Product Description

The Macintosh Plus

It has more memory, double-sided disks, and an SCSI interface

By Phillip Robinson

June 1986

Editor's note: The following is a BYTE product description. It is not a review. We provide an advance look at this product because we feel that it is significant. A complete review will follow in a subsequent issue.

The Macintosh introduced personal computer users to technology that has now become commonplace: bitmapped displays, 3-1/2-inch floppy disks, and iconic desktop environments. The Macintosh Plus adds welcome double-sided drives, a megabyte of RAM, an industry-standard SCSI interface, a numeric keypad, and a larger, faster ROM of operating system routines. The Mac Plus is also much faster than the 512K Mac, partly because of new software routines and partly because of the additional RAM. One feature that the 512K Macintosh had--free MacWrite and MacPaint software--is missing from the Mac Plus. Both programs are available separately for $125 apiece.

System Description

Most of the technical details in BYTE's February 1984 description of the original, skinny (128K-byte) Macintosh are still apt descriptors for the Mac Plus. It is a desktop, 7.8336-MHz 68000based microcomputer in a small-footprint case containing a built-in 3-1/2-inch floppy disk drive and a 9-inch diagonal, bit-mapped, monochrome display of 512 by 342 pixels. It doesn't have any expansion slots, and it doesn't have a cooling fan. It possesses a number of I/O ports, a detachable keyboard. and a mouse.


At first glance, the Mac Plus keyboard looks like a Mac keyboard with an added numeric keypad on the right side. But there are other changes as well: Four cursor keys are located at the bottom right of the alphabetic section; there is only one Option key; the Enter key is placed with the numeric keypad; and the Return key is larger than on previous Macs. Other adjustments in key size and position are minor. Incidentally the cursor keys aren't recognized by all applications or even by the Mac's own desktop.


The Macintosh Plus has two main circuit boards, analog and digital. Although the analog board is little changed from the 512K Mac design, the digital board has been thoroughly modified. It is still designed around the 68000 CPU, but it has more RAM and more ROM--both socketed--than previous Macintoshes.

One full megabyte of RAM sits on the Macintosh Plus digital board. This memory takes up less space and consumes less power than the 512K in the Fat Mac. Apple's engineers accomplished this by employing four CMOS SIMMs (single in-line memory modules), each of which holds 256K bytes in eight 256K-bit CMOS dynamic RAMs. These RAM chips are enclosed in surface-mount packages and then soldered onto tiny PC (printed circuit) boards.

The RAMs most of us are familiar with are packaged in DIP cases. Surface-mount packages are smaller, so more chips can be squeezed into less PC board space . They are called "surface-mount" because the legs don't have to stick through a PC board to be soldered on the opposite side (as DIPs are). Instead, the legs fold over and contact the same side the chip packages sit on. For the Mac Plus, these packages are soldered onto four small boards that insert at an angle-and overlap-into special sockets on the Mac Plus digital board.

When 1-megabit surface-mount CMOS DRAMs are available, it will be relatively easy for manufacturers to create SIMM strips that hold a full megabyte. Such strips could replace the current Mac Plus SIMMs to upgrade the system to 2 or 4 megabytes of RAM. Applications that follow Apple's design rules can use all of the additional RAM.


With 128K bytes of ROM code, the Macintosh Plus has twice what the 128K and 512K Macs had. The old 64K-byte ROM contained equal parts of Macintosh operating system. QuickDraw graphics routines, and the User Interface Toolbox. The 128K ROM keeps trimmed and optimized versions of those routines, adds new routines, and holds some routines that used to sit in the disk system files.

QuickDraw--which draws everything you see on a Macintosh display--was rewritten to be faster. New ROM drivers were written for the double-sided disk drives and for the Hard Disk 20. The operating system now allows the Mac Plus to boot from a hard disk, a feat the previous Macintoshes couldn't perform. And the new ROM has numeric computation routines: Applications aren't forced to have their own such routines and can depend on floating-point arithmetic and transcendental functions in the ROM.

The operating system shows up in some subtle environmental effects. An icon called a "zoom box" toggles a window between full-screen size and whatever other size it was last set to. There is also an additional choice under the View pull-down menu: Small Icon. This menu selection preserves the information about the file type that icons provide, but it allows many more icons to fit onto a single display.

The System file is listed as version 3.0. and the Finder is version 5.1. Most programs should work with both old and new ROMs, but the 128K ROM routines are still being integrated into the calls from applications software.

Desk Accessories

The Control Panel desk accessory contains some new controls. The biggest change is the addition of a RAM cache controller that lets programs load, run, and quit up to twice as fast as conventional operational methods. Frequently used routines can be read from disk only once and then run from RAM. The cache can be turned on or off and can employ anywhere from 32K to 768K bytes of RAM.

The Choose Printer desk accessory is replaced by the more general Chooser that lets you select any of the serial ports or the SCSI port. The AppleTalk network is now connected and disconnected via the Control Panel instead of Choose Printer.


The HFS (Hierarchical Filing System) first appeared in Finder 5.0 in the fall of 1985 with Apple's HD2O external 20-megabyte hard disk drive. It is now built into the new Finder 5.1 of the Mac Plus ROM.

The previous Macintosh Finders used a flat filing system (now called MFS for Macintosh Filing System). All files were listed in a single directory. Now the HFS puts files within directories and subdirectories, just as MSDOS does for many microcomputers. You move through the directories by manipulating slightly more complex versions of the dialog boxes used by earlier Finders. You can pull down lists of the directories-shown iconically as nested folders on the Mac Plus-to see a path list. The number of files the HFS can handle is limited only by the disk space. Because it runs from ROM it is also faster than the MFS. Some programs will have to be rewritten to avoid path problems with the subdirectories of HFS.

The Mac Plus comes with a disk of system tools that includes an Installer utility for updating the system files on your older start-up disks.

Floppy Disk Drives

The original Mac depended on 3-1/2-inch floppy disk drives, built by Sony, that recorded 400K bytes on a single side of the disk. The Mac Plus drives are still 3-1/2-inch Sonys, but they are double-sided and so can read and write 800K bytes per disk. They can also read and write the single-sided format of the older drives. In effect, the double-sided disks provide more than twice the effective space of the old 400K-byte drives because much of the 400K was occupied by system files.

The Sony drives use a special. variable-rate rotation scheme where the spin of the disk depends on the track being accessed. The new drives use the same scheme but are twice as fast as the old drives for reading and writing. The transmission speed between the drives and the Mac (about 500.000 bits per second) hasn't changed.


The biggest change in ports from the 512K Mac to the Mac Plus is the addition of the SCSI port for fast communications. The other ports have been moved around, and some have different connectors. The external disk port can handle single-sided or double-sided floppy disk drives as well as the HD2O hard disk drive. The audio jack, the mouse port, and the external disk drive port retain their shape and size but have been moved. The two serial ports typically used for modem and printer connections now employ mini DIN-8 connectors instead of the 9-pin D-type connectors used before. The new circular connectors eat up less room--a necessity with the addition of the large SCSI connector.


The 128K and 512K Macintoshes relied upon an Apple-specific serial port that communicated at 230,400 bps using the Zilog 8530 SCC (serial communications controller) chip. Apple hoped that the general-purpose serial port would lead to "virtual slots," with add-on devices attached to the serial port instead of inserted within the computer. Unfortunately, the port just didn't have enough speed for many peripherals, including hard disks, tape backup systems, and scanners. So the Mac Plus design team looked around for a faster, more standard interface.

SCSI is an industry-standard, parallel, system-level interface bus for connecting peripherals to a variety of personal computers. It descended from the old SASI disk drive standard to become ANSI X3T9.2. Many peripherals such as hard disk drives already contain SCSI interfaces. All the Macintosh Plus needs to use those peripherals is the proper software drivers.

Up to seven devices can be daisy-chained to the single Mac Plus SCSI port. If any SCSI devices are attached to the Macintosh Plus, at least one of them must be switched on for the Mac Plus to start.

The Macintosh Plus SCSI interface is designed around an NCR 5380 SCSI controller IC and communicates at 320K bytes per second: about 10 times faster than the old serial port. It uses an 8-bit parallel data connection, a variety of control lines, and a parity line for error checking.


Apple devised a thorough upgrade program for those who have 128K or 512K Macintoshes and want Mac Plus power. You upgrade a system by buying and installing special kits.

The first kit--$299--includes the new 128K ROM and an internal double-sided disk drive. This drive replaces the single-sided drive and is only available to be installed by certified Apple dealers. (External double-sided floppy disk drives sell for $499.) This kit can be installed in Macs that have third-party RAM additions. but Apple doesn't guarantee that the upgraded Mac will be able to use all of the third-party RAM.

The second kit--$599 for 512K Macs and $799 for 128K and third-party-enhanced Macs--contains a new digital logic board and a new backpanel. The board replaces the old digital board. You need the panel with this kit because the new set of port connectors on the Mac Plus demands differently placed and sized openings.

The third so-called kit--$129--is simply the Mac Plus keyboard.

The only way to get the new ROM is to buy kit 1. And you cannot use kit 2--the digital logic board--without kit 1: The new digital board is built for a system with the 128K ROM and the double-sided internal drive.

The 512K Macintosh suggested retail price was dropped to $1999: the 128K Macintosh is no longer available from Apple. For Lisa or Macintosh XL owners, from April 15 through August 31, 1986, you can ante up $1498 and your machine and receive a Macintosh Plus with an HD2O.

In Brief

Name:  Macintosh Plus
Apple Computer Inc.
20525 Mariani Ave.
Cupertino, CA 95014
(408) 996-1010
Price:  $2599

Motorola 68000, 32-/16-bit microprocessor (32-bit internal data path
and registers, 16-bit external data bus) running at 7.8336 MHz

Main Memory
1 megabyte of RAM, expandable to 4 megabytes in the future 128K-byte
ROM containing operating system code 256-byte EEPROM for
user-settable parameters

9-inch diagonal built-in screen that displays bit-mapped 512 by 342

Four-voice sound from 8-bit D/A conversion (22-kHz sampling rate)

Disk Memory
800K-byte 3-1/2-inch double-sided floppy disk drive built-in

Detachable 78-key typewriter-style keyboard plus numeric keypad and
cursor keys; offers 2-key rollover and software mapping

CMOS custom chip built-in with 4.5-volt battery

Two RS-232C/RS-422 serial ports with maximum speed of 230,400 bps
SCSI port capable of 320K-byte-per-second communications
Mouse port (for mechanical mouse)
External disk port
Synchronous serial keyboard port
Loudspeaker jack

Optional Peripherals
800K-byte 3-1/2-inch double-sided floppy disk drive
Imagewriter dot-matrix printer
Apple Modem 1200 (300/1200-bps modem)
Hard Disk 20, 20-megabyte hard-disk drive (operates through serial port)

Bundled Software
System tools

Macintosh Plus

BYTE June 1986

Copyright 1986 CMP Media LLC