an RCA connector but the rest are mini-DINs except for the video, which uses a DA-15 connector. (The keyboard port should fit a PS/2 keyboard connector but these keyboards will not work and may damage the system or the keyboard, so only use official keyboards)

The JX was generally available in three configurations, the JX1 with 64KiB (primarly oriented toward schools), the JX2 with 128KiB and one 3.5" drive and eventually the JX3 with two 3.5" drives and 256KiB.  

The most notable differences between the PCjr. and the JX are the color (black vs. off-white), the keyboards' 98 or 76 keys to the PCjr's keyboard and the default drive is a 3.5" drive.  The drives may be in a physical 3.5" form factor, but to the BIOS they appear as 5.25" 40-track drives with the track head double-stepped.  There is a BIOS upgrade which adds support for 80-tracks, as can loading PC-DOS 3.2 or above.  

The JX's keyboards are wireless infrared keyboards with optional wired cable support like the PCjr.  The 76-key keyboard adds function keys and a navigation cluster compared to the 62-key PCjr. keyboard.  The 98-key keyboard extends the 76-key keyboard with a number pad.  There is still an Fn key for End, Page Up and Page Down.

Not only is the keyboard experience significantly improved with the greater number of keys, but the build quality of the JX keyboard is superb.  The keycaps are double-shot moulded keycaps, so the white lettering on the black keys will not wear away from use like pad printing or white infill.  The switches are Alps Green SKCL and the function keys use Alps Green SKCL Compact switches.  These are very smooth linear switches, the PCjr. used a very stiff rubber dome membrane mat.

The JX came with 64KiB of RAM on the motherboard, which has to serve as both CPU RAM and Video RAM.  The JX will boot into 40-column text mode, but unlike the PCjr. if there is no floppy disk or hard drive to boot, it will boot into Cartridge BASIC, not Cassette BASIC.  This is because the JX has 128KiB of ROM compared to the 64KiB of ROM the PCjr. had.  The PCjr. had to insert Cartridge BASIC (32KiB) into one of its cartridge slots to obtain the extra functionality of Cartridge BASIC.  

Expansion on the JX has both similarities and differences to the PCjr.  There are four expansion slots on the JX.  The first one (silkscreened EXRAM64) is for a 64KiB expansion which behaves just like the PCjr.'s 64KiB expansion, it adds extra CPU and Video Memory.  The second is an RS-232 serial port expansion (RS232C), which adds a standard DB-25 pin port and there is a slot cover on the back of the JX that is removed to accommodate the connector.  The Serial Port was the port built into the PCjr, but IBM wisely replaced it with a parallel port on the JX and had all the circuitry needed to drive the parallel port build into the base system.  

The third expansion connector (EXRAM128) is intended for a 128KiB Memory expansion.  This expansion connector is the closest to an ISA/Sidecar general purpose expansion slot in the JX.  128KiB and 256KiB + Clock memory expansions were available.  The 128KiB memory expansion cards were configured via jumper similar to how the IBM PCjr. Memory expansion sidecars were, but they do not need their own DRAM refresh controller.  Just like a PCjr., the JX will not recognize memory above 128KiB without loading a device driver that configures the memory like  

The fourth expansion connector (DSKTA) is for the diskette drive adapter.  This board mounts horizontally with a chassis that holds the 3.5" disk drives.  Two 3.5" drives are supported, and they are assigned as Drive A: and Drive B: by PC-DOS.  There are two headers on the adapter for disk drive signals, one connects the signals to the 3.5" drives.  

Obviously in 1985 there was no IBM PC compatible 3.5" disk software beyond what IBM provided, so it released an Expansion Unit, Model 5519, with added a 5.25" drive.  The expansion unit stacked on top of the base JX, not unlike how a Racore expansion sat on top of a PCjr.  This 5.25" drive was connected by the 2nd set of disk drive signal pins on the adapter board, and assigned by PC-DOS as Drive C:  

Alternatively, the Expansion Unit could be equipped to add a 10MiB Hard Drive.  This connected to the base unit by an extension card which connected the 128KiB Memory expansion connector to a PCB in the Expansion Unit.  The Hard Drive is assigned as Drive C: in this configuration.  The Hard Drive Expansion Unit permitted access to five Memory Expansion connectors, but two were taken up by the MFM Hard Drive Controller boards.  This Expansion Unit does not add DMA support to the JX.  With three 128KiB Memory expansion cards, you could expand the JX officially to a maximum of 512KiB of RAM. Unofficially you should be able to manage 640KiB with the right combination of cards.  As you can only stack one Expansion Unit above the System Unit, so you can officially have either the hard drive or the 5.25" floppy drive, but not both.

The power supply is completely internal to the JX, no big black external transformer brick like the PCjr.  The power supply sits in a metal cage, unlike the card-design the PCjr. used.  It is a 40W supply.  The Expansion Unit provides another power supply but does not require a separate power plug, the base unit's power supply has an AC output port where the Expansion Unit connects via a short extension cable.  The JX uses a standard 3-prong cable unlike the PCjr.'s unique power adapter.  The Australian JX requires 240V, so if you import one you must have a step-up transformer or be prepared to plug it into your dryer socket.

The video connector carries the standard RGBI signals and the standard monitor is the IBM JX Color Monitor, Model 5515.  This is the equivalent to the PCjr. Display, Model 4863.  The connector and the monitor support audio output.  The connector also supports the signals needed to construct a composite video signal.  This is done with an RF unit which is an external box that provides both composite and RF output.  It is unlikely that the RF unit would have been sold in Australia as the JX is essentially a 60Hz NTSC machine and Australia is a 50Hz PAL Country.  Unless IBM put the effort in to convert NTSC to PAL that would have been a monochrome signal.

When it comes to compatibility with PCjr. software, the news is pretty much good.  The system maintains the contention between the CPU and Video memory, so the speed when running a 128KiB PCjr. or JX is very close.  Games which detect a PCjr. may adjust their speed to compensate for the slower PCjr., which can make those games too fast when played back at PC speeds.  Running PCjr-unaware programs with the above 128KiB memory expansions will fix their speed issues.  Joystick reading routines are also affected by system speed differences, but the speed differences between the PCjr. and JX are slight enough that the joystick trimmers should be able to handle any centering issues.  Unfortunately the JX's official joysticks do not provide trimmers, something IBM warned of in game documentation.

The cartridge ports of the JX are compatible with PCjr. cartridges.  The 3.5" drives can act just like 5.25" drives to software if disk images are written back to disk.  If you wish to use original PCjr.-compatible software, be aware that almost all software which takes advantage of PCjr. graphical and sound capabilities came on 5.25" disks.  You will need a 5.25" (double density) drive to run this software on the JX.  Just swap the drive cables and set the drive terminator jumpers so that the 5.25" drive is seen as the boot drive.  Use straight-through cables here, the JX has the twist in the drive cable for the B: drive only.  If you wish to run a 5.25" drive outside the machine in order to use the Hard Drive, then the 5.25" drive will be Drive C: and the Hard Drive will be Drive D:

The Japanese JX

Japan's computing needs were different compared to Western countries.  While western countries only needed to have an occasional letter or character added to the keyboard and the character set, Japan's input needs were far more complex.  The Japanese writing system uses three scripts, Katakana, Hiragana and kanji.  Katakana and Hiragana are phonetic characters, each character corresponds to a vowel or a consonant plus a vowel (or occasionally just a consonant).  Katakana and Hiragana have 46 characters each, but the two systems have equivalent syllabary.  English has 52 letters between upper and lower case.  English, Katakana and Hiragana can be reproduced in an 8x8 text cell, although Hiragana's more circular characters could stand a few extra pixels here and there.

Kanji was a rather more difficult issue for early home computers to tackle.  Kanji characters are far more detailed than Hiragana and Katakana characters, often using many strokes.  The standard 8x8 text box of early PCs and home consoles cannot really do justice to most Kanji characters, there simply is not enough detail to distinguish many characters.  A 16x16 text box was generally considered the minimum text cell size for legible kanji.  With a 640x200 maximum resolution for the PCjr., that allows for 40x12 kanji characters on the screen, which is somewhat less than ideal.

Additionally, there are a lot of kanji, approximately 3,000 different characters are in regular use in Japan.  Assuming 1-bit per pixel, storing a 16x16 kanji character takes 32 bytes.  So while the standard IBM ASCII character set can be stored in a measly 2,048 bytes, you need more than 64KiB to store a decent kanji set.  The traditional CGA/MDA text modes and character sets were simply not designed for kanji in mind.

IBM set about addressing these issues in two ways in the Japanese JX, which was finalized before the Australian JX.  First, the JX Japanese keyboard has four keys where the Australian JX keyboard just had the spacebar.  (It also has an extra key for Yen).  The functions of the keys are designed to assist Japanese text input without losing any ability to enter English text.  Japanese does not need spaces nearly as often as English, so the Japanese spacebar is only three units wide.  Other keys will switch between English, Hiragana and Katakana.  As all Kanji characters can be expressed in Hiragana (furigana), there is a key that signals the program to convert the typed hiragana to kanji or leave it as hiragana.  

The second addition was internal.  The Japanese JX has more memory than the Australian JX.  In addition to the 128KiB System ROM, there is also a 128KiB Kanji ROM.  There is also an extra graphics processor (VP2) which is designed to display Japanese characters.  VP2 uses an additional 32KiB of DRAM installed only in the Japanese JX.  There is also a 2KiB SRAM intended to store "Gaiji" (external mechanism) characters, which is necessary to display any kanji not stored in the ROM (drawn by software).  As the Kanji ROM is mapped in the area where the RAM would occupy from 512KiB to 640KiB, the Japanese systems are probably truly limited to 512KiB.  

VP2 has the same graphics modes as the PCjr. compatible VP1, but instead of the normal 40-column and 80-column (by 25 row) text modes, VP2 can display Kanji, Hiragana and Katakana at 20 and 40 columns (by 11 rows) using 16x16 text cell sizes.  VP2 can also display 8x8 Katakana in the standard 40-column and 80-column modes.  Whereas regular alphanumeric characters only require one byte to designate and one byte for attributes, Japanese characters take up two bytes each (because there are so many of them).  

IBM realized that the 200-line resolution limitation would be a hindrance in achieving market domination in Japan, so it provided an upgrade path for higher resolution graphics.  On only Japanese JX motherboards is a fifth internal expansion slot for a high resolution video expansion.  This expansion adds a third graphics processor, VP3, and 32KiB extra DRAM for graphics memory.  This video upgrade required a special monitor which used an effective resolution of 360x512 or 720x512 pixels.  The monitor came in monochrome and color varieties.  This permitted 40-column by 25-line Kanji display.  Half-width (8x16) katakana and alphanumeric characters were supported for a 80-column display.  In addition to the 2-color foreground/background per cell text modes, it also supported 360x512 and 720x512 graphics modes with 4-color and 2-color support.  

As the 32KiB of VP2 and the 32KiB 32KiB of the VP3 are not addressed in the same manner as the 128KiB of RAM addressed by VP1, it is possible that programs running in these video modes may run without the PCjr. speed impact.  

This flyer details five Japanese models of the JX, named JX1-JX5 :

JX1 - 64KB of RAM/32KB VRAM (smaller keyboard)
JX2 - 128KB of RAM/32KB VRAM and one 3.5" drive
JX3 - 128KB of RAM/64KB VRAM and two 3.5" drives
JX4 - 256KB of RAM/64KB VRAM and two 3.5" drives
JX5 - 384KB RAM/64KB VRAM and two 3.5" drives

JX3-5 came with the video expansion card.  The Japanese JX came in white or black chassis and keyboard colors.  The JX5 appears to be a later model in the series, Model 5510, despite the earlier model number.  The JX5 has the ability to run the 8088 CPU at either 4.77MHz or 7.2MHz (probably 7.159MHz in actuality).  

Final Words

Why was the JX a failure in both Japan and Australia?  There was nothing inherently bad about the systems, IBM had fixed many of the PCjr.'s flaws in the JX, including the terrible keyboard and allowing for memory expansion on day one.  If the system was flawed, it was because buyers expected IBM PC compatibility for IBM PC prices.  For the JX2, IBM wanted  270,000円 or AU$2861, roughly equivalent to $2200USD back in the day.  The JX simply was a PCjr. with a new design, it retained all the compatibility issues of the PCjr. and its lack of speed, plus the scarcity of 3.5" software in 1985-86 made it extremely unattractive to buyers.  Australian users found the Commodore 64 to be a much more inexpensive option and Japanese computer buyers did not see enough benefit to pull them away from the PC-8801 and similar machines.

Essential resources for the IBM JX include :


  1. I came across this recently when searching through Japanese auction history. Wouldn't you love to know what's on this disk?

  2. The p/n on the diskette i.e. 6239030 refers to the diskette itself. If you google it, you can see that other programs on diskettes for the IBM JX have the same p/n. Of more interest is the number under the version number. The Japanese diskette has the number 5601-JVS. The Australian/NZ version of the same game i.e. Kings Quest has the number 5601-SCX. I can only assume it's the same code.