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USB Daisy Chain

Whiteout

So the max number of devices you connect via a usb hub or daisy chaining is 127. Somebody humor me and give me an example of when you would need such an extrodinary high number of USB devices connected.

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Comments

demonfurbie

Swag usb drives and a raid

Whiteout

demonfurbie wrote: »

Swag usb drives and a raid

What are swag usb drives. And you talking like a massive hdd raid?

demonfurbie

Swag is the free stuff vendors give out, it was about 128gig hd when I was done

Devilsbane

I don't think you ever would. I know some people that have tried to max it out but when you start getting to large numbers there becomes some reliability issues. Not to mention the amount of external power you would need.

JDMurray

USB is both a shared power bus and a shared data bus. So the problem isn't with the number of devices on a USB bus, but with how much communications bandwidth those devices are using, and how much power they are drawing from the bus itself.

Using USB devices that have their own external power supplies, such as USB hubs powered by a wall wart, helps quite a bit. But insisting on running many USB devices that communicating large amounts of data simultaneously(such as printers, scanners, headsets, hard drives, etc.) slows down the bus for all connected USB devices, like Internet access speed slowing to a crawl when too many people are on the cable at 6PM. Many gaming aficionados will not use a USB (or wireless) keyboard/mouse for this reason. Access to the shared USB medium can be spotty when under a high load.

Whiteout

@Furbie - Ha ha, thats funny. So you just slowly started a train of USB's from all the free ones you got?

@JD - Ah good way of putting it. I never thought of the communication bandwith issue, only just the power issue. Man you have to be pretty hard core to buy a non-usb mouse just for gaming so you don't have to worry about miliseconds of lag. I recently just bought a gaming mouse and was doing some research and never once came across a non USB one. I did however not buy a wireless mouse, I am that nitpickey about reaction time. Also wired ones are cheaper.

demonfurbie

yep, it was slow but it was there

JDMurray

Whiteout wrote: »

I recently just bought a gaming mouse and was doing some research and never once came across a non USB one.

Hard-core gamers will limit the number of USB devices plugged into their rigs, so a USB mouse isn't necessarily a source of micro-lag. I use USB-to-PS/2 converters for mouse/keyboard and a headset with audio/mic plugs and plug it directly into the sound card's jacks. I'm no fast-twitch gamer, but I do run a lot of USB-connected hard drives for forensics work, so I prefer controller devices off my USB bus.

saspro

The 127 device limit was decided during "plugfest". They basically kept plugging things in until usb crashed (whilst drinking beer I imagine)

JDMurray

saspro wrote: »

The 127 device limit was decided during "plugfest". They basically kept plugging things in until usb crashed (whilst drinking beer I imagine)

...or the USB standards committee decided a 7-bit device ID was sufficient. I'm glad the IP committee back in the 70's didn't think that way.