Apple IIgs #30
Apple IIgs Hardware Reference Updates

Revised by Jim Luther (September 1990)
Written by Rilla Reynolds & Jim Luther (October 1987)

This Technical Note includes updates to the Apple IIgs Hardware Reference, published by Addison-Wesley. Please contact Apple II Developer Technical Support at the address listed in Apple II Technical Note #0 if you have additional corrections or suggestions for these manuals.

Changes since July 1990: Changed the description in "Signals at the Serial Ports and the Serial Communications Controller" to correctly note that the SCC can support a maximum asynchronous transmission rate of 57,600 bits per second (bps) in X16 clock mode.


There are two editions of the Apple IIgs Hardware Reference, the first edition (July 1987) which covers the original Apple IIgs only, and the second edition (1989) which covers both original Apple IIgs and the 1 MB Apple IIgs. Because page numbers have changed between the two editions and because an update to one edition may not be needed in both editions, this Note organizes corrections by chapter, always noting corrections to the Second Edition followed by corrections to the First Edition.

Chapter 3: Memory

Second Edition -- Page 40, Table 3-2, Bits in the State register
First Edition -- Page 36, Table 3-2, Bits in the State register

Switch the given values and descriptions for bits 7 and 2 as follows:


    Bit  Value  Description
    _________________________________________________________________________
    7    1      ALTZP: If this bit is 1, then bank-switched memory, stack, 
                and direct page are in auxiliary memory.
         0      If this bit is 0, then bank-switched memory, stack, and 
                direct page are in main memory.
    2    1      LCBNK2:  If this bit is 1, language-card RAM bank 2 is 
                selected.
         0      If this bit is 0, language-card RAM bank 1 is selected.
    _________________________________________________________________________

Chapter 6: The Apple Desktop Bus

Second Edition -- Page 148, after final paragraph

Add a new heading and description:

Control Panel Control Jumper
The ADB microcontroller provided with the 1 MB Apple IIgs includes an input that disables the text Control Panel (normally available via the Classic Desk Accessory menu). This feature allows the system parameters to be set and then protected from changes made via the text Control Panel. A jumper across the pins of connector S1 removes the text Control Panel from the Classic Desk Accessory menu. All other installed classic desk accessories are still available in the Classic Desk Accessory menu when the S1 jumper is installed. The S1 connector is located near the ADB microcontroller at motherboard location F12.
Note: The S1 jumper does not prevent the system parameters from being changed with the graphic Control Panel (a new desk accessory normally available from the Apple menu of the Finder or of any other application that includes the Apple menu).

First Edition -- Page 130, Table 6-9, Command byte syntax

The first row in the table should read:


    x    x    x    x    0    0    0    0    Send Reset

and not


    A3   A2   A1   A0   0    0    0    0    Device Reset

First Edition -- Page 131, Device Reset

Replace "Device Reset" with "Send Reset." The paragraph should be: "When a device receives a Send Reset command, it will clear all pending operations and data, and will initialize to the power-on state. The Send Reset command is not device-specific; it is sent to all devices simultaneously."

First Edition -- Pages 138-139, Collision detection

The fourth sentence in the last paragraph should be: "By using the Listen register 3 command, the host can move the device with the activator pressed."

Chapter 7: Built-In I/O Ports and Clock

Second Edition -- Page 154, Table 7-3, Disk-port soft switches
First Edition -- Page 146, Table 7-3, Disk-port soft switches

$C0E8
Drive disabled
$C0E9
Drive enabled
$C0EA
Drive 1 select
$C0EB
Drive 2 select

In addition to the corrections listed for Table 7-3, the reference to "spindle motor switches" in the paragraph following the table should be replaced with "drive enable switches."

Second Edition -- Page 155, Table 7-4, IWM states
First Edition -- Page 146, Table 7-4, IWM states

Change the table to the following:


    Q7    Q6    Drive     Operation
    _________________________________________________________________________
    0     0     enabled   Read Data register
    0     1     -         Read Status register
    1     0     -         Read Handshake register
    1     1     disabled  Write Mode register
    1     1     enabled   Write Data register
    _________________________________________________________________________

    1 = asserted state    0 = negated state    - = do not care

First Edition -- Page 146, after Table 7-4, IWM states

The following text and table should also be added:

The drive enable switches and the drive select switches control the state of the disk port signals DR1 and DR2. The following table shows the relationship between these.

              Soft Switches           |  Disk Port Signals
    $C0E8    $C0E9    $C0EA    $C0EB  |  DR1    DR2
    __________________________________|_______________________________________
      1        -        -        -    |    0      0
      -        1        1        -    |    1      0
      -        1        -        1    |    0      1
    __________________________________|_______________________________________

    1 = asserted state    0 = negated state    - = do not care

First Edition -- Page 147, The Mode register

The IWM Mode register is a write-only register, so disregard the advice to use only a read-modify-write instruction sequence when manipulating bits.

Second Edition -- Pages 156-7, Table 7-5, Bits in the Mode register
First Edition -- Pages 147-8, Table 7-5, Bits in the Mode register

For Second Edition, change the description for bit 2, value 0 as shown. For First Edition, switch the given values and descriptions for bits 1, 2, and 4 as shown.


    Bit  Value  Description
    _________________________________________________________________________
    4    1      8-MHz read-clock speed selected.
         0      7-MHz read-clock speed selected.  Set to 0 for all Apple IIgs 
                disk accesses.
    2    1      1-second timer is not selected.
         0      1-second timer selected.  When the current disk drive is 
                deselected, the drive will remain enabled for 1 second if 
                this bit is clear.
    1    1      Asynchronous handshake protocol selected; for all except 
                5.25-inch Apple disk drives.
         0      Synchronous handshake protocol selected; for 5.25-inch Apple 
                disk drives.
    _________________________________________________________________________

Second Edition -- Page 159, The serial ports
First Edition -- Page 150, The serial ports

The first sentence should read: "The Apple IIgs has two serial ports located at the back of the computer, which may provide synchronous and asynchronous serial communications."

Second Edition -- Page 160, Table 7-9, Pins on a serial-port connector
First Edition -- Page 151, Table 7-8, Pins on a serial-port connector

Replace the table title and table with this table title, table and note:


    Table 7-x    Signal assignments for the mini 8-pin serial port connectors

    Pin Number  Signal name  Signal Description
    1           HSKo         Handshake output.  Driven uninverted from the 
                             SCC's /DTR output.
                             Voh = 3.6V; Vol = -3.6V; Rl - 450 ohms
    2           HSKi         Handshake input or external clock.  Received 
                             inverted at SCC's /CTS and /TRxC inputs.
                             Vih = 0.2V; Vil = -0.2V; Ri = 12K ohms
    3           TxD-         Transmit data (inverted).  Driven inverted 
                             from SCC's TxD output; tri-stated when SCC's 
                             /RTS is not asserted.
                             Voh = 3.6V; Vol = -3.6V; Rl = 450 ohms
    4           GND          Signal ground.  Connected to logic and 
                             chassis ground.
    5           RxD-         Receive data (inverted).  Received inverted 
                             at SCC's RxD input.
                             Vih = 0.2V; Vil = -0.2V; Ri = 12K ohms
    6           TxD+         Transmit data.  Driven uninverted from SCC's 
                             TxD output; tri-stated when SCC's /RTS is not 
                             asserted.
                             Voh = 3.6V; Vol = -3.6V; Rl = 450 ohms
    7           GPi          General-purpose input.  Received inverted at 
                             SCC's /DCD inputs.
                             Vih = 0.2V; Vil = -0.2V; Ri = 12K ohms
    8           RxD+         Receive data.  Received uninverted at SCC's 
                             RxD input.
                             Vih = 0.2V; Vil = -0.2V; Ri = 12K ohms

Note: Absolute values of specified voltages are minimums; Ri is a minimum, Rl is a maximum.

Second Edition -- Page 164, after Figure 7-9
First Edition--Page 155, after Figure 7-9

Add a new heading and description:

Signals at the Serial Ports and the Serial Communications Controller
The Apple IIgs has two serial ports which are compatible with most RS-232-C devices. This section describes the input and output signals provided at the serial ports. This section also discusses some input signals to the 8530 Serial Communications Controller (SCC) chip that are not described in the Apple IIgs Hardware Reference.
The transmit-data and receive-data lines of the Apple IIgs serial interface conform to the EIA standard RS-422, which differs from the more commonly used RS-232-C standard in that, whereas an RS-232-C transmitter modulates a signal with respect to a common ground, an RS-422 transmitter modulates the signal against an inverted copy of the same signal (to generate a differential signal). The RS-232-C receiver senses whether the received signal is sufficiently negative with respect to ground to be logical 1, whereas the RS-422 receiver simply senses which line is more negative than the other. An RS-422 signal is therefore more immune to noise and interference, and degrades less over distance, than an RS-232-C signal. If you ground the positive side of each RS-422 receiver and leave unconnected the positive side of each transmitter, you have essentially converted to EIA standard RS-423, which can be used to communicate with most RS-232-C devices over distances up to fifty feet, as illustrated in Figures 7-x1 and 7-x2.

  ................................................
  .    8530   26LS32 Receivers  IIGS Mini 8-pin  .      RS-232-C DTE Device
  .    SCC    & 26LS30 Drivers  Serial Connector .        DB-25 Connector
  .  _______                                     .         ____________ 
  . |       |        /|             ______       .        |            |
  . |       |       / |            |  8   |      .        |            |
  . |       |      / +|____________| RxD+ |________       |            |
  . |   RxD |_____/   |            |______|      . |      |            |
  . |       |     \   |            |  5   |      . |      |            |
  . |       |      \ -|____________| RxD- |________|______| TxD pin 2  |
  . |       |   |\  \ |            |______|      . |      |            |
  . |       |   | \  \|            |  6   |      . |      |            |
  . |       |   |  \_______________| TxD+ |________|_ NC  |            |
  . |   TxD |___|   \              |______|      . |      |            |
  . |       |   |   /              |  3   |      . |      |            |
  . |       |   |  /O______________| TxD- |________|______| RxD pin 3  |
  . |       |   | /  /|            |______|      . |      |            |
  . |       |   |/  / |            |  4   |      . |      |            |
  . |       |      / +|_______o____| GND  |________o______| GND pin 7  |
  . |  /CTS |__o__/   |       |    |______|      .        |            |
  . |       |  |  \   |       |    |  2   |      .        |            |
  . | /TRxC |__|   \ -|_______|____| HSKi |_______________| DTR pin 20 |
  . |       |   |\  \ |       |    |______|      .        |            |
  . |       |   | \  \|       |    |  1   |      .        |            |
  . |       |   |  \__________|____| HSKo |_______________| DSR pin 6  |
  . |  /DTR |___|   \         |    |______|      .        |            |
  . |       |   |   /         |    |  7   |      .        |            |
  . |       |   |  /O      ___|____| GPi  |____________o__| RTS pin 4  |
  . |       |   | /  /|   |   |    |______|      .     |  |            |
  . |       |   |/  / |   |   |                  .     |  |            |
  . |       |      / +|___|___o                  .     |__| CTS pin 5  |
  . |  /DCD |_____/   |   |   |                  .        |            |
  . |       |     \   |   |   |                  .        |            |
  . |       |      \ -|___|   |                  .        |            |
  . |       |       \ |     signal               .        |            |
  . |_______|        \|     ground               .        |____________|
  ................................................

  Figure 7-x1 - Apple IIgs Connection to an RS-232-C DTE Device


  ................................................
  .    8530   26LS32 Receivers  IIGS Mini 8-pin  .      RS-232-C DCE Device
  .    SCC    & 26LS30 Drivers  Serial Connector .        DB-25 Connector
  .  _______                                     .         ____________ 
  . |       |        /|             ______       .        |            |
  . |       |       / |            |  8   |      .        |            |
  . |       |      / +|____________| RxD+ |________       |            |
  . |   RxD |_____/   |            |______|      . |      |            |
  . |       |     \   |            |  5   |      . |      |            |
  . |       |      \ -|____________| RxD- |________|______| RxD pin 3  |
  . |       |   |\  \ |            |______|      . |      |            |
  . |       |   | \  \|            |  6   |      . |      |            |
  . |       |   |  \_______________| TxD+ |________|_ NC  |            |
  . |   TxD |___|   \              |______|      . |      |            |
  . |       |   |   /              |  3   |      . |      |            |
  . |       |   |  /O______________| TxD- |________|______| TxD pin 2  |
  . |       |   | /  /|            |______|      . |      |            |
  . |       |   |/  / |            |  4   |      . |      |            |
  . |       |      / +|_______o____| GND  |________o______| GND pin 7  |
  . |  /CTS |__o__/   |       |    |______|      .        |            |
  . |       |  |  \   |       |    |  2   |      .        |            |
  . | /TRxC |__|   \ -|_______|____| HSKi |_______________| DSR pin 6  |
  . |       |   |\  \ |       |    |______|      .        |            |
  . |       |   | \  \|       |    |  1   |      .        |            |
  . |       |   |  \__________|____| HSKo |_______________| DTR pin 20 |
  . |  /DTR |___|   \         |    |______|      .        |            |
  . |       |   |   /         |    |  7   |      .        |            |
  . |       |   |  /O      ___|____| GPi  |_______________| DCD pin 8  |
  . |       |   | /  /|   |   |    |______|      .        |            |
  . |       |   |/  / |   |   |                  .        |            |
  . |       |      / +|___|___o                  .        |            |
  . |  /DCD |_____/   |   |   |                  .        |            |
  . |       |     \   |   |   |                  .        |            |
  . |       |      \ -|___|   |                  .        |            |
  . |       |       \ |     signal               .        |            |
  . |_______|        \|     ground               .        |____________|
  ................................................

  Figure 7-x2 - Apple IIgs Connection to an RS-232-C DCE Device

The serial inputs and outputs of the SCC are connected to the external connectors through differential line drivers (26LS30) and receivers (26LS32). The output line drivers are tri-state devices and can be put in the high-impedance mode between transmissions to allow other devices (i.e., AppleTalk devices) to transmit over those lines. A line driver is activated by lowering the SCC's Request To Send (/RTS) output for that port.
The Handshake Output signal (HSKo, pin 1) for each Apple IIgs serial port originates at the SCC's /DTR output for that port and is driven uninverted by an RS-422 line driver (26LS30). Each port's Handshake Input signal (HSKi, pin 2) is received and inverted through a differential receiver (26LS32). The output of the differential receiver is connected to the SCC's Clear To Send (/CTS) and Transmit/Receive Clock (/TRxC) inputs for that port. HSKi is designed to accept an external device's Data Terminal Ready (DTR) handshake signal through the /CTS input. The /CTS input to the SCC can be polled by software or can be used to generate an interrupt. The HSKi line is connected to the SCC's Transmit/Receive Clock (/TRxC) input for that port, so that an external device can perform high-speed synchronous data exchange. Note that you can't use the HSKi line for receiving DTR if you're using it to receive a high-speed data clock.
Each port's General-Purpose input (GPi, pin 7) is received and inverted through a differential receiver (26LS32). The output of the differential receiver is connected to the SCC's Data Carrier Detect (/DCD) input for that port. This input can be used to provide a handshake signal from an external device to the computer. The /DCD input to the SCC can be polled by software or can be used to generate an interrupt.
Note: Because a 26LS32 differential receiver is used for the external handshake or clock signals to the SCC, the signals must be bipolar, alternating between a positive voltage and a negative voltage with respect to the internally grounded input. If a device uses ground (0 volts) as one of its handshake logic levels, the receiver interprets that level as an indeterminate state, with unpredictable results.
The SCC's Receive/Transmit Clock (/RTxC) inputs for both ports are driven by a single crystal oscillator circuit. This is accomplished by connecting a 3.6864 MHz crystal between the /RTxC and Synchronization (/SYNC) input of port A. Port B's /RTxC pin is connected to port A's /SYNC pin to drive port B's clocks from port A's oscillator circuit. Because of this single circuit, Write Register 11 (WR11) bit 7 must be set to 1 for SCC port A and must be set to 0 for SCC port B. The SCC itself is clocked at 3.58 MHz by the Apple IIgs' Color-Reference clock (CREF) at the SCC's PCLK clock input. The maximum asynchronous transmission rate supported by the SCC is 57,600 bits per second (bps) in X16 clock mode (WR4=01xxxxxx).
The SCC's Interrupt Enable In (IEI) and Interrupt Acknowledge (/INTACK) inputs are both tied to logical high in the Apple IIgs. Keeping the SCC's IEI input high enables the SCC to always generate interrupts if interrupt modes are enabled through software. Keeping the SCC's /INTACK input high leaves the SCC in Interrupt Without Acknowledge interrupt mode.

Chapter 8: I/O Expansion Slots

First Edition -- Page 167, Direct memory access

DMA bank register location is $C037.

Further Reference:


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