computer 1 usb 1<====>usb 2 computer 2

computer 1 usb <====> computer 2 usb

i bought a usb cable with two male ends. then plugged that in to an xp box and a vista box.

nothing.

so how come the computers do not identify them selves to each other ? if you buy a usb memory device it will identify itself to the computer. they will see each other over wifi but not usb ? think there is something not designed in to the computers that should be there. future suggestion. include usb to usb id.

thought it would make a good quick null modem to transfer files from one computer to another.

I think those kinds of devices are available.(can't remember the name).
All I remember is that it had a little cube somewhere in the middle.
Some extra driver is needed though, and I could not get it working for a linux distro.

You need a special cable for this... Computer-to-Computer USB Networking Cable | Overstock.com ... note the electronics pod in the middle. Something has to handshake with the usb ports before the computer will recognize them.

Also be aware that with power in the connectors it is possible to damage both computers with the wrong cable.

A modem is a serial/bit-streaming device. A usb port is more abstract, like an IDE or SATA connection. It represents a block device. So this is like saying, "Why can't I plug two hard drives directly together with an IDE cable to transfer data? Why does it have to go thru the computer?"

Ie, you need some additional hardware and software to accomplish this (eg, the cables mentioned, which I am sure require software drivers to be useful).

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Originally Posted by MK27

A modem is a serial/bit-streaming device. A usb port is more abstract, like an IDE or SATA connection. It represents a block device. So this is like saying, "Why can't I plug two hard drives directly together with an IDE cable to transfer data? Why does it have to go thru the computer?"

Ie, you need some additional hardware and software to accomplish this (eg, the cables mentioned, which I am sure require software drivers to be useful).

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Yes they do.... But Windows since XP/SP2 has the driver in it's built in repository. If the OS has the data transfer module (used to move stuff from old computer to new computer) the computer to computer cable should work.

Also, a little known USB trick is that a device can actually store it's own driver which is uploaded to the host as part of the Plug and Play handshaking sequence...

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Item#: 934629

This cable provides the perfect Plug-and-Play solution for networking. It is USB 2.0 compliant for transfer speeds up to 480Mbps, but also works in USB 1.1 ports at USB 1.1 speeds!

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Nowhere on the product description, the packaging, the installation CD or on the cable itself is the manufacturer listed. This was a big problem when I found that the installation CD was blank, and could not load the drivers -rendering the product useless.

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serilously ? <bangs forehead on wall> why meow ?

by the one and only review i would guess that the cable is older ie made before roxio changed the cd format which is why i think the cd is not blank just not readable with the newer format.

[img]www.starmount.co.uk/usb_skta.gif[/img]
this is only four lines. vc+ data+ data- and gnd so the data lines are only one way ? ok i see that the - needs to connect to the + data line ie one in and the other out line for data flow. i knewd that the xp had a driver for the usb but not sure if it would work connecting two computers.

the 5v to 5v should not do any damage. it is when the v is diff is when probs happen.

thank you for the replies and suggestions.

What you try can not be done without any in the middle electronics. What you are trying to do is hook up 2 USB Host devices together, but the Host is expecting a Client device to be plugged in, so you have 2 Hosts waiting for a client to be plugged in. This has nothing to do with the wiring and everything to do with the specification of the USB protocol.

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this is only four lines. vc+ data+ data- and gnd so the data lines are only one way ? ok i see that the - needs to connect to the + data line ie one in and the other out line for data flow. i knewd that the xp had a driver for the usb but not sure if it would work connecting two computers.

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NO! USB is NOT serial port (where you have to exchange Rx and Tx).

D+ and D- are used in differential signaling. Meaning, they only carry one signal. Theoretically, USB can be made a 3 pin connector, with just 1 data line. They used 2 to improve high speed signal integrity. Differential signaling - Wikipedia, the free encyclopedia

I don't know what will happen if you swap the 2 lines. In the very best case, it just won't work. Worst case is a lot worse.

The USB protocol is host/client based. A device has to either be host, or client (there is a new class of USB devices that can work in what's called On-The-Go mode, as either device or client, but they can still only do 1 at a time).

5V to 5V will most certainly do damage. In the real world, you'll never get 5V +-0%. In fact, I believe USB specifies something like 4.3V to 5.3V. One side will always be higher, and will be feeding current backwards into the lower side, and it probably won't like that. In the best case, the USB controller will just lock up and shut off the port. In the worst case...

The USB standard says the host must supply power to the device, and the device must not feed current back into the host (draw 0-500mA depending on power negotiation, negative current is NOT allowed).

As you can see, it's a REALLY bad idea to plug host into host.

The USB people thought about that, so to make sure that never happens, the standard says all hosts should have Type-A ports (the USB port on your computer), and all devices should have Type-B ports (including mini and micro variants, which are electrically identical). And cables will all to A to B, and no one can plug anything wrong. If it fits, it's right. It's physically impossible to make wrong connections.

And then of course some bastards decided to make A to A cables... like the one you have. They are designed for non-standard compliant devices, and should never be used to connect 2 standard-compliant devices (like PCs) together. For their one time convenience, they broke the robust protection built into USB standard, and now every unsuspecting person can ruin their devices with those cables.

At very high level, how USB works is you have a host that awaits connection from up to 127 devices, optionally through a tree of hubs.

Every time a device is connected, the host gives it a unique address. Each device identifies itself with 2 16-bit IDs - VID (Vendor ID, indicating the, well, vendor), and PID (Product ID, allocated by the vendor, to distinguish between their own devices). The OS then uses those 2 IDs to attempt to find a driver to talk to the device. If it's successful, the OS hands control to the driver that says it can handle the VID/PID pair.

When the driver, through the host controller, wants to talk to a device, it sends the message along with the address to ALL devices. All except the addressed device ignores it. If the message is a read request, the addressed slave then starts transmitting back to the host.

All transactions are initiated by the host. There is absolutely no way a device can initiate a transaction (it could lead to very bad things if multiple devices try to transmit at the same time). They only speak when spoken to.

As you can see, there is no way 2 hosts can talk to each other. You should just throw out the cable (it has absolutely no use for you unless you have a non-standard device that happens to need it) and send unpleasant things to the manufacturer.

The cables with an electronics box in the middle work because they have a microcontroller inside that talks to both computers as slaves (everything else is driver-level magic on the computer side).

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Also, a little known USB trick is that a device can actually store it's own driver which is uploaded to the host as part of the Plug and Play handshaking sequence...

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Where did you hear that from? I have never heard of anything like that. Last time I checked, USB is completely platform-independent.

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Originally Posted by kryptkat

the 5v to 5v should not do any damage. it is when the v is diff is when probs happen.

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You wanna try that again?

If you have a simple wire cable --no interface chip-- you can have some major problems with the power connections...
You're talking about a cable that most likely has an end to end resistance of less than 2 ohms... So, lets do just a little math, based on normal variations in supply voltage... Computer A's +5vdc supply is actually 5.25v (within tolerance)... Computer B's +5vdc supply is running 4.75v (also well within tolerance)...

Ohm's law tells us that current = voltage / resitance ... so we can create a formula here...

current = (5.25 - 4.75) / 2 = .5 /2 = .250 AMPS ... and that's only from normal, in tolerance, supply variations...

You don't just simply plug stuff in, like that...

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Originally Posted by cyberfish

Where did you hear that from? I have never heard of anything like that. Last time I checked, USB is completely platform-independent.

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Oh gees, it's been a while... I don't have the link anymore. But there was at least one chip design I saw with memory space for imbedded drivers.

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Originally Posted by CommonTater

Oh gees, it's been a while... I don't have the link anymore. But there was at least one chip design I saw with memory space for imbedded drivers.

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Ah, it probably enumerates as a composite device with 2 "virtual" interfaces. One is mass storage device that just contains the driver, and the second one is the real interface. This way driver can be delivered with the device, and computer will (initially) see it as a USB drive and an unknown device.

I'm pretty sure that's not part of the USB standard, though. They worked very hard to achieve platform-independence and forward-compatibility. Allowing this in the standard would kill both things by tying a device to one (or a few) version of one (or a few) OS.

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Originally Posted by cyberfish

Ah, it probably enumerates as a composite device with 2 "virtual" interfaces. One is mass storage device that just contains the driver, and the second one is the real interface. This way driver can be delivered with the device, and computer will (initially) see it as a USB drive and an unknown device.

I'm pretty sure that's not part of the USB standard, though. They worked very hard to achieve platform-independence and forward-compatibility. Allowing this in the standard would kill both things by tying a device to one (or a few) version of one (or a few) OS.

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I've got to visit a few hardware sites over the next few days (audio parts) and while I'm surfing I'll try to locate that chip again... No promises.

What you say makes complete sense to me but I'm not exactly a USB protocal expert...

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Originally Posted by CommonTater

You wanna try that again?

If you have a simple wire cable --no interface chip-- you can have some major problems with the power connections...
You're talking about a cable that most likely has an end to end resistance of less than 2 ohms... So, lets do just a little math, based on normal variations in supply voltage... Computer A's +5vdc supply is actually 5.25v (within tolerance)... Computer B's +5vdc supply is running 4.75v (also well within tolerance)...

Ohm's law tells us that current = voltage / resitance ... so we can create a formula here...

current = (5.25 - 4.75) / 2 = .5 /2 = .250 AMPS ... and that's only from normal, in tolerance, supply variations...

You don't just simply plug stuff in, like that...

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While this may be true (and definitely a safe practice to follow)...
I'm pretty sure that so long as both computers USB bus' are running off of the same reference voltage, it's perfectly safe to run a high logic voltage from each computer directly the other, head on. (In the case of high to low logic, it should also be fine given adequate short circuiting protection)

You mean both computers running on the same PSU...?

It's also not logic voltage. Logic voltage for USB is much lower. 3.3V for low and full speed, something lower for high speed.

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Originally Posted by cyberfish

You mean both computers running on the same PSU...?

It's also not logic voltage. Logic voltage for USB is much lower. 3.3V for low and full speed, something lower for high speed.

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Logic levels are generally current limited by resistors in the chip's outputs... while it's not smart and never successful, connecting logic outputs together is seldom destructive...

However, the USB 5 volt lines are not buffered or fused and the most common result of current races is damage to the USB host devices (which may be on your motherboard!)

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Originally Posted by cyberfish

You mean both computers running on the same PSU...?

It's also not logic voltage. Logic voltage for USB is much lower. 3.3V for low and full speed, something lower for high speed.

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Well, I guess that example works. Though, I was actually thinking of even different hardware, (2 computers of the exact some model, running from the same outlet, for (safe) example).

Yeah, I guess it was a bad idea to mention logic voltage. What I mean, is, so long as the cross-voltage stays within both buses' voltage reference range.

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Originally Posted by CommonTater

while it's not smart and never successful, connecting logic outputs together is seldom destructive...

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What do you mean by never successful? What could one hope to achieve by connecting 2 (assuming constant) outputs?

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Well, I guess that example works. Though, I was actually thinking of even different hardware, (2 computers of the exact some model, running from the same outlet, for (safe) example).

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Then the voltages will most certainly be different.

Transformer has some (non-zero) tolerance, inductors have some tolerance, regulators have some tolerance, and voltage will be affected by what the computer is doing at that time. Even for identical computers.

In the real world, nothing will ever be perfect. We always talk about values with tolerances, and tight tolerance gets expensive, so we always aim for lowest tolerance acceptable.

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Originally Posted by Yarin

Well, I guess that example works. Though, I was actually thinking of even different hardware, (2 computers of the exact some model, running from the same outlet, for (safe) example).

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Tell you what... go visit the Dell assembly plant, or any big computer store that has more than one of exactly the same models... Take some nice careful power supply readings... If you find any two *exactly* the same, it will be a miracle second only to walking on water. I'm not talking about material defects or different models... I am talking about perfectly ordinary and expected manufacturing variations --tolerances-- even in identical make and model.

The only connections that are guaranteed safe are the grounds.

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What do you mean by never successful? What could one hope to achieve by connecting 2 (assuming constant) outputs?

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My friend, you should see some of the mindnumbingly stupid things I've seen people do to electronic devices...

For example: Ham radio operator come technician, wants several receivers on his desk at the same time so he gets coaxial T connectors and hooks them all together... Then he can't figure out why keying up his transmitter blew the inputs on *every receiver* he owned.

For example: A so-called audiophile who connected three different amplifiers to the same speakers, with the logic that "they're never all on at once" and couldn't figure out why he kept blowing output transistors... he had a working impedance of less than .5 ohms...

For example: The rather extreme case of a house fire started because a supposed computer expert put 24 volts into a 16 volt laptop and exploded the batteries.

For example: The CBer who kept blowing fuses every time he hooked up his radio; "The black wire is positive, right?" So, buddy puts in heavier fuses until he finally gets one strong enough to blow out the "idiot" diode in the radio... and sets his dash on fire.

And on and on and on...

Trust me on this... consumers can be extremely stupid people.

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Originally Posted by cyberfish

Then the voltages will most certainly be different.

Transformer has some (non-zero) tolerance, inductors have some tolerance, regulators have some tolerance, and voltage will be affected by what the computer is doing at that time. Even for identical computers.

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Excellent point. Do you think the same applies to microcontrollers or the same model running from the same power source?

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Originally Posted by cyberfish

In the real world, nothing will ever be perfect. We always talk about values with tolerances, and tight tolerance gets expensive, so we always aim for lowest tolerance acceptable.

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Well of course, this is also hew programming (should) work. Nothing new ;)

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Excellent point. Do you think the same applies to microcontrollers or the same model running from the same power source?

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Power has to go through many many stages before reaching low voltage DC devices in a computer. All of those stages will introduce variations.

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Originally Posted by cyberfish

Power has to go through many many stages before reaching low voltage DC devices in a computer. All of those stages will introduce variations.

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Yep... even the length of a wire or size of a foil trace can affect it...

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Originally Posted by CommonTater

Yep... even the length of a wire or size of a foil trace can affect it...

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Okay, but outside of the computer, when 2 microcontrollers of the same model are running straight from the same power source do this, is it safe then? (I ask because I'm doing this, and I don't want bit in the butt later down the road ;))

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Originally Posted by Yarin

Okay, but outside of the computer, when 2 microcontrollers of the same model are running straight from the same power source do this, is it safe then? (I ask because I'm doing this, and I don't want bit in the butt later down the road ;))

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If they are both grounded to the same powersupply and exchanging only data signals, yes it's safe.

But I would not consider connecting the supply voltages together, especially through a chip or regulator. Any difference in voltage could be damaging, may cause ground currents (which switching power supplies really hate) and might result in system failure.

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Originally Posted by CommonTater

If they are both grounded to the same powersupply and exchanging only data signals, yes it's safe.

But I would not consider connecting the supply voltages together, especially through a chip or regulator. Any difference in voltage could be damaging, may cause ground currents (which switching power supplies really hate) and might result in system failure.

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I'll keep this in mind; thanks.

In electronics in general, you always only want at most 1 driver driving a node, whether it's supply or digital or analog IO. There are rare exceptions, but it's true 98% of the time.

A node is a "point" in your circuit that is connected together, and should be at the same potential. Node (circuits) - Wikipedia, the free encyclopedia

Usually for digital IO, one side will be sending, and one or more sides will be receiving. If multiple parties transmit at the same time, the voltage level on that node will be indeterminate, and you most certainly don't want that.

That's why in protocols where multiple devices are connected to the same bus, like USB, multi-slave SPI, or I2C, there is always some kind of arbitration logic to make sure only 1 party transmit at a time.

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Originally Posted by cyberfish

Usually for digital IO, one side will be sending, and one or more sides will be receiving. If multiple parties transmit at the same time, the voltage level on that node will be indeterminate, and you most certainly don't want that.

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Yeah, I wanted to multiplex I/O onto 1 common grounded wire.
I figured that so long as the TX and RX cycles on each side were properly sequenced, each one would have time to receive the peer's bit in each given frame. In the event that both sides are TXing, the indeterminate voltage isn't being read anyway, so it wouldn't matter. After hearing you and Tater, I'm glad I didn't waste any time in implementing this :)

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Originally Posted by Yarin

Yeah, I wanted to multiplex I/O onto 1 common grounded wire.
I figured that so long as the TX and RX cycles on each side were properly sequenced, each one would have time to receive the peer's bit in each given frame. In the event that both sides are TXing, the indeterminate voltage isn't being read anyway, so it wouldn't matter. After hearing you and Tater, I'm glad I didn't waste any time in implementing this :)

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In a case like that you have two choices... place an OR gate at the receiver to isolate the outputs or in some cases a "diode or"... using two diodes to isolate the respective lines can be used. Just don't be directly connecting outputs to outputs...

As they say... "When you let all the smoke out, the circuit stops working."

5v + 5v = 10v what happened to voltage doublers ? or voltage multiplying ? or adding two voltages together ? if you tried to read or measure the voltage on the power line of two usb connected together from two separate power supplies from two different computers would you read 10v assuming that the power supplies were 5v ?

what about the circuitry ? voltage regulators ? or clipping diodes like the zener ? that would send any volts over 5v to ground leaving the 5v. so then you would only measure 5v ? assuming both power supplies have the same 5v regulator circuit.

is .125 watt from tolerance example really enough to blow a half watt resistor or half watt regulator ? i doubt any decent designer would use half watt parts in a power supply.

it is the current that blows parts. you can have a potential difference of charges extremely high as long as there is no current flow ie capacitor. same cap with a change of volts current will flow. ie sin or on off . the rating on the component says what it can take.

so the data + and data - are one way ? like full duplex ? the data + transmits while the data - receives ? or is it data + is a control line only two way and the data - is two way data only ?

i can see how that would be a problem for the latter data + as control line switching the lines.

i guess it is how the data - and + work. i will have to read up on that. usb dot org has some good docs but it is not easy to find an answer to some questions. like what will happen if you swap the data + and data - lines.

You'll only get 10V if you connect ground of one side to 5V of the other side, and leave the 5V side floating. This is assuming they are both isolated (not from the same transformer). What does it have to do with this?

Even if you can somehow get 10V on the USB power pin, it will probably just blow up. The USB spec says current cannot be fed into the host. What 0.125W from where? Most regulators are not rated for reverse current. I have no idea what you are talking about. If you want to talk about a specific circuit, draw the circuit. Don't pull numbers out of thin air and start arguing random things. Have you tried logarithm?

D+ and D- are a differential pair, half-duplex. They always carry opposite voltage (read up on differential signalling).

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i guess it is how the data - and + work. i will have to read up on that. usb dot org has some good docs but it is not easy to find an answer to some questions. like what will happen if you swap the data + and data - lines.

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Like I already mentioned in previous post, that is not how it works. If you swap D+ and D- all bits will be flipped, so a high-speed device would be mis-identified as a low-speed device, and vice versa. Then, J and K states will be flipped, which may mess up bit stuffing, etc (USB inserts extra transitions when there's no transition in the data, eg. sending all 0's, to make it easier for receiver to recover clock from the signal). The doc also doesn't say what will happen if a cat chews on the USB cable. It's undefined behaviour if it's not specified.

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Originally Posted by kryptkat

5v + 5v = 10v what happened to voltage doublers ? or voltage multiplying ? or adding two voltages together ? if you tried to read or measure the voltage on the power line of two usb connected together from two separate power supplies from two different computers would you read 10v assuming that the power supplies were 5v ?

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Nope... you'd probably read 0, because you'd be damaging stuff. On the off chance it survived you have two current sources in parallel not series, so you would only read 5 volts.

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what about the circuitry ? voltage regulators ? or clipping diodes like the zener ? that would send any volts over 5v to ground leaving the 5v. so then you would only measure 5v ? assuming both power supplies have the same 5v regulator circuit.

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The one thing you do not do is back feed power into a current source... it lets all the smoke out and the part stops working.

And.. in this case the current source is the USB controller chip... not the power supply directly. The USB standard allows for 500ma (1/2 amp) per connector... 1/2 amp at 5 volts is 2.5 watts.

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it is the current that blows parts. you can have a potential difference of charges extremely high as long as there is no current flow ie capacitor. same cap with a change of volts current will flow. ie sin or on off . the rating on the component says what it can take.

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And what about that first instant when power is turned on but the capacitor is discharged, it's effective resistance is almost 0... there can be a huge burst of current... A 1uf electrolytic capacitor going abruptly from 0 to 10volts with a 1 ohm source resistance can pass a momentary burst of up to 10 amps!

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so the data + and data - are one way ? like full duplex ? the data + transmits while the data - receives ? or is it data + is a control line only two way and the data - is two way data only ?

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No it's a differential pair... when data+ is going positive data- is going negative at the same time. It's half-duplex communication.

was not pulling values out of the air. it was calculated with the tolerance numbers in the example.

ok "same time" that is what was missing. that is redundant. but i see how that would keep better integrity of the signal quality. both data lines go high or both go low and the signal goes back and fourth between devices as necessary. like a sin 180 degrees for one data line mirroring the other data line. that "differential signaling" wiki page has to be the worst page yet for explaining things.

walking in a store and see a male usb connector on both ends of the cable and think "null modem". future computers could be made to detect the presents of another computer like a device by setting it up like a client and server. the usb on the computer would start in client mode. if nothing is detected switch to server mode. but that would slow down the initialization process. a detector on the 5v + keeping the voltage at 0 and waiting for another computer. when it detects voltage act as a client or other device. if after a moment nothing then send 5v + in server mode. some day it could work like that.
for now i understand why it did not work.

did you see penny as a cat ?

Problem with that is, if the device is an unpowered slave, like a USB driver, it can't talk to you if you don't give it power.

There is already a protocol for that. It's the On-The-Go extension to USB 2.0.