View Full Version : IBM PCjr (4860)

February 8th, 2010, 02:59 AM
The IBM PCjr (model 4860) is a home computer announced November 1st, 1983 and available through 1985. Two models were availabe:
Model 4860-004: Base model (128K, no diskette controller or diskette drive)
Model 4860-067: Same features of the base model plus a 64KB memory expansion card, a diskette controller card and a 5.25 inch diskette drive
Near the end of production, IBM eventually shipped a unit equipped with a sidecar that added a parallel port and a second disk drive.

Standard features for both models included:
Intel 8088 CPU running at 4.77 Mhz
CGA+ graphics including a 320x200 16 color mode and a 640x200 4 color mode ("VGA" = Video Gate Array)
TI sound chip with three voices and a white noise generator
62 key wireless keyboard with an optional keyboard cable
Two cartridge ports for software on ROM chips
External ports: RGB monitor port, television adapter output port, composite video output port, Serial port, joystick ports (2), light pen port, cassette port, audio output port, keyboard port
Internal slots: 64KB memory card slot (used on the enhanced model), modem slot, floppy controller slot (used on the enhanced model)
Bus connection/expansion though a 'sidecar expansion bus'

The PCjr was designed for a home or small business user, and it was significantly less expensive. Some of the major differences between the PCjr and the IBM PC (5150) are:
The PCjr was physically much smaller, using a plastic case with a metallic coating on the inside for electromagnetic noise suppression.
The PCjr was BIOS compatible with the other members of the PC family, but not fully hardware compatible. This made for some challenges in getting software titles to run, if they could be patched to run at all.
The PCjr was limited to 128KB of memory and one diskette drive. Both of these limitations were later worked around.
The PCjr video subsystem supported all of the standard CGA modes plus some enhanced modes that used more memory, but provided far superior graphics when compared to standard CGA.
The PCjr video subsystem did not have its own memory - it borrowed main system memory. This reduced the memory by a varying amount (usually 16KB) and slowed the system down because the CPU and the video subsystem could not access the memory at the same time.
The PCjr had a real sound subsystem, which other members of the PC did not. (They had a crude speaker that could generate single tones.)
The PCjr had cartridge ports which made it more like other home systems at the time. Cartridges were thought to be more durable and harder to duplicate.
The PCjr had a non-standard keyboard layout. The implementation of the keyboard was also controversial - the 'Chiclet' keyboard was considered hard to type on and looked more like a toy than a computer keyboard.
Expansion on the PCjr was severely limited. IBM included a lot of standard hardware and ports, but also did not allow the PCjr to use standard expansion cards available for the IBM PC family.
The PCjr lacked a DMA controller which negatively affected performance when doing diskette drive reads and writes.

The PCjr was a fine machine with good technical specifications, but when compared to other home machines of the era it was expensive, and when compared to the more expensive PC line it was too limited. IBM had effectively introduced a not-so compatible PC clone which caused some software compatibility problems. The irony is that it was an IBM produced machine that was not fully IBM compatible!
More information than you ever want to know can be found here: Mike\'s IBM PCjr Page (http://www.brutman.com/PCjr)