USB to Parallel port

Nothing. They don't go anywhere. A few are completely unused and the others control things such as the secondary channel that are almost never used. They aren't implemented on most devices even with the DB25 connector - this goes for PCs with 25 pin serial ports. BTW the connector conspiracy is more complicated than you seem to envisage. DB25 ports can be used for other serial standards or completely different protocols - Apple used them for SCSI which wasn't such a good idea (the connectors aren't really suited to SCSI speeds). For RS232 on DB25 the convention is for DTE wired devices to have male connectors and DCE devices to have female, but again this is flexible

- some manufacturers, e.g. Sun used female connectors on DTE machines with the rationale being that they are less likely to be damaged than male connectors.

Primarily for legacy reasons. However using connectors with more pins than required is actually quite common. The 6 pin PS/2 keyboard and mouse connectors only use 4 pins for instance. That one really is oddball because they could have just as easily made a combined keyboard-or-mouse port by putting the signals for one device on the unused pins (in the same way laptops do) and saved the hassle of plugging each device into its correct port when the machine is against the wall and you can't see what you're doing...

Others have mentioned the latency involved but the driver is also tricky for the general case. Some device use I/O ports in ways that were never envisaged in the original design, using low-level bit banging of individual signals to create the desired effect. This is usually acomplished by dealing with the hardware directly - it isn't as simple as wishing up a new driver. While simple stuff like a printer will usually work fine with a USB parallel port things like Zip drives and scanners may not. Some circuits are even more dependent on this low level control - the minimalist parallel port PIC programmers will not work at all for instance.

-- Andrew Smallshaw snipped-for-privacy@sdf.lonestar.org

Quite a few years ago RS-232D tried to start doing that, but I beleive=20 most of it was advisory.

The pre-dominace of DB25 on modems and serial came from the use of modems and PTT standards, hence CCITT (now ITU).

--=20 Paul Carpenter | snipped-for-privacy@pcserviceselectronics.co.uk PC Services Timing Diagram Font GNU H8 - compiler & Renesas H8/H8S/H8 Tiny For those web sites you hate

When applied to a serial channel, far fewer than 25 pins are needed. Often only 3 are used, or sometimes 5. A 9 pin connector is adequate to carry that. The 9-to-25 adapaters connect the signals that are conventionally used in serial applications, and leave the rest unconnected.

In the old days, before the IBM PC, serial ports were on 25 pin connectors (and often female on the computer side), and parallel ports were on centronics connectors, as the printer end of a printer cable often still is. IBM started a bit of a new connector scheme, which became a defacto standard itself, but the original standards remained too.

USB is both faster and slower than a traditional parallel port. It's faster in the sense that it's designed to let you shove more data down it per unit time - it has high "throughput". But it's also slower, in that it takes substantially longer to set up a data transfer - it has substantial "latency". So USB is terrible for some classic parallel port hacks, like controlling motors on hobby-scale comptuerized machine tools, and also for applications that involve rapid back-and- forth "conversations" of very short messages betweeen device, which includes some kinds of programmers for embedded devices. Sometimes you can get around this by moving the "device interactive" part of the program from the host PC into a micro controller sitting on the other side of the USB, and have the PC give it higher-level work orders.

USB to parallel adapters exist, but generally they only work well for what they were designed to do: talk to printers. They tend to be either slow, or completely unworkable with many of ht other gizmos that have been designed to hang off a PC parallel port over the last

Some laptops had a PS/2 "keyboard-or-mouse" port. The electrical interface is exactly the same, but the command set is slightly different, so I guess that is used to detect which is connected.