Max number of USB device - power limitation

[tony359]

Hi there,

I have normally 2 USB-C devices plugged in my Mac Mini M4. One is a USB-4 SSD enclosure with an nVme drive in it. The other one is a powered Targus dock (10Gbps) where many other devices are plugged in, including two extra monitors via DisplayLink. A third monitor is directly connected to the HDMI output. Resolution is 1440p. I'm using the ports at the back.

I wanted to get rid of the DisplayLink driver (it can be taxing) so I bought two USB-C to HDMI dongles from Cable Matters.

Unfortunately, with those plugged in and working, the SSD refuses to work.

I've had a look online and I've learnt that there is a power budget for those ports and I've learnt that Apple might not deliver the full budget required by the specs.

I'm just puzzled that two USB-C to HDMI dongles can break the power budget for the mini.

Do you think there is a workaround for this issue (maybe my dongles draw too much power?) or not?

I'd imagine a powered USB enclosure might help?

 

[throAU]

Yes, if you're plugging in multiple things into a USB hub you should probably make sure it is powered.

This isn't just a Mac thing, any machine with USB ports has a limited power budget on each port.

 

[tony359]

The hub is powered. That is not my issue. The issue does not involve the hub 🙂

The SSD refuses to work with the following devices plugged in at the back:
2 x HDMI dongles
1 x USB SSD

Also, there seems to be a total budget as I can have 2 of those devices working - just not all three simultaneously.

BTW, Just tested the same configuration (2 x HDMI dongles + USB SSD) on an M1 Max MBP and it works with zero issues.

I'm disappointed Apple doesn't post clearer specs so one can make an educated decision when purchasing accessories.

 

[throAU]

The hub is powered. That is not my issue. The issue does not involve the hub 🙂

The SSD refuses to work with the following devices plugged in at the back:
2 x HDMI dongles
1 x USB SSD

Also, there seems to be a total budget as I can have 2 of those devices working - just not all three simultaneously.

BTW, Just tested the same configuration (2 x HDMI dongles + USB SSD) on an M1 Max MBP and it works with zero issues.

I'm disappointed Apple doesn't post clearer specs so one can make an educated decision when purchasing accessories.

If the hub is powered and the device is plugged into the hub, the power limitation is nothing to do with your Mac?

 

[tony359]

I have normally 2 USB-C devices plugged in my Mac Mini M4. [...]. I'm using the ports at the back.

The SSD refuses to work with the following devices plugged in at the back:
2 x HDMI dongles
1 x USB SSD

"at the back" = "at the back of the mac mini"

No hub involved 🙂

 

[Bigwaff]

I'm disappointed Apple doesn't post clearer specs so one can make an educated decision when purchasing accessories.

Saw this when I was researching whether to get one...
"In the case of the Mac Mini, all ports, both Thunderbolt and USB-C, can deliver a maximum of 12.5 Watts, 5V, and 2.5 amps. We connected two MacBooks for charging and the Mac is able to provide a total of 25 Watts output from the two ports. If a third device is connected, the total power it can deliver decreases."

Mac Mini M4 review:super performance, impressive efficiency. Desktop revolution

The new version of the Mac Mini is even smaller, and thanks to the M4 processor, it's even more powerful. Today, just under 1000 euros is enough to get a computer that can be taken anywhere and excels in every field.

global.dday.it

 

[tony359]

Thanks @Bigwaff

So it looks like there is a total budget of 25W across all ports - not a lot!

Now, I've been unable to accurately measure the USB power draw of my devices (my USB power meter doesn't seem to work with them) so I did a ballpark evaluation by looking at the "total system" power on Stats in macOS.

It looks like the SSD is drawing about 10W when plugged in and 12W when being used which is consistent with the max power per port mentioned in the link.

The USB-> HDMI dongles seem to read about 2W each.

So with the SSD and two dongles, I should still be well below those 25W. Maybe there are spikes which I cannot catch by looking at that slow sensor.

I don't see a clear solution here. For 2 monitors, I definitely need two USB-C dongles as macOS doesn't support multiple monitors on a single link.

What I could do is to plug the SSD into a USB4/TB4 *powered* hub. But that would also introduce an extra link in the chain and, honestly, I'm not entirely sure it would resolve my problem.

The USB4 enclosure I've been using has been FLAWLESS for 2 years. I have all my Resolve assets and the database so it's a well stressed unit.

Is there a powered USB4/TB4 SSD enclosure which is known for being good quality without breaking the bank?

 

[FreakinEurekan]

The USB4 enclosure I've been using has been FLAWLESS for 2 years. I have all my Resolve assets and the database so it's a well stressed unit.

I don't know that it really makes a difference, but I'm using a Thunderbolt NVMe enclosure for my 4TB card with my M4 Pro mini for the past year or so; it was used on my M1 mini before that and it's been working flawlessly with both computers. Perhaps Thunderbolt is more forgiving on power draw?

Mine's TB3 (since it came from my M1) but if buying now you'd look at TB4 or TB5 of course. But for what it's worth, here's what I'm using: https://a.co/d/0bVXrxpb

 

[tony359]

@FreakinEurekan

Thanks. I'd love to be able to determine whether it's going to work or not as my own enclosure has also been working flawlessly for 2 years.
Yours is a nice enclosure BTW: no fan and TB3, nice!

 

[BigBlur]

I have no idea if this will help, but it might be worth a try. Do your monitors support DisplayPort? If so, get a couple of USB-C to DisplayPort cables and use them. They require a little less power as the Mac can just send native DP Alt Mode signals to the monitors. HDMI adapters need to convert the signal (active conversion), which requires additional power.

Also, out of curiosity, does anything change if you were to use one or both of the ports on the front?

 

[Allen_Wentz]

The hub is powered. That is not my issue. The issue does not involve the hub 🙂

The SSD refuses to work with the following devices plugged in at the back:
2 x HDMI dongles
1 x USB SSD

Also, there seems to be a total budget as I can have 2 of those devices working - just not all three simultaneously.

BTW, Just tested the same configuration (2 x HDMI dongles + USB SSD) on an M1 Max MBP and it works with zero issues.

I'm disappointed Apple doesn't post clearer specs so one can make an educated decision when purchasing accessories.

The Mini was Apple's lowest end (until the Neo) Mac; IMO all devices being plugged in to it should be externally powered if possible. MY MBP cost more than 3x what the Mini cost and I still try to have everything being plugged in be independently powered. That helps reduce heat in the Mac, increasing longevity while reducing fan noise and/or throttling.

Following that protocol for all regularly used devices usually means that if one does temporarily plug in something not powered like a backup SSD or a USB stick it will work just fine.

 

[tony359]

I have no idea if this will help, but it might be worth a try. Do your monitors support DisplayPort? If so, get a couple of USB-C to DisplayPort cables and use them. They require a little less power as the Mac can just send native DP Alt Mode signals to the monitors. HDMI adapters need to convert the signal (active conversion), which requires additional power.

Ah, that is a good idea. Yes, I have DisplayPort on all my monitors. I might want to give that a go, thanks!

Also, out of curiosity, does anything change if you were to use one or both of the ports on the front?

No unfortunately and when the SSD was misbehaving, the front ports would fail to recognise the USB-C dock, suggesting that the power budget for ports is one for all of them?

The Mini was Apple's lowest end (until the Neo) Mac; IMO all devices being plugged in to it should be externally powered if possible.

Of course - though having a USB4 dock/hub with external power is almost as expensive than the whole Mac Mini! 🙂

 

[bzgnyc2]

The Mini was Apple's lowest end (until the Neo) Mac; IMO all devices being plugged in to it should be externally powered if possible. MY MBP cost more than 3x what the Mini cost and I still try to have everything being plugged in be independently powered. That helps reduce heat in the Mac, increasing longevity while reducing fan noise and/or throttling.

Note that the Studio also has as similar issue:

Why can't I use all 4 Thunderbolt ports o… - Apple Community

discussions.apple.com

Studio Ultra - Simultaneous use of Bus Powered TB devices limited? (title edited)

Today I discovered that I cannot connect more then three thunderbolt drives (and the BenQ display using USBC) to my Studio Ultra. On the back four ports, I have connected: 1. BenQ SW321C 2. ProGrade TB3 CF Express card reader 3. & 4. Orico TB4 enclosure w/Samsung 980 Pro NVMe SSD If I connect...

forums.macrumors.com

The numbers are slightly different but the unfortunate summary is that Apple doesn't generally support driving all USB4/TB4/5 ports at full spec power on any their current or recent models. On the MacBook Air (haven't heard about the latest), only one port can pull full ThunderBolt 4 power at a time.

Understand the engineering challenges with a full square design (hitting all specs simultaneously) but then it would be good if Apple documented the limitations on the spec page similar to how they document resolution limits.

Following that protocol for all regularly used devices usually means that if one does temporarily plug in something not powered like a backup SSD or a USB stick it will work just fine.

I understand the logic of that but two challenges:
-I don't generally see simple self-powered NVMe enclosures -- one has to jump to a TB3+ dock or hub that includes an NVMe slot
-Some docks/hub had a slight overhead versus an NVMe-only enclosure which is fine but Apple's engineering decisions here mean we then have to pay more for less to workaround an undocumented hardware limitation

 

[bzgnyc2]

Saw this when I was researching whether to get one...
"In the case of the Mac Mini, all ports, both Thunderbolt and USB-C, can deliver a maximum of 12.5 Watts, 5V, and 2.5 amps. We connected two MacBooks for charging and the Mac is able to provide a total of 25 Watts output from the two ports. If a third device is connected, the total power it can deliver decreases."

Mac Mini M4 review:super performance, impressive efficiency. Desktop revolution

The new version of the Mac Mini is even smaller, and thanks to the M4 processor, it's even more powerful. Today, just under 1000 euros is enough to get a computer that can be taken anywhere and excels in every field.

global.dday.it

Thanks for sharing. Interesting if that review's measurement of a maximum of 12.5W on the Mac Mini's TB ports holds up as that would mean it's not hitting TB3/4/5 spec. We can debate whether a system should be required to support 15W on every TB port simultaneous in order to meet TB specs but certainly it should hit the spec on at least one of them regardless.

 

[tony359]

Note that the Studio also has as similar issue:

Thanks, I found that link this morning and it's when I started thinking it could be a power problem

Understand the engineering challenges with a full square design (hitting all specs simultaneously) but then it would be good if Apple documented the limitations on the spec page similar to how they document resolution limits.

I am 100% with you.

Particularly on a base model, I could understand if limitations (physical or artificial) were in place.
Just be clear about it so I can make an educated decision!

I really like @BigBlur idea. I had a look and it looks like with DP-Alt mode, there wouldn't be any conversion so the cable would not draw any power. In theory, it should work.

 

[bzgnyc2]

Of course - though having a USB4 dock/hub with external power is almost as expensive than the whole Mac Mini! 🙂

Agree with you that Apple should document this better. Or ideally at least on the higher-end models, support all ports at full spec.

Unfortunately, people find out after the fact (often only when they try to upgrade possibly long after they buy their machine) and as you note good dock/hubs with NVMe drive support tend to be expensive.

And while I do see TB docks/hubs in the $150-170 range, they often have performance limitations such as running at 1/2 speed (i.e. only giving 2 out of 4 PCIe lanes to any one SSD) and/or requiring RAID0 to get full performance (with resulting greater data risk).

 

[tony359]

question: my monitors have DP-out as well so in theory I can daisy-chain them. However, that would require Multi Stream Transport which Mac doesn't support, correct? 😀

 

[Bigwaff]

question: my monitors have DP-out as well so in theory I can daisy-chain them. However, that would require Multi Stream Transport which Mac doesn't support, correct? 😀

Correct.

 

[tony359]

I'd like to update this thread for future reference: I've purchased two USB-C to mini-DP from StarTech.

I now have the following devices connected:

2 x USB-C to mini-DP monitors
1 x USB4 SSD
1 x USB hub (into a monitor USB hub, yes a mess, but it's temporary!)
1 x old USB-C DisplayLink hub (without monitors connected)

The Mini looks like a spider but everything works, zero issues (though for now macOS seems to have some trouble enumerating the screens when booting, I get the login on the wrong screen for a few seconds...)

I was also thinking something: with my previous configuration I had my iPhone or iPad connected to the Mac Mini via USB-C - together with the USB SSD - and never had a power issue. I feel the issue I was having was due to the adaptors somehow.

In any case, a USB-C to mini-DP is a much cleaner and better solution. Also I wasn't using mini-DP on the monitors so the new path is not disrupting existing connections!

Thanks again @BigBlur for the nice suggestion!