This page is for answering some of the more common questions about the Jr. If your question isn't covered here, please feel free to email me (mbbrutman at gmail.com) or use the web forum (https://www.brutman.com/forums).
This page is broken up into several sections:
Q: Can I add another diskette drive to a PCjr?Yes, there are several options.
I have tried connecting newer 1.44MB floppy drives to the PCjr and the worked. You can only use them with 720KB diskettes, which makes them behave like double density drives. The good news is that you don't have to go rummaging around trying to find the relatively rare 720KB drives if a standard 1.44MB floppy drive will work. I used a 1.44MB drive from a Compaq 4/33i (1992 vintage) and a Sony 1.44MB drive (2000 vintage) and both behaved well. I had problems reading the floppy under Windows 98, but Linux did just fine with it. I suspect it is because I wrote a 360KB diskette image onto the drive, which set the boot sector up to make it look like a 360KB diskette. Windows 98 was able to read the directory but nothing else - Linux did everything correctly.
If you use the parallel port solution, you can probably
floppy drive that you want, as long as you can run the drivers.
This include high density drives. Keep in mind that
standard parallel port can send eight bits at a time, but it can only
read four bits at a time - writes will be twice as fast as reads, and
reads will be very slow! (See the section on the
parallel port modification below for a way to improve this.)
Q: Can I use a high density diskette drive on a PCjr?It depends.
The PCjr controller card can handle 360KB 5 1/4 inch diskette drives and 720KB 3 1/2 inch diskette drives.
The PCjr controller card can not generate the proper data for high density diskette drives. (These are the 1.2MB and 1.4MB types.) I think that the data rate on the controller card has to be significantly faster than what the PCjr can do. This is not a design flaw on the part of the PCjr; Only the PC AT had high density drives at that time, and it was considered a heavy duty piece of equipment.
You can connect a high density drive to the PCjr, but you will have to use double density media to make it usable. (Double density media forces the drives to operate in double density mode, which the Jr can use.) Connecting a 1.44MB 3.5 inch diskette drive works well with 720KB double density diskettes. Connecting a 1.2MB 5.25 inch drive is not recommended because of track width differences, even if double density media is used.
PC Enterprises had two solutions for running a high density diskette drive on the Jr. One solution used the slow, low data rate controller built into the Jr. It read and wrote 1.2MB diskettes, but at a slower rate than a real high density controller. (They must have spun the disk more slowly to compensate for the low data rate.) Later on they developed a new controller that worked at a faster data rate.
Parallel port solutions - anything goes, as long as you
drivers to work.
Q: Can I add a hard drive to a PCjr?Yes. (And read the part about jrIDE - this is the one you want!)
One option is the traditional hard drive upgrade for a machine. You get a standard hard drive that you can boot from, just like a PC, XT or AT. There were several flavors of this option.
One flavor of this option was an expansion chassis that included the hard drive and room for other options, such as a parallel printer port, diskette drive, extra memory, clock/calendar, etc. The expansion chassis usually sat on top of the PCjr, making the PCjr look like a "double decker." The expansion chassis used a special sidecar that connected it to the system bus on the right side of the machine. Companies such as Racore, Legacy , and PC Enterprises sold this type of solution. The expansion chassis usually had it's own power supply. Hard drives were usually limited to low profile models no more than an inch tall, as the expansion chassis was usually not very tall.
Another flavor of this option was an externally mounted hard drive in another box. The other box might have had the full system bus brought to it by a sidecar, or it may have been connected via a SCSI bus, making it a storage solution only.
The TMC 850jr is amazing - it was a real, honest-to-goodness SCSI adapter capable of controlling seven devices. It also patched the BIOS of the PCjr to allow booting directly from the hard drive, without first booting from a floppy. DOS recognizes a SCSI hard drive attached to it with no problems. It truly is an amazing adapter. The Future Domain DOS ASPI drivers seem to work on the PCjr, although they are not needed for operating a hard drive. (They are needed for SCSI CD-ROM support.) I have tested several SCSI hard drives, a NEC Multispin CD-ROM, and an Iomega Zip 100 SCSI drive with the TMC 850jr.
(Not to short sell the Seagate based adapter - it was probably pretty decent too. I just haven't seen one, so I can't rave about it.)
A company called RIM also had a SASI/SCSI based solution. The upgrade consisted of a small interface card that installed in the modem slot and a larger SASI/ST506 bridge board that sat in an external chassis. The bridge board would then permit the use of a relatively standard MFM hard drive, such as the Seagate ST-225. The RIM solution does not have a BIOS extension so the machine is not bootable with it. However, they sold a cartridge known as the "HARDBIOSjr" that added the necessary BIOS extensions to make it a bootable solution.
The ultimate solution for the PCjr is a new project called jrIDE. jrIDE adds an IDE interface, memory, and a real time clock. It fits in an existing PCjr sidecar and lets you add a relatively modern large capacity drive to the PCjr. Although most of the drive will not be used by the PCjr, these drives are cheap, newer, and should have a fairly long lifespan compared to the old MFM clunkers from the mid 1980s.The jrIDE is a new project, designed in late 2011 and first functional in early 2012. For more information on the jrIDE see http://www.brutman.com/jrIDE .
The other major option is to use a hard drive or other storage device that is attached via parallel port. These are generally slower and not as well integrated as the solutions described above; the machine still has to boot from a floppy diskette and you need to load device drivers. Most PCjrs have a parallel port side car, so this is a feasible option. The only barrier is finding working device drivers for DOS. (Most of the devices do have device drivers that work under DOS.) Tested devices include hard drives, parallel-to-SCSI adapters, parallel port CD-ROMs, SparQ removable storage, and the Iomega Zip drive. Instructions for the SparQ can be found at (www.micro-zone.com ).
If you are going to do the parallel port route, see the
parallel port modification below for a way to improve the speed of the
Hard drive vendors for the PCjr from the ancient past:
Q: Can I use a CD-ROM/Iomega Zip/SparQ/Syquest drive on a PCjr?Yes.
People have reported success with the following types of devices on the parallel port:
One particularly notable device driver is PalmZip by Klaus Peichl. This device driver for the standard parallel port Zip 100 works on every version of DOS from 2.1 to 5 (and probably 6.x as well). The speed isn't great, but it works well and has never crashed my machines or dropped a byte of data.
Let me know what types of removable storage you've used!
Q: How about Compact Flash?Yes! By this point you should have figured out that the parallel port is the key.
In particular, the Datafab MDCFE-SR is known to work well on the PCjr. There is a slight problem - it needs power supplied to it and the mechanism they provide is to use a vampire tap from a standard keyboard port. You will have to cobble together some form of alternative power source for it. Performance was adequate - about 40KB/sec for reads and writes on a bi-directional parallel port.
Compact Flash is also a possibility if you have a jrIDE. (Compact Flash with an adapter can be used to emulate an IDE hard drive.)
Q: How much memory can I add to a PCjr?A PCjr has a minimum of 64KB of memory, which is soldered onto the motherboard. (Eight chips, each 64 kilobits - no parity protection in this setup.) To get to 128KB, you need the internal expansion card. To go beyond that, you need to start adding sidecars or other boards.
Some of the sidecars were sold as 128KB units, which would bring memory up to 256KB. The IBM and Microsoft Booster sidecars could be modified to go up to 512KB, giving the PCjr up to 640KB of memory.
The way the PCjr memory map is laid out, it is possible to get up to 736KB on a PCjr system. Of course you need that much memory installed and some software patches to make DOS recognize the extra memory.
Remember that on a PCjr, the graphics subsystem "steals" memory from the system to use for the video display. Therefore, you are usually short 16KB or 32MB on a PCjr. Device drivers to use extra memory usually reserve the unused memory in the first 128KB for performance reasons; that means on a 640 KB system you have at best 628KB or 608KB using some slow memory, and at worst only 512KB is you reserve the slow memory for a RAM disk. This is usually enough for most old software.
Here are some memory expansion vendors for the PCjr from the ancient past:
The first improvement that you can make is to add memory via a sidecar, and use software to reserve the first 128KB of memory for DOS, a ram disk, and the video graphics memory. This forces your programs to run above the 128KB mark, which makes them faster because the access to this memory is faster. See the section on " The Memory Subsystem " for the details on why this is so.
The second thing that you can do is to replace the Intel 8088 microprocessor with the NEC V20 microprocessor. The NEC V20 was an Intel 8088 clone that ran a little faster. Some software was not compatible with the NEC V20, most of it being games. (Broderbund's Lode Runner is one of them.) The NEC V20 can give you a noticeable improvement; some users have reported their PCjrs running almost 15% faster. (Obviously it depends on the benchmark.) It's going to be a little hard finding a NEC V20 now though .. at least you don't have to worry about voiding your warrantee. ;-)
Another option was the PC-SPRINT kit. PC-Sprint was designed by Douglas A. Severson in 1985 and it was intended for the PC and the XT, however it works on the PCjr as well. It involves replacing the microprocessor with a version that is rated for 8Mhz, and changing the 8245 timing chip. The effect is to give you a 7.33Mhz system. The memory and the disk drives will still be at their same old speed, but processor operations will be much faster.
Another option was the jrExcellerator, a board sold by PC
The jrExcellerator was a daughtboard board that plugged into
socket on the motherboard. It came with a replacement NEC
the switch flipped on, the Jr would run at 9.54Mhz. It also
onboard BIOS chip that would allow the Jr to recognize and use extra
without a device driver.
Q: Can I change the clock crystal in a PCjr to speed it up?
No. The PCjr uses the clock crystal for a lot of things. Changing it by even a few percent will leave you with a dead screen and a dead keyboard.
However, you can use the PC-Sprint modification which is almost equivalent.
Q: Can I add an 8087 math coprocessor to the PCjr?Yes. There is no socket on the motherboard for the 8087, so you just can't plug the chip in. A company named TIAC Manufacturing (from Canada) made a daughterboard that used the 8088 socket, and had the 8088 and 8087 sit on the daughterboard. You will need to find one of these (or construct one!) to add an 8087 math coprocessor. The original cost of the TIAC jr-87 in late 1984 was $90, without the 8087 chip. Legacy technologies also made an 8087 daughterboard.
Be aware that you may be stymied by software. An 8087 on a PCjr would be extremely rare, and if the software recognizes that it is running on a PCjr it may not even bother to look for an 8087. (A Racore PC ID cartridge could get around a software limitation like that.) You will have to run a program to change the equipment word in memory to note the presence of an 8087, as the PCjr BIOS won't look for one and won't set the equipment word right if one is present.
Q: What kind of printer port does the PCjr use?The parallel port sidecar uses a fairly standard parallel port implementation that was common on early PCs. The parallel port can write 8 bits at a time and read 4 bits using control lines. It is implemented in TTL logic with approximately 13 chips. The output is a standard DB25 connector. The port and IRQ line are not adjustable, meaning that two parallel port sidecars can not be used on the same machine. (And of course there are ways around this ... keep reading.)
Q: Can I use a bi-directional printer port?Yes. There was a pretty common hack to modify the parallel port to turn it into a PS/2 compatible bi-directional printer port. This isn't any use for printers, but is great for storage devices connected via the parallel port.
The hack involves cutting a pin on two ICs used in the parallel printer port, and connecting them together via wire. After that - voila, you have a bi-directional printer port. The trick is knowing the two pins. :-) Click here for the details. The great thing about a bi-directional parallel port is that it is much faster than a standard parallel port, especially when using a device like a hard disk or Ethernet adapter on the port.
Q: Can I have more than one parallel port on a PCjr?Yes. The parallel port sidecar sold by IBM does not have an adjustable address. However, other parallel ports may. The Racore expansion unit features jumpers that can set the parallel port address; other vendors may have the same feature.
If not, you can always modify the IBM sidecar to change the addresses. Click here for the details.
Q: Can I use better types of parallel ports on a PCjr? (EPP, ECP?)With some work, yes.
An EPP parallel port uses a few extra I/O registers to work, but those are readily available on the PCjr and do not conflict with other hardware. An ECP parallel port will not work though, as that requires DMA capability.
The tricky part is getting the parallel port onto the system. EPP parallel ports were invented long after the PCjr failed as a machine. To get an EPP parallel port onto a PCjr you need to use an 8 bit ISA bus to PCjr adapter. The pinout for such an adapter can be found here.
If you are connecting a hard disk or other non-printer peripheral through a parallel port, this can be a much faster solution than the standard parallel port or a bi-directional parallel port. With my hard disk setup I have seen that sequential disk reads are three times faster than on a normal bi-directional parallel port, and sequential disk writes are 2 times faster.
Q: What options do I have for hooking up a monitor to the PCjr?There are three options available:
Q: Can I use an old CGA monitor on the PCjr?Yes. IBM sold an adapter to go from the rectangular BERG type connector on the PCjr to a standard CGA type connector. If you can not find such an adapter, constructing one should not be too hard - just find the pinouts, and hack away.
Q: Can I use an old monochrome monitor on the PCjr?It depends.
If you are talking about a high quality monochrome monitor (TTL style), such as one designed to be used with a Hercules card, forget it. The PCjr has CGA only (a superset actually), and it can not drive such a monitor. Even if you could connect a monochrome adapter onto the PCjr using an ISA bus adapter the PCjr does not have the BIOS programming necessarily to set the card up and make it produce output.
If you are talking about a lower quality (but still
monitor that uses the composite video output of the PCjr, then yes, you
can connect it. You will have CGA graphics, but in 16 shades
grey. I used an amber monochrome monitor on my PCjr for
and I loved it.
Q: I'm having diskette read errors - help!Is your monitor sitting within six inches of the diskette drive? Or on top of it? This is a no-no ... IBM often showed pictures of PCjr's with the PCjr Color Monitor proudly sitting on top of it. Yet there is a warning in the technical reference guide that says the diskette drive should be at least six inches away from the monitor. Monitors generate a lot of RF noise. Move your monitor and try again.
If it's not your monitor causing the problem, you either have a bad diskette or a bad diskette drive. Try a new/different diskette.
Rick Ryan (email@example.com) adds an interesting point:
"I was the technical director for a PC retailer when these things came out. Customers who thought they could get the better PC/XT Color Display instead of the PCjr Color Display found their floppy drives stopped working if the larger display was placed on top of the PCjr."Apparently the PCjr Color Monitor was safe to sit on top of a PCjr, but the bigger IBM Color displays were not.
Q: A game from an IBM PC seems to run, but the screen is all messed up. Why?Welcome to the wonderful world of not being exactly compatible with the IBM PC.
The graphics subsystem of the PCjr is compatible with the CGA standard at a high level (BIOS), but software that directly accesses registers or memory is not guaranteed to work. Software might do this for performance reasons, which is good, except that it limits portability.
Sometimes the bad program can be patched to interact
the video hardware. I will be digging this patch out and
it available soon. (Email me if "soon" hasn't arrived yet for
you.) If these patches don't work, you're out of luck.
Q: I have two cartridges in my PCjr, and it doesn't work. If I pull one cartridge out it works. Why?Cartridges are ROMs, and they have to tell the PCjr BIOS where to locate them in the memory map. There are a limited number of locations that they can go to. If two cartridges are inserted and they both require the same memory locations, then they can not work.
" for more details.
Q: I have a cartridge without a label - how do I find out what it does?The easiest way is just to try it:
Some carts acted silently: They enhanced the BIOS of the machine in some way that is hard to detect. Some examples of these carts:
A thorough discussion of cartridge 'ripping' can be found here. My cartridge ripper (PCJRCART) can be downloaded by going to the downloads page or by clicking here. Another PCjr cartridge ripper is included in the tools distributed with Tand-Em, the Tandy 1000 emulator. (The Tandy 1000 was very close to the Jr.)
Q: I've got extra memory on my system, but if I run CHKDSK to see how much is available it doesn't show it?A few possible answers:
Q: What versions of DOS will run on a PCjr?Officially:
Q: Can I run Windows on my PCjr?Yes. I have a reliable report, and have seen a screen shot!
Your Jr needs to be running DOS 5 or better, 640KB of RAM, and some sort of hard disk support. That will let you run Windows 3.0. The hard disk support can be provided using the parallel port if a better solution isn't available.
Q: Can I run Unix/Linux or OS/2 on my PCjr?No. Please be serious. ;-) Those are real operating systems. The PCjr can run DOS and maybe Windows, which imply that those are toy operating systems. ;-)
You know, come to think of it, somebody has probably ported Linux to the PCjr ... ;-)
October 2000, Last updated November 25th,
(C)opyright Michael B. Brutman, mbbrutman at gmail.com
Return to Mike's IBM PCjr Page main page