Classic Mac Ports

Older computers often have simple serial ports instead of USB or FireWire connections. These only accommodate one device at a time, although you can use a serial port switch box to choose one of several devices.

Most ‘classic’ Mac OS models have two such ports, usually designated as Modem and Printer ports, as well as Apple Desktop Bus (ADB) connections. All of these convey data in serial form, with each bit of data transported in turn along a common circuit, which makes them rather slow, although they employ convenient low-cost connectors and lightweight cables.

The Modem and Printer ports are based on the RS-422 standard, with data sent in asynchronous form, consisting of a start bit, followed by eight data bits and a stop bit. These short bursts minimise timing problems, avoiding the need for a separate clock circuit. Hence the data can be eventually sent over a two-wire circuit, such as a telephone line connected via a modem.

To ensure data is sent reliably over a link, some form of handshaking is normally used. Software handshaking, operated by special control codes sent between the devices, is effective, but slows down the interface. Hardware handshaking, as employed, for example, between a modem and a computer, is faster but requires additional wires in the cable.

Unlike the standard serial ports, the ADB interface on older Mac OS machines is synchronous. Hence it includes an extra wire that conveys a clock circuit, which provides a timing reference for the receiving device.

Connection Systems

A Modem or Printer port can provide a point-to-point connection, such as linking a printer to a computer, as shown below:-

To use several devices you must fit a manually-operated serial port switch box or serial port expander, the latter using software to fool the computer into seeing only one device at a time. The box or port expander is connected as shown here:-

The ADB interface is different in that it acts as a bus, allowing several devices, such as a keyboard, mouse or modem to be connected at once. Connections can be made using a daisy chain, as shown below:-

Modem and Printer Ports

Although the Modem and Printer ports are essentially standard RS-422 interfaces, they can also be wired for compatibility with RS-232, allowing them to operate with almost any serial device. The ports appear on an 8-way mini-DIN socket or 9-way mini-DIN socket, although very early Macs are fitted with a 9-way miniature D socket, also known as a DB9 connector. Unfortunately, the wiring of these connectors is entirely different to those found on non-Macintosh equipment.

These ports default to a rate of 9.6 kbit/s, which is fast enough for most printers, although they can run at up to 57.6 kbit/s or higher. With some computers, an external clock can be applied to the General Purpose input (GPi) pin, allowing speeds of 115.2, 230.4, 460.8 or even 921.6 kbit/s. When used for a LocalTalk network, the ports run at 230.4 kbit/s.

• ‘Classic’ Mac OS machines use a serial port controller (SCC) to transfer data between the processor and both ports.
• Serial devices often work with ports built into a machine, provided by an external serial adaptor or fitted on an internal expansion card, although the ports on older hardware may not be compatible with modern devices.
• With DOS compatibility software in the Classic Mac OS you can use each port as a PC port, although hardware handshaking may not work. A PC mouse, as designed for the COM1 or COM2 serial ports on a PC, can also be used, providing a suitable hardware adaptor and software is available.

Although the ports are designated as Printer or Modem, you can usually plug any device into any port, although some Printer ports aren’t GeoPort-compatible and may lack a GPi connection (see below).

• Most computers give timing priority to the Modem port, so this should be used for fast devices.
• Older versions of the Classic Mac OS require you to use the Printer port for LocalTalk. If in doubt, connect the modem to the Modem port and the printer, or printer network, to the Printer port.

GeoPort Compatibility

Later ‘classic’ Mac OS computers often have a 9-way mini-DIN socket for one or both of the serial ports. This accepts a matching GeoPort Telecom adaptor (a special modem that also works as a speakerphone and answering machine), any other GeoPort-compatible device, such as a printer or ISDN terminal adaptor, or a standard serial device.

GeoPort-compatible devices are frequently powered from the + 5 volt supply that appears on pin 9 of the socket. Older ports only accommodate 8-way mini-DIN plugs and therefore can’t provide such a feed. In theory, you can supply the device with an external source of power, although most GeoPort devices also need a GPi connection (see below).

General Purpose Input (GPi) Compatibility

In modern equipment, pin 7 of the mini-DIN connector, supposedly assigned to the DCD signal for a modem, is also used as a General Purpose input (GPi). Some devices, especially those that transfer data at an unusual rate, apply an external clock to this circuit, directly controlling the speed of the port. A GPi input is required by many products, including GeoPort-compatible devices, some modems, ISDN adaptors and MIDI adaptors, as well as certain printers and scanners.

• Before using a serial device it’s worth checking your computer port for GPi compatibility. Those that don’t provide a GPi connection include all ports with a DB9 connector, the 8-way mini-DIN ports on older Mac OS machines (but sometimes only the Printer port), those on some types of expansion card and ports on a USB to serial port adaptor.
• A typical MIDI adaptor injects a 500 kHz or 1 MHz signal into the GPi pin. The computer hardware then divides this by 32, producing the standard MIDI data rate of 31.25 kbit/s.

Adding Extra Serial Ports

Special hardware can be used to provide extra ports, all of which usually work with the Communications Toolbox (CTB) element of the Classic Mac OS, thereby making them appear in all CTB-compatible applications and control panels.

Several options exist, including:-

• PCI Expansion Card

  Suitable for most desktop PCI-based machines, often providing four ports running at a maximum speed of 115.2 or 230.4 bit/s. Unfortunately, such ports may not be GeoPort-compatible and can lack a GPi connection.

• PC Card

  Similar to a PCI card, but designed for the matching slot or slots in a laptop machine.

• Modem Slot Card

  A small single-port card, replacing the internal modem card in an Apple G3 or G4 computer. This is a very effective solution for those who don’t need the original built-in modem.

• USB to Serial Adaptor

  Suitable for machines with USB ports no PCI slot. This usually has two 8 way mini-DIN sockets for Mac OS devices or a DB9 plug for non-Apple peripherals. Although often limited to 57.6 kbit/s, this shouldn’t cause problems with most equipment. Unfortunately, adaptors of this kind can’t be used for LocalTalk connections, GeoPort products or devices that need a GPi input. This means that some models of printer, modem or MIDI adaptor can’t be used.

• Serial Port Switch Box

  A low-cost solution for machines that have too few serial ports. The switch can only be moved when the port isn’t carrying data, so you can use it to choose a printer between printing sessions but not for switching a LocalTalk circuit.

• Serial Port Expander

  Effectively an intelligent serial port switch box. One variety, known as a PortXpander, provides three serial ports, using special software to fool the computer into thinking that only one device is actually connected to the port. Fortunately, this particular expander can also be used to connect GeoPort products or a LocalTalk network.

Modem and Printer Ports in Detail

Standard Connections

Most Modem or Printer ports appear as an 8-way or 9-way mini-DIN socket, wired as shown below:-

Pin 2, which is normally used as a handshake input, can also accept an external clock signal for use with devices that run at non-standard speeds. The GPi input on pin 7 isn’t available on all models.

Older Mac Connections

The earliest Mac OS models have a 9-way miniature D socket, also known as a DB9, wired as follows:-

Standard Interconnections

For compatibility, the two-wire (balanced) data circuits on the serial port conform to the RS-422 standard whilst the single-wire (unbalanced) handshake circuits comply with RS-232 signal levels.

To further simplify matters, the pin connections on the socket at the back of a printer are arranged to be the same as those on the back of a computer. This means that a standard printer cable, which is actually wired with crossover connections between the data and handshake pins, can be used to connect any two devices. For example, you can instantly create a LocalTalk network between two computers by plugging them together with an ordinary printer cable.

The wiring for a standard printer cable is shown below:-

Where software handshaking is used the HSKo and HSKi circuits aren’t really necessary. And if you’re short of multi-way cable you can also omit the TXD+ and RXD+ wires over short distances. As mentioned above, the GPi input is essential for some devices but can be omitted for others.

If you want to connect to an elderly Mac OS computer with a DB9 serial port you’ll need the following cable:-

Again, you may be able to reduce the number of wires. However, you should be aware of the lack of a HSKo circuit on the DB9 connector. This means that the computer can’t provide hardware handshaking for any device that’s sending data to it.

The adaptor shown below converts the standard connector on a modern machine into the older form:-

• This adaptor doesn’t give a +5 or +12 volt supply, as provided by older Mac OS machines.

Connecting Devices to a Classic Mac

A variety of adaptor cables can be used to connect different RS-232 and RS-422 devices to the serial ports on a ‘classic’ Mac OS machine. For full compatibility with RS-232 levels, you should link the RXD+ and Gnd wires (as shown in the diagrams below) although many devices work without such a link.

• As well as a suitable cable you’ll need matching software for a peripheral device. For a printer, the Classic Mac OS normally uses a Chooser Extension file that’s kept in the Extensions folder.

Connecting an RS-232 Modem

The cable shown below can be used to connect a Mac OS computer to a modem, also known as a DTE device:-

• The GPi connection is often ignored by software and may not be connected. However, some devices, such as a synchronous modem, do require this connection.
• The DTR connection at the modem is usually provided but isn’t necessary. However, if you want to use hardware handshaking, as required for high-speed modem operation, you must configure the modem to ignore the DTR line.
• For high-speed modem operation there shouldn’t be a link between pin 2 of the mini-DIN plug (normally CTS) and pin 6 on the DB25 socket (DSR).
• Any cable used to extend a modem’s connection cable must be wired pin-for-pin. A crossover cable, also known as a null-modem cable, shouldn’t be used.

Older modems that only require software handshaking may use a cable wired as follows:-

• The DTR connection at the modem is usually provided but isn’t normally necessary.

For fans of the antique, here are the connections for wiring an old Mac OS computer with a 9-way D connector to a modem:-

Connecting an RS-232 Printer

The following adaptor works with a DTE printer, including the original ImageWriter:-

With some devices you must also connect pin 1 on the mini-DIN connector (HSKo) to both pin 6 (DSR) and pin 8 (DCD) on the 25 way D plug. This extra handshaking, working in the opposite direction to normal, isn’t usually necessary for a printer.

Apple Desktop Bus (ADB) Ports

ADB is a synchronous port that operates as a bidirectional serial bus at 4.5 kbit/s. In theory, up to 16 devices can be connected, including the computer itself. In practice no more than three devices, excluding the keyboard or trackpad in a portable computer, should be wired into a single daisy-chain.

An ADB splitter cable can be used for devices that only have one socket. Alternatively, an ADB switch box can be used to select particular devices, as long as the switch isn’t moved while useful data is in transit. The total cable length of the daisy-chain shouldn’t exceed 5 metres.

• According to some sources, connecting an ADB device to a Mac OS computer whilst it’s powered can short out the port’s +5 volt power supply, possibly causing damaging to the machine.
• If you start up a computer without an ADB mouse, the machine may not respond to one if you plug it in later. However, if started with a mouse, you can replace it with another, although this is risky (see above).
• Some monitors can be controlled via an ADB connection, although modern machines, apart from some early G3 models, don’t have the necessary port, requiring you to obtain a USB to ADB adaptor.

ADB Connections

The ADB connection appears on a 4-way mini DIN socket wired as follows:-

As well as data, it conveys a +5 volt power supply for other devices, which has a total current capacity of 500 mA. Fortunately, a mouse only uses 80 mA whilst a keyboard takes between 25 and 80 mA. In some instances this supply is protected by a black fuse on the computer’s motherboard.

• Apple PowerBook computers normally only accept low power ADB devices (denoted by a D symbol), which prevents you from using older keyboards or other high-power devices.
• The remote operation of the computer’s (Power) key via ADB can be useful. In fact, some commercial products use this feature to restart an unattended machine whenever it crashes.

Apple Floppy Drive Interface

Some very old Mac OS computer can accommodate an external floppy disk drive via a special serial interface. This runs at 62.5 KB per second and is connected via an external 19-way miniature D connector, also known as a DB19, or an internal 20 way ribbon connector, wired as follows:

• The power feed to the drive is sometimes protected by a yellow fuse on the motherboard.
• This interface is different to the 38-way connection used for floppy disk drives in a PC.
• Some Apple PowerBook models also accommodate an external floppy drive but use a different connector.

©Ray White 2004.