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Use of SCSI Command Set by USB

Q: I think it would be useful to mention that USB uses the SCSI command set, despite adopting a different physical architecture. This is actually highlighted in the SCSI article. As the current article stands SCSI and USB seem like completely unrelated technologies. Nick 08:50, 20 Aug 2004 (UTC)

A: See USB Mass Storage Rationale of SCSI over USB.

Can someone please explain usb cable types

I know there are at least two and maybe more types of cables for USB - Could someone differentiate them? Is one type of plug/cable for USB 1 and another for USB 2? Or does the cable/plug type not matter? I know they are compatible, does one cable/plug limit the bandwidth?

I have tried to find this on the internet but my searches only turn up vendors trying to sell stuff - it is overwhelming.

Could someone please research this and maybe even put pictures of the USB plug types up on the page?

Oh, and I know that a USB 1 hub would have to be replaced to handle the bandwidth of USB 2, but I do not know if the cables make a difference.

The link to the USB.ORG site includes access to the specs for cables which include pictures (drawings). The A end hooks to the host and the B end hooks to the device. As noted above, in the extension called On-the-go, there is also a hermaphroditic socket which will accept either the A or the B end of the mini-version of the standard cables.

There is no plug/socket change for High speed (480Mpbs) but the cable spec was tightened in the 2.0 version to allow for the higher transfer rates. A 1.1 Spec. hub will work on a 2.0 system but will limit the maximum speed of any down-stream devices to 12Mbps regardless of whether the downstream devices are High-speed capable or not. - richard

USB A-A cables

Can someone write in detail on the topic of USB A-A cables? From what I know, there are 2 types of these:

• Plain A-A cablle
• A-A with electronics

I don't know what the first one is used for, it costs ~$5 (nominal cost - cable and 2 terminals). The latter can be used to connect 2 computers and includes some kind of device, so that files can be copied (~$50). I have some info that files cannot be copied from computer to computer on the plain ones because USB is an asynchronous protocol, but I don't understrand the technical details. Any info on this? Any software workarounds for file transfers using the cheap cables possible?

Helix84 13:25, 3 Nov 2004 (UTC)

There shouldn't be any topic of USB A-A cables. The ones that have electronics in the middle are just another device. The electronics just expose a bulk interface on each side, one to each host controller. These usually require some special software that sends files or whatever over the bulk interface to the same software running on the other machine. These aren't deserving of their own topic, they're just another device.

Those that don't have electronics in the middle are invalid according to the spec. Since the first machine you plug one into is sending 5v and the next machine you plug it into EXPECTS to send 5v, you're just likely to blow the motherboard of one or both machines. Just bad, bad, bad. Of course you occasionally run into some no-name stupid device that uses an A-A as a device cable, avoid them like the plague. If they couldn't even get the cable spec right it's unlikely the device will work well AT ALL. -- SchmuckyTheCat 8 Dec 2004

USB 2 HS vs. FireWire

I would like to comment on USB2.0 being in direct competition with IEEE 1394. Here are a list of features that makes then resolutely different:

Application domain

• USB addresses needs for a wide range of devices (mouse, keyboard, modem, hard disk drives, scanner, printer...) that do not exist in IEEE 1394.
• IEEE 1394 addresses needs of audio-video devices such as videorecorder, digital camera that have no real equivalent on USB.

USB's isochronous mode does apply to streaming devices including videorecorders and audio devices such as speakers, microphones, etc. The High-speed (480Mbps) use of isochronous is directly comparable to use of IEEE 1394/Firewire for these devices - richard

Communication paradigm

• USB provides host to peripheral communication. A host computer is required in the system.
• IEEE 1394 provides non-centralized networking. You may build a network with simply a VCR and a TV tuner.

Sylvwild

Yes, the system requres a host - but the USB On-the-go extension allows for the possibility of a unit that is usually used as a device to become a host for the purposes of a point to point conversation as you describe. The initial setup is determined by which end of the cable is plugged in to which unit but is switchable under software control so that the initial device may assume host and vice versa. The USB OTG sockets are hermaphroditic - can accept either the A or B end of the cable. - richard

"Throughput"

"USB 2.0 boasts 480Mbps throughput"

I don't think so. USB 2.0 does signaling on the wire at 480 megacycles per second. The physical layer transports up to 480 million bits in one second. But that doesn't mean it has 480 Mbps throughput. In fact, it pretty much guarantees the throughput is less than that.

Looking at the spec, the fastest way to transfer data seems to be with a high-speed bulk transaction with a data payload of 512. This gets you (see p.55) 53248000 bytes/second of bandwidth, or just under 426 Mbps throughput. And that's assuming you can saturate the line with 100% high-speed bulk transactions; I think you have to have other transactions going on to request all that data, which is why even the theoretical throughput is lower than 426 Mbps.

Am I missing something?

I haven't seen the Firewire spec, but it probably has something similar going on. I doubt it has 400 Mbps of throughput. But it does seem to perform better than USB 2.0: this should be a good indicator that USB doesn't really "boast" more "throughput".

USB 1.0, 1.1, 2.0

"Confusingly, the USB Forum has renamed USB 1.1 to USB 2.0 Full Speed; and USB 2.0 to USB 2.0 High Speed."

Are those definitely absolutely the same? Please provide a reference. I find conflicting info online. - Omegatron 22:39, Nov 6, 2004 (UTC)