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About Macintosh Serial Expansion

Why more serial ports?

Most Macintoshes have two built-in serial ports which can support a printer and a single modem, sufficient for many users. Newer Powerbooks and some low-end machines may have only one, but have the capacity for adding an internal modem. Many Mac users have discovered that they need a third or fourth serial port to add more serial devices, such as additional modems or printers, voice mail handlers, Visioneer scanners, Wacom graphics tablets, digital cameras, label makers, MIDI controllers, etc. An increasing number of Mac users need even more than four serial ports to handle multiple modems for E-mail or fax servers, small bulletin board systems, or voice mail handlers. The simplest situation is someone who never needs to run more than two serial devices at the same time - a simple switch is usually sufficient to use two printers, for example. The more vexing problem is the user who needs to run more than two serial devices *at the same time*, and can only be solved by adding additional serial ports.

How do I add more serial ports to my Mac?

1.  Serial Switches

You can use a mechanical or electronic switch box. Mechanical switchers should work fine, allowing you to switch one of your Mac ports among three or four devices. Of course, this does not allow more than two simultaneous serial connections. Some devices, like fax modems, scanners and graphic tablets install software at Mac startup and do not like being switched without restarting the Mac. There are a couple of electronic "switch boxes," using a control panel to switch ports or doing it automatically when you switch applications. The unit from Silicon Valley Bus Company (voice: 406-623-2330, email: silibus@aol.com) is the only unit currently available that allows switching of GeoPort devices, and allows switching of both the Mac serial ports. If using are considering using an electronic or automatic switcher on a PCI Macintosh, be sure to inquire with the manufacturer about compatibility with latest Apple OS software.

2.  NuBus Serial Expansion cards

  • Many desktop Macintoshes can accommodate one or more NuBus internal expansion cards. In mid-1995, Apple abandoned NuBus in favor of PCI bus expansion cards (more about Macintosh PCI bus). Three companies supply most of the NuBus cards for additional serial ports: Advanced Logic, Creative Solutions, Inc., and Digiboard.

  • 3. PCI serial expansion cards

    4.  External via SCSI

    You can use a Creative Solutions SCSI-based external serial expansion unit. These add two or four ports each and appear to work reasonably well with PCI Macs. For multiple line, high load applications (like busy BBS, fax or mail servers), only one or two of these devices can be added per Mac before excessive motherboard CPU slowdown begins to occur (typically 10% slowdown per SCSI box). Thus this method is only practical for two to four ports at 115,200 baud or up eight ports at 38,400 baud.

    5.  External via Ethernet.

    You can add an external Ethernet serial expander or wide area router. This may be an ideal solution for adding up to eight ports to a medium to large network. However, these ports are exclusively for wide area servers and typically are not useful for connecting to printers, scanners, fax modems, or voice mail handlers. High "traffic" with several high-speed modems or ISDN adapters will put a heavy load on the Ethernet channel.

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