UNIX System Administration Handbook - Evi Nemeth [76]
A cheap breakout box can be worse than no breakout box at all. Our favorite is the BOB-CAT-B made by Black Box. It is an easy-to-use box that costs around $250. You can reach them at (724) 746-5500 or www.blackbox.com.
See page 391 for more information about Black Box.
7.15 OTHER COMMON I/O PORTS
Serial ports used to be the unchallenged standard for attaching low-speed peripherals to UNIX systems, but now that UNIX is increasingly seen on PC hardware, we have inherited several additional options from the PC world.
PC-style parallel ports are similar in concept to serial ports, but they transfer eight bits of data at once rather than just one. They’re significantly faster than serial ports but require bulkier cabling and connectors. Parallel interfaces are most commonly found on printers, but in the Windows world they’re also used to connect Zip and tape drives, which require more bandwidth than a serial port can deliver. UNIX support for parallel devices other than printers is scant, however.
USB, the Universal Serial Bus, is a more recent innovation that puts traditional serial and parallel ports to shame. It’s fast, architecturally elegant, and uses standardized cables that are both simple and cheap. Unfortunately, it will take years for organizations to get rid of their existing serial and parallel devices. For now, we can only dream of living in the USB promised land (and hope that Microsoft doesn’t build on all the good lots first).
Parallel ports
PCs have had parallel ports for decades, but only recently have they started to make their way onto UNIX systems. Any UNIX system based on PC hardware will have a parallel port, of course, but some manufacturers have also begun to add them onto dedicated UNIX workstations so that UNIX wireheads can use printers made for the Windows market.
Parallel ports have adhered to five or six different protocol standards over the years, but contemporary parallel interfaces are compatible with all earlier versions. The best standard now available is IEEE-1284, which incorporates compatibility with most prior standards (both de facto and written).
To achieve the fastest throughput speeds, modern parallel ports can be set to operate in either EPP (Enhanced Parallel Port) mode or ECP (Extended Capability Port) mode, both allowing speeds of 2 MB/s and beyond. The two high-speed modes are more or less equivalent, except that ECP supports DMA. It’s questionable whether this makes any difference in practice.
Computers usually provide a female DB-25 connector for the parallel port, and peripherals tend to have a female 36-pin Centronics connector. Therefore, most parallel cables are male DB-25 to male Centronics. A third connector type, mini-Centronics, is also permitted by IEEE-1284.
Parallel cables can be up to 10 meters long. Since cable lengths are limited and only two types of connectors are in common use, it’s more cost-effective to buy prefabricated parallel cables than to make them yourself.
Although there are many Windows peripherals that can connect to a PC’s parallel port, only printers are widely and generically supported under UNIX. Other peripherals, such as Zip drives and video cameras, need a device-specific driver to be usable. Drivers are available for a number of popular devices, but support varies by operating system, and the drivers must usually be installed by hand. As might be expected, Linux is miles ahead in this arena.
For better or worse, UNIX support for parallel devices will probably never get much better than it already is because development effort is likely to be concentrated on USB in the foreseeable future.
For a useful list of parallel port FAQs and related parallel port information, see the entry under “parallel port” at www.webopedia.com.
USB: the Universal Serial Bus
USB is a generic peripheral interconnect system designed by Compaq, DEC, IBM, Intel, Microsoft, NEC, and Northern Telecom. The first USB standard was published in 1995.