Connection extension for USB hard drives

michael baxter

michael baxter

michael baxter

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8 Nov 2005
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668
I've got a number of hard drives mounted in USB cases, and I use them for offline backups of my main rig.

However, I'm moving PC onto a filing cabinet which is about 5 meters away from my desk. I have extended my VGA cable to suit. I have also daisy chained two 1.8 meter USB cables and two usb hubs to my mouse and keyboard, as below:

PC - 1.8m cable - hub - 1.8 meter cable - hub - mouse+keyboard

I'm on the understanding 1.8 meters is the recommended limit for a single USB cable, hence the hubs. This setup seems to work fine.

My question is, will it be safe to plug in a USB hard drive as well? Or will I be asking for trouble and possible data failures?

Bandwidth isn't a huge issue, but reliable operation is!
 
So long as the hubs are self powered i.e. with their own PSU's it's fine. If not, then you are asking for trouble. I know USB drives are powered themselves, but wanting data to reliably travel along a length already out of spec and running on the power in the cable, might be pushing it.

As I say, so long as those hubs are powered, I wouldn't have any qualms about connecting up a USB drive. Maybe test it on one drive first and see how it is for a week. MD5 sum some data transfers on large files - if it consistently fails, then it may not be the best idea.
 
Please don't take what I say as gospel here though - it might be completely fine without the hubs or it may fall flat on its face. This is something so bespoke, only you could really test it to see functionality - and what I say is only based on the fact that you are going a bit out of spec. In fairness, it should work fine, but then again I'm assuming this is data which is worthy of being backed up so precautions never go amiss. MD5'ing large file transfers seems the best method of conducting a crude test on functionality, but maybe you could use even more stringent error checking. Command prompt has some bit checking commands so these might be worth looking into.

Hope it works out :).
 
I have my printer connected via a 10-metre USB cable and never had any problems... although I imagine data transmission would be much more sensitive for something like a HDD!

[edit] An extract from USB.org:

Q1: How long of a cable can I use to connect my device?
A1: In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).

Q2: Why can't I use a cable longer than 3 or 5m?
A2: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.

Q3: How far away from a PC can I put a USB device?
A3: With the maximum of 5 hubs connected with 5m cables and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route, you will probably be able to reach out 25m or so from the PC.

Relevant for you -> Q4: I need to put a USB device X distance from my PC. What do I do?
A4: If X is less than 25m or so (see previous question), buy a bunch of hubs and connect them serially with 5m cables. If you need to go farther than that, put another PC, or maybe a laptop, out where you need the device to be and network it with the first PC using something that's intended to be a long-range connection, such as Ethernet or RS-485. If you need to use nothing but USB, consider using USB based Ethernet adapters to hook the PCs together.
Click to expand...
 
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Thank you both for the input, I'll be putting the setup through some tests first. I'll also get some decent quality cables too, as the ones I have were only a couple of quid, so probably won't have good screening.

Thanks again!

Michael
 
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