R&S RTB2004 front USB 5V power output?

[gdr771]

Does anyone know how much power the front USB connector on a RTB2004 can output?  I have a usb-powered Micsig DP10013 differential probe that seems to draw 90mA at 5V (according to a cheap USB meter), but it keeps resetting every second unless I power it directly from a wall adapter.  (I was also not able to power a keyboard or mouse from the front connector, so I'd like to know if this is a fault or expected.)  Thanks!

[tooki]

The user manual expressly claims USB keyboards and mice are supported (though the datasheet does not).

[Hydron]

Mine lets me power two of them at the same time, but you have to plug them in sequentially otherwise the inrush trips the scope

[Rydda]

I usually have a usb rf dongle ( which seems to draw <10mA) on the front panel with no problems. A Micsig DP1007 (which draws 240 mA) also works OK.

[langwadt]

if it is USB you should be guaranteed atleast 100mA

[nctnico]

Sounds like a defect in the oscilloscope to me. Unless there is some extra hardware like a USB hub connected in between.

[gdr771]

Thanks for the info everyone.  I'll take it up with R&S product support.

[gdr771]

Trusting R&S a bit more than Micsig, I decided to test some more with the bench instruments. 

The RTB2004 front USB port can supply 500mA without breaking a sweat, but a >2A inrush of the 2024 Micsig DP10013 probe that occurs ~1sec after power-up causes a voltage dip to <1V for a couple of msec.  This causes the Micsig to reset before the voltage can recover.  Rinse & repeat.

I don't know what the USB spec is, so I can't say if the RTB2004 supply is too weak or if the Micsig probe is too sensitive.

Note that this is the newer two-part version of the DP10013 where the control module with buttons contains the BNC connector and the small measurement module has the two short measurement leads.

[langwadt]

Trusting R&S a bit more than Micsig, I decided to test some more with the bench instruments. 

The RTB2004 front USB port can supply 500mA without breaking a sweat, but a >2A inrush of the 2024 Micsig DP10013 probe that occurs ~1sec after power-up causes a voltage dip to <1V for a couple of msec.  This causes the Micsig to reset before the voltage can recover.  Rinse & repeat.

I don't know what the USB spec is, so I can't say if the RTB2004 supply is too weak or if the Micsig probe is too sensitive.

Note that this is the newer two-part version of the DP10013 where the control module with buttons contains the BNC connector and the small measurement module has the two short measurement leads.

probably not within USB spec, back in the day it was big hassle to make the PSU for USB GSM modems because the transmit pulses could be 2A

[Hydron]

Ah yeah my test with Micsig probes were the very first all-in-one-box design. Maybe check with some alternative mice etc - it really should work with them at least!

[tooki]

The RTB2004 front USB port can supply 500mA without breaking a sweat, but a >2A inrush of the 2024 Micsig DP10013 probe that occurs ~1sec after power-up causes a voltage dip to <1V for a couple of msec.  This causes the Micsig to reset before the voltage can recover.  Rinse & repeat.

I don't know what the USB spec is, so I can't say if the RTB2004 supply is too weak or if the Micsig probe is too sensitive.

The RTB2004 is behaving correctly. A USB device (that isn't using USB Power Delivery or Battery Charging protocols) must draw only 100mA without performing power negotiation. If power negotiation succeeds, then it may begin to draw up to 500mA for USB 1.1 and 2.0, and (IIRC) 900mA for USB 3.

Many "dumb" gadgets just pull from USB ports without negotiating, but this isn't really allowed.

It's common for USB hosts to protect their ports with overcurrent protection devices, be it something like a polyfuse, or (what is more likely in this case because it happens so quickly) some kind of active protection.

So Micsig kinda messed up by drawing so much power.

[nctnico]

Not quite. A dedicated charging port can supply several amps without any form of negotiation. Also, USB spec say that a capacitance of up 10uF should be tolerated on a USB device.
The 100mA max. really is a deprecated part of the standard for a long time. If you want to design a somewhat useful USB port on a device, make sure it can supply somewhere between 500mA and 2A and that it allows the rush-in current from a 10uF capacitor (which some margin).

The MicSig device is only at fault if the capacitance exceeds 10uF.

[tooki]

Not quite. A dedicated charging port can supply several amps without any form of negotiation. Also, USB spec say that a capacitance of up 10uF should be tolerated on a USB device.

But the RTB2004 does not have a dedicated charging port. It has a normal USB port, which is what my statements are about. No need to be a pedantic whataboutist.

The 100mA max. really is a deprecated part of the standard for a long time.

Well that is certainly a bold claim. I don't think it's correct, though.

If you want to design a somewhat useful USB port on a device, make sure it can supply somewhere between 500mA and 2A and that it allows the rush-in current from a 10uF capacitor (which some margin).

The MicSig device is only at fault if the capacitance exceeds 10uF.

I suppose.

But I kinda trust R&S to have been more fastidious about standards-compliance than Micsig, if I had to make a guess as to who messed up.

[nctnico]

The 100mA max. really is a deprecated part of the standard for a long time.

Well that is certainly a bold claim. I don't think it's correct, though.

Lets put it differently: try and find a USB port which enforces it... I have never found a USB port which doesn't deliver less than 500mA. It just causes the user grief to limit the current based on negotiation at the cost of extra parts and software. It never made sense to negotiate power at this level. Maybe the very, very first USB host ports had variable current limiting but I guess they found out quickly that it wasn't a good idea.

Also, there are dual-use ports for both data and charging (Charging Downstream Port) defined in the USB battery charging standard. The whole USB power delivery situation is a huge mess; better design something that has wide margins for as long as it doesn't make attached devices go up in smoke.

[skander36]

Micsig DP10013 tested with RTB2002 works without problems.
Maybe you have Micsig DP1500 from description.

[langwadt]

The 100mA max. really is a deprecated part of the standard for a long time.

Well that is certainly a bold claim. I don't think it's correct, though.

Lets put it differently: try and find a USB port which enforces it... I have never found a USB port which doesn't deliver less than 500mA. It just causes the user grief to limit the current based on negotiation at the cost of extra parts and software. It never made sense to negotiate power at this level. Maybe the very, very first USB host ports had variable current limiting but I guess they found out quickly that it wasn't a good idea.

I had a laptop eons ago it definitely didn't have any current limiting, if you shorted Vbus you got a scream from the supply and instant reset of the PC

negotiating the the current limit sorta makes sense when you have a bus powered hub, if you have a five port hub with four devices using 100mA each and the plug in something that needs 500mA should you deny that or crash all five devices because the hub can only deliver 500mA in total?
 

[tooki]

The 100mA max. really is a deprecated part of the standard for a long time.

Well that is certainly a bold claim. I don't think it's correct, though.

Lets put it differently: try and find a USB port which enforces it... I have never found a USB port which doesn't deliver less than 500mA. It just causes the user grief to limit the current based on negotiation at the cost of extra parts and software. It never made sense to negotiate power at this level. Maybe the very, very first USB host ports had variable current limiting but I guess they found out quickly that it wasn't a good idea.

I guess you've never used an unpowered USB 1.1/2.0 hub. It has 500mA for the whole hub and all unpowered downstream devices.

But you also seem to mistakenly think that it's the host's responsibility to enforce the negotiated current. It  isn't. It's the device's responsibility to obey. If it doesn't follow this, it's noncompliant.


A port can be designed to protect itself, and that's OK. I've never heard of the protection being variable, but I have seen hosts that shut down the port if a device pulls more than the port's maximum current.

Also, there are dual-use ports for both data and charging (Charging Downstream Port) defined in the USB battery charging standard.

That is true, but so what? I am discussing the port on the RTM2004, not every type of USB port that exists in the universe.

The whole USB power delivery situation is a huge mess; better design something that has wide margins for as long as it doesn't make attached devices go up in smoke.

Well, find a time machine and tell early-90s Intel that USB will become the dominant DC charging port. Back when they designed it, nobody even distantly envisioned it as a de-facto DC power supply standard. The little bit of power envisioned was just a courtesy so small gadgets wouldn't need a separate power supply. Remember, it was conceived as a replacement for the keyboard and mouse ports, the serial port, and the printer port. Every other application basically got tacked on later. And of those original applications, only the keyboard and mouse ports had power at all. So the original power architecture was designed for that kind of thing. The 500mA power level was itself generous overkill for all the original applications.

At the time, storage devices and the like were envisioned to use FireWire, which had up to 1.5A of power (at 9-30V), though actual available power varied wildly by system.

It's so easy to forget that USB only accidentally became the de-facto DC power supply standard (and later still a de-jure one in the EU!), since it's now everywhere. Had we known that back then, we doubtless would have designed it differently. But it's disingenuous to decry as poor the decisions made ages ago in a context where those decisions absolutely made sense.

[tooki]

The 100mA max. really is a deprecated part of the standard for a long time.

Well that is certainly a bold claim. I don't think it's correct, though.

Lets put it differently: try and find a USB port which enforces it... I have never found a USB port which doesn't deliver less than 500mA. It just causes the user grief to limit the current based on negotiation at the cost of extra parts and software. It never made sense to negotiate power at this level. Maybe the very, very first USB host ports had variable current limiting but I guess they found out quickly that it wasn't a good idea.

I had a laptop eons ago it definitely didn't have any current limiting, if you shorted Vbus you got a scream from the supply and instant reset of the PC

negotiating the the current limit sorta makes sense when you have a bus powered hub, if you have a five port hub with four devices using 100mA each and the plug in something that needs 500mA should you deny that or crash all five devices because the hub can only deliver 500mA in total?

I've seen both Mac and Windows machines that displayed a message that a USB port has been shut down because too much power was drawn. Definitely better than just resetting!!! 

[gdr771]

Micsig DP10013 tested with RTB2002 works without problems.
Maybe you have Micsig DP1500 from description.

Micsig completely redesigned the probe into a two-part device but used the exact same model number.  I have the new version shown in the attached photo.



Which version do you have?

[gdr771]

Here's a capture of the inrush in question.  Channel C2 is the 5V line from the USB port.  C3 is the voltage drop across a 0.1-ohm 5% 5W resistor in series on the ground line scaled for current.  I confirmed the scaling by comparing the DC steady-state draw against both a Fluke 87 and a Siglent SDM3045X meters.  You can see the initial inrush at plug-in isn't bad at all:



Then, after about 1 second there is a second inrush around 2.5A, which lasts for a millisecond:



That's when the Micsig probe resets, and the whole process starts over again, resetting every second.

[skander36]

Micsig DP10013 tested with RTB2002 works without problems.
Maybe you have Micsig DP1500 from description.

Micsig completely redesigned the probe into a two-part device but used the exact same model number.  I have the new version shown in the attached photo.

(Attachment Link)

Which version do you have?

I have the old version, the white one.
Strange thing, on their site this black model is not listed as a new model. Instead is a mention: "*The previous model DP10013 has been upgraded to DP1500. "

[nctnico]

The whole USB power delivery situation is a huge mess; better design something that has wide margins for as long as it doesn't make attached devices go up in smoke.

Well, find a time machine and tell early-90s Intel that USB will become the dominant DC charging port. Back when they designed it, nobody even distantly envisioned it as a de-facto DC power supply standard. The little bit of power envisioned was just a courtesy so small gadgets wouldn't need a separate power supply.

At the time, storage devices and the like were envisioned to use FireWire, which had up to 1.5A of power (at 9-30V), though actual available power varied wildly by system.

It's so easy to forget that USB only accidentally became the de-facto DC power supply standard (and later still a de-jure one in the EU!), since it's now everywhere. Had we known that back then, we doubtless would have designed it differently. But it's disingenuous to decry as poor the decisions made ages ago in a context where those decisions absolutely made sense.

Now you are pulling my words a bit out of context I'm afraid  The reason for the mess is not due to bad decissions in the past as it was impossible for the original designers to even dream about how USB turned into a universal power delivery standard. The reason for the mess is manufacturers and the USB design commitee comes up with standard upon standard. Nobody is to blame but it is a mess nonetheless. I'm involved in the design of a device which tries to deal with it all and that is far from easy. Besides USB-C there are about a dozen manufacturer specific charging solutions (example: Qualcom's Quickcharge aka QC in 5 different iterations).

Hence my suggestion that if you design something with a USB socket which could be used to power some kind of device, be as generous as you can where it comes to inrush currents and current consumption. As a designer of a USB powered device (which doesn't require USB-PD), try to stay below 500mA if you can as the majority of the 'dumb' USB sockets are likely to be able to supply at least 500mA. These are just simple guidelines which will make life of the user of your devices easier.

But in the end it is logical that a host port limits the current to its design limit. If you use an unpowered hub, then the host port -assuming it has current limiting- will shut everything down when all the devices connected to the hub (together) start drawing more current than the host port can supply.

[gdr771]

I have the old version, the white one.
Strange thing, on their site this black model is not listed as a new model. Instead is a mention: "*The previous model DP10013 has been upgraded to DP1500. "

Here's the alleged specs of my new-style DP10013, also straight from their website.  The photo above this chart on their webpage is of the new-style probe:

[skander36]

Yes Micsig site is a mess, when is loading ...
On the DP page, DP1013 cannot be found anymore, just the mention that it was upgraded. Only your direct link got me to the DP1013.
They excel at this behavior lunching products that easy disappear like UPI bus or DP750 or not appear anymore like Remote for TO oscilloscopes
The old version of DP1013 draw about 147 mA from USB.

[gdr771]

Thanks for looking at this.  And, nice scope!

Today this new-version DP10013 probe reset a few times with the Micsig-supplied wall adapter before luckily getting past the inrush at the 1-second mark.  So I'll either send the probe back or make an inline power buffer.