Apple IIGS

Apple IIGS

Apple IIGS, Woz limited edition (1986)
Specifications
Manufacturer	Apple Computer, Inc.
Type	Personal Computer
Released	September 1986
Discontinued	December 1992
Intro price	US$999 at introduction, bundled with an Apple 3.5 drive and no monitor
CPU	Western Design Center 65C816, selectable 1.024 MHz or 2.8 MHz
Memory	256 KB on the original main logic board (1 MB on the ROM 3 board); up to 4 MB of expansion RAM
Storage	3.5-inch 800 KB and 5.25-inch 140 KB floppy drives; hard disk via a SCSI card
Display	Super Hi-Res 320×200 or 640×200, 16 colours per line from a palette of 4096, plus all previous Apple II modes
Sound	Ensoniq 5503 Digital Oscillator Chip, 32 oscillators, 64 KB of dedicated sound RAM
OS / Firmware	GS/OS, ProDOS 16, ProDOS 8
Predecessor	Apple IIe
Codename	Phoenix, Rambo, Gumby, Cortland

The Apple IIGS is the 16-bit member of the Apple II family, introduced in September 1986 at $999 bundled with an Apple 3.5 drive and no monitor, and dropped from Apple's price lists in December 1992.

Every hardware figure on this page comes from Apple's own Apple IIGS Hardware Reference (1989), hosted on this wiki, except where another source is named.

The one thing to do first: the battery

The Apple IIGS carries a lithium cell on the main logic board, and a leaking cell is the single most destructive fault this machine has. It is worth dealing with before anything else, because everything else on this page is repairable and a board eaten through by electrolyte often is not.

Apple designed the machine around battery-backed RAM: the Control Panel settings and the real-time clock are held in RAM kept alive by that cell. Apple told owners to watch for two symptoms, and both are a low battery:

• the Control Panel settings revert to the factory defaults, and revert again after you reset them
• the clock stops keeping accurate time

Apple's answer to both was "have your authorized Apple dealer replace the battery", rather than telling the owner to do it, because on most machines it is soldered down.

How the cell is fitted depends on which board you have:

• ROM 0 and ROM 1 boards: a 1/2 AA axial-leaded lithium thionyl chloride cell soldered directly to the board, sitting barely under the power supply where it is easy to miss. Apple's original was usually a Varta.
• ROM 3 (1 MB) boards: a 1/2 AA holder with a clip-on plastic cover, with a plain barrel cell in it. Apple's original was usually a Maxell, and Maxell cells of this type have a poor reputation for leaking.

When one of these leaks, the electrolyte attacks every metal it reaches, including the copper traces, and the result is often not economically repairable. On a ROM 3 board you can simply pull the cover and lift the cell out. On a ROM 0 or ROM 1 board no desoldering is needed either: the leads can be cut close to the cell body and the stubs dealt with afterwards.

The machine runs perfectly well with no battery at all. You lose the clock and you have to set the Control Panel after each power-up. For a machine that is used occasionally, that is a better trade than a cell sitting on the board for another decade. If you do want the clock, replace like with like - lithium thionyl chloride, not alkaline - and if you want it outside the case, solder leads to the board and put the cell in a holder somewhere it cannot drip onto anything.

See Battery Explosion, Capacitor or Corrosion Damage for cleaning up after one that has already gone.

Architecture and Processor

The IIGS is built around a Western Design Center 65C816, with the system speed selectable between 1.024 MHz and 2.8 MHz. The machine is split in two: an FPI side running fast, and a Mega II side running at the 1.024 MHz an Apple II needs for its video and I/O timing. The FPI synchronises to the Mega II whenever the two have to exchange data, which is what makes Apple II software run at the right speed.

2.8 MHz is not the speed you get. The FPI generates the refresh cycles the fast DRAM needs, and "the time required for the refresh cycles reduces the effective processor speed for programs in RAM by about 8 per cent". Only programs running from ROM see the full 2.8 MHz.

Memory

• 256 KB on the original main logic board - 128 KB of fast RAM under the FPI plus 128 KB on the Mega II side
• 1 MB on the 1 MB (ROM 3) logic board
• 128 KB of ROM (256 KB on the 1 MB board)
• Expansion through the memory expansion slot: up to 4 MB of RAM and up to 1 MB of ROM

Apple's figure is 4 MB of expansion RAM, not 8 MB. The 65C816 can address 16 MB, but that is a property of the processor, not of this machine's memory expansion slot.

ROM versions

Apple's own naming is ROM 0, ROM 1 and ROM 3. There is no ROM 2: Apple skipped it "since there was already enough confusion about the first version, ROM 0, and the second version, ROM 1".

Version	ROM size	Notes
ROM 0	128 KB	The original 1986 machine. Apple's system software stopped supporting it; dealers could upgrade machines with serial numbers below E704... to ROM 1 on request
ROM 1	128 KB	Large numbers of firmware fixes across ADB, AppleTalk, the Memory Manager, QuickDraw II, SANE, Serial I/O, SmartPort and Sound. Requires System Software 2.0 or later
ROM 3	256 KB	From August 1989. A different logic board, not just different ROMs. Requires System Software 5.0 or later

What is different about a ROM 3 machine

The ROM 3 is the 1 MB Apple IIGS of Apple's own documentation, and the board differs in more than memory size:

• 1 MB of RAM and 256 KB of ROM, with the toolbox utilities that used to load from disk now resident in ROM
• A new ADB microcontroller, which adds sticky keys and the keyboard mouse and allows finer mouse tracking
• A new CYA chip, which handles text page 2 shadowing and the power-up bits
• Shadowing of text page 2 becomes available
• A bit in the Speed register that lets software tell whether a cold boot began at the power switch
• It no longer supports the built-in Apple IIe style keyboard. Only ADB keyboards work.
• The clock battery is in a holder rather than soldered down (see above)

That last keyboard point matters when swapping boards between machines, and the battery difference matters when buying one.

Display

Super Hi-Res offers 320 or 640 pixels across by 200 lines, displaying 16 colours per line chosen from a possible 4096. The display buffer, its palettes and its scan-line control bytes occupy 32 KB at $2000-$9FFF in bank $E1; each scan line uses 160 bytes.

The two modes differ in how a pixel selects its colour:

• 320 mode - four bits per pixel, two pixels to a byte. A pixel can be any of the 16 colours in the palette.
• 640 mode - two bits per pixel, four pixels to a byte. A pixel can be one of only four colours. The 16 palette colours are divided into four groups of four and each of the four pixels in a byte draws from a different group, so different pixels along a line can still take any of the 16.

All the earlier Apple II text and graphics modes are present as well, and the machine has analogue RGB outputs as well as composite.

Sound

The IIGS uses the Ensoniq 5503 Digital Oscillator Chip: 32 independent oscillators, volume control and digitising, with 64 KB of RAM dedicated to waveform storage, talking to the 65C816 through the Sound GLU.

The built-in sound output is a single mixed channel. Apple's own wording is that "sound is output via the built-in speaker, external speaker jack, or molex connector on the main logic board". The oscillator control registers carry channel-address bits, but those bits drive an optional external demultiplexer: "Connecting a demultiplexer to the 7-pin molex connector will allow you to use up to eight separate sound channels." That connector is J-25 and it also carries the analogue-to-digital converter input.

In other words, the stereo and multi-channel output people associate with the IIGS comes from a third-party card plugged into J-25. A stock machine is mono, and there are no "two 8-bit DACs" on the board.

Expansion

Seven peripheral slots, numbered 1 to 7, plus the memory expansion slot. Each built-in device is assigned to a slot, and only one of the built-in device or a card can be selected for a given slot - the Slot register at $C02D decides. A card that appears dead in slot 2 when the modem port is enabled is not faulty.

Apple's current budget for all seven slots together:

• 500 mA at +5 V
• 250 mA at +12 V
• 200 mA at −5 V
• 200 mA at −12 V

That is 500 mA of +5 V shared between every card in the machine. A IIGS that misbehaves with a full complement of cards, and behaves with two of them removed, is very likely running out of slot current rather than failing.

The support circuitry is designed for two LS TTL loads and no more than 15 pF per slot pin.

Power supply

Apple describes the IIGS supply as having "the same four-supply, switching, load-sensing design as the Apple II, II Plus, and IIe models".

Parameter	Apple's published figure
Input	107 to 132 V AC
+12 V	1 A
−12 V	0.25 A
+5 V	4 A
−5 V	0.25 A

The chirp is the protection working, and a no-load condition sets it off too. Apple's own description: the load-sensing circuitry stops the supply "whenever this circuitry detects a short or a no-load condition", and "the supply will emit two audible chirps per second" until the situation is corrected or the supply is switched off.

Two chirps a second therefore means one of three things: something downstream is shorted, nothing is connected at all, or the supply itself has failed. Bench-testing a IIGS supply with nothing attached will make it chirp, and that is correct behaviour, not a fault. Put a load on it before drawing conclusions.

Things that are not faults

• Two chirps per second from a supply with nothing plugged into it - the load-sensing circuit needs a load
• Real-world speed noticeably below 2.8 MHz for code in RAM - about 8 per cent goes to refresh
• A card in slot 1 or 2 that is ignored while the printer or modem port is enabled - only one device per slot can be selected
• Mono sound from the headphone jack on a stock machine - separate channels need an external demultiplexer on J-25
• An Apple IIe style keyboard that does nothing on a ROM 3 board - that board supports ADB keyboards only

Gallery

• 
  Apple IIGS (front view)
• 
  Apple IIGS logic board and power supply
• 
  Apple IIGS with peripherals

See also

• Apple IIGS Hardware Reference
• Apple IIGS ROM Revision Summary
• Apple IIGS Owner's Reference
• Battery Explosion, Capacitor or Corrosion Damage
• Apple II General Maintenance