kartable
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video output with VGA-C connector

Fri Jul 17, 2020 12:03 pm

Hi to all,

I have a PC screen which I intend to use with my new Raspberry Pi 3 B+ and I exeprience some flickering on this "old" screen, whereas it's perfect on my TV screen with HDMI.

Thanks to this thread viewtopic.php?f=91&t=83372&p=589714#p635072 I had been warned that a VGA output could not work with the Pi's HDMI. I then purchased a cable with a VGA-C connector, also called DVI connector. And at first glance, it works.

But as I say, the video output is flickering sometimes with a black screen for as long as a second. Can this be solved?

hippy
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Re: video output with VGA-C connector

Fri Jul 17, 2020 1:30 pm

Maybe it's an issue with the monitor ?

I, and I'd guess many people, use monitors with DVI inputs so use HDMI-to-DVI adapters and I've not experienced such problems, nor recall seeing reports of the problem from others.

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rpdom
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Re: video output with VGA-C connector

Fri Jul 17, 2020 2:45 pm

I don't know about DVI-C (I've never heard of one of those), but a DVI-D cable should work fine with most monitors with DVI connectors.

Some older monitors (PCs and cables) use DVI-A (analogue), which is basically just VGA on a DVI plug. It is not compatible with DVI-D (Digital).

There is also a DVI-I, which has both types of DVI connections.
Last edited by rpdom on Fri Jul 17, 2020 5:22 pm, edited 2 times in total.
Unreadable squiggle

njh
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Re: video output with VGA-C connector

Fri Jul 17, 2020 3:06 pm

It sounds like the OP may be using a HDMI-to-VGA converter rather than just a DVI cable?
What kind of adaptor is it exactly?
Some of them can be fussy about video modes (try changing the screen resolution in "preferences") or power supplies.

kartable
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Re: video output with VGA-C connector

Sat Jul 18, 2020 7:24 am

njh wrote:
Fri Jul 17, 2020 3:06 pm
Some of them can be fussy about video modes (try changing the screen resolution in "preferences") or power supplies.
Changing the video modes could have been a right guess. I've tried most of them, also with different Hz settings, but no way.
rpdom wrote:I don't know about DVI-C (I've never heard of one of those), but a DVI-D cable should work fine with most monitors with DVI connectors.

Some older monitors (PCs and cables) use DVI-A (analogue), which is basically just VGA on a DVI plug. It is not compatible with DVI-D (Digital).

There is also a DVI-I, which has both types of DVI connections.
Re this https://components101.com/connectors/dvi-connector, I have a DVI-D input on my device.
And my cable is this one https://fr.aliexpress.com/item/32830411 ... 6c37lJar6V
hippy wrote:Maybe it's an issue with the monitor ?
This monitor works quite well with my PC running Windows 10.
For some reason, a flash appears on the top right corner which means a high consumption, as far as I understand. So I modified the config.txt file with an additional line "max_usb_current=1", but this doesn't help.

EDIT : I realise that I can switch the input from the screen itself from VGI digital to analog, should I try that?

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B.Goode
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Re: video output with VGA-C connector

Sat Jul 18, 2020 9:54 am

kartable wrote:
Sat Jul 18, 2020 7:24 am
For some reason, a flash appears on the top right corner


D0 you mean the Yellow 'lightning flash' show here - https://www.raspberrypi.org/documentati ... g-icons.md


If so, your problem is due to an inadequate power supply for the RPi board itself, probably worsened by the current drawn by the video interface.

kartable
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Re: video output with VGA-C connector

Sat Jul 18, 2020 11:58 am

B.Goode wrote:
Sat Jul 18, 2020 9:54 am
D0 you mean the Yellow 'lightning flash' show here - https://www.raspberrypi.org/documentati ... g-icons.md
That's it
B.Goode wrote:
Sat Jul 18, 2020 9:54 am
If so, your problem is due to an inadequate power supply for the RPi board itself, probably worsened by the current drawn by the video interface.
I am using a 5V 2.5Amps power supply for my Raspberry. Is it possible that my "old" PC screen device draws much current from the Pi when both have their separate power supply?
The flash icon never shows up on my TV screen.

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Re: video output with VGA-C connector

Sat Jul 18, 2020 12:23 pm

The 3B+ needs three amps. The new (at the time) PMIC on that one has always been a bit power hungry and sets the lower power warning early.
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Re: video output with VGA-C connector

Sat Jul 18, 2020 12:28 pm

kartable wrote:
Sat Jul 18, 2020 7:24 am
For some reason, a flash appears on the top right corner which means a high consumption, as far as I understand. So I modified the config.txt file with an additional line "max_usb_current=1", but this doesn't help.
max_usb_current only does something on the Pi 2 where it changed the USB port current limiter from 600mA to 1.2A. It does nothing for every other Pi. It doesn't help with undervoltage issues anyway.

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rpdom
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Re: video output with VGA-C connector

Sat Jul 18, 2020 12:59 pm

trejan wrote:
Sat Jul 18, 2020 12:28 pm
kartable wrote:
Sat Jul 18, 2020 7:24 am
For some reason, a flash appears on the top right corner which means a high consumption, as far as I understand. So I modified the config.txt file with an additional line "max_usb_current=1", but this doesn't help.
max_usb_current only does something on the Pi 2 where it changed the USB port current limiter from 600mA to 1.2A. It does nothing for every other Pi. It doesn't help with undervoltage issues anyway.
Wasn't it the Pi 1B+ that had that? I thought the Pi 2 had it set to high anyway. In fact I think later "firmware" set that option as standard on all Pi that support the option.
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kartable
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Re: video output with VGA-C connector

Sat Jul 18, 2020 1:08 pm

DougieLawson wrote:
Sat Jul 18, 2020 12:23 pm
The 3B+ needs three amps. The new (at the time) PMIC on that one has always been a bit power hungry and sets the lower power warning early.
Well I purchased the official power supply that's rated 2.5 Amps for the 3B+ so I don't understand at all.

trejan
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Re: video output with VGA-C connector

Sat Jul 18, 2020 1:13 pm

rpdom wrote:
Sat Jul 18, 2020 12:59 pm
trejan wrote:
Sat Jul 18, 2020 12:28 pm
max_usb_current only does something on the Pi 2 where it changed the USB port current limiter from 600mA to 1.2A. It does nothing for every other Pi. It doesn't help with undervoltage issues anyway.
Wasn't it the Pi 1B+ that had that? I thought the Pi 2 had it set to high anyway. In fact I think later "firmware" set that option as standard on all Pi that support the option.
Looks like it was for the 1B+ and 2B. The 3B onwards had no choice and it was 1.2A always.

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Re: video output with VGA-C connector

Sat Jul 18, 2020 1:23 pm

kartable wrote:
Sat Jul 18, 2020 11:58 am
I am using a 5V 2.5Amps power supply for my Raspberry. Is it possible that my "old" PC screen device draws much current from the Pi when both have their separate power supply?
It's possible, but the 2.5A recommended for the 3B+ includes some 1.2A spare capacity for powering USB devices.

You seem to be using a standard passive HDNI-to-DVI cable so it is hard to see how a monitor would draw so much current through the HDMI port that it puts the Pi into an under-voltage situation -- which is presumably what's causing the monitor to blank its display. The Pi then recovers, the monitor re-displays, rather than rebooting.

Drawing just a little current could however tip the Pi over the edge if current were being drawn elsewhere. What else do you have connected via USB or to the 40-way connector ?

It might be worth trying with keyboard and mouse unplugged, nothing attached via USB, seeing if that improves things.

hippy
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Re: video output with VGA-C connector

Sat Jul 18, 2020 1:28 pm

DougieLawson wrote:
Sat Jul 18, 2020 12:23 pm
The 3B+ needs three amps.
2.5A recommended according to the official documentation - https://www.raspberrypi.org/documentati ... /README.md

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Re: video output with VGA-C connector

Sat Jul 18, 2020 1:58 pm

kartable wrote:
Fri Jul 17, 2020 12:03 pm
it's perfect on my TV screen with HDMI.
One thing I have noticed with my collection of TV's is they do not seem to draw power from pin 18 of the Pi's HDMI port.

The power for their internal Eeprom appears to be provided only internally, and often only when turned on. When turned on the EDID information can be read. Also for a period after putting the TV in standby. But not after being in standby for some time, when the Eeprom power rail reservoir will have discharged. Took me some time to figure out what was going on there.

Monitors on the other hand do seem to draw current from pin 18, or power the Eeprom internally when connected to mains, even in standby or sleep modes, or showing the other input.

It could therefore point to a pin 18 issue, the monitor drawing too much current perhaps.

I had noticed, when using Pi's on sub-optimal, under-rated PSU's, I do see more under-voltage flashes on my monitors than on my TV's. I hadn't investigated why, but this could explain it.

Alternatively it could be down to back-powering of the Pi through pin 18.

Despite the RT9741CGV, which current limits the pin 18 voltage, often being described as if 'a diode', preventing back-flow of current; the datasheet clearly indicates otherwise. This is why some Pi's remain powered until the HDMI lead is unplugged.

I am not entirely sure what the failure mode would be but back-powering will happen if the monitor is putting out a higher voltage on pin 18 than the Pi's 5V rail. This can happen whenever current draw causes the Pi 5V rail to reduce.

If back-powering occurs it is unlikely a monitor can supply the power a Pi requires which could lead to the Pi's voltage collapsing, until the attached Pi PSU takes over again. It could be the over-current draw which is causing the monitor to momentarily blank, or low-voltage Pi-side.

kartable
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Re: video output with VGA-C connector

Sat Jul 18, 2020 2:15 pm

hippy wrote:
Sat Jul 18, 2020 1:23 pm
Drawing just a little current could however tip the Pi over the edge if current were being drawn elsewhere. What else do you have connected via USB or to the 40-way connector ?

It might be worth trying with keyboard and mouse unplugged, nothing attached via USB, seeing if that improves things.
Removing the keyboad and mouse does help, you're right. But my installation is supposed to get a PS3 eye for voice recording and a speaker.
What would it be if I used a 6V 3A power for all this ? I can't find a 5V 4A which would be probably better.

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B.Goode
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Re: video output with VGA-C connector

Sat Jul 18, 2020 2:33 pm

kartable wrote:
Sat Jul 18, 2020 2:15 pm

What would it be if I used a 6V 3A power for all this ?

That would probably be a 'pop', or a quiet sigh, maybe some smoke, and your frustrations with this particular board will be over... for good...


- DON'T -

hippy
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Re: video output with VGA-C connector

Sat Jul 18, 2020 2:53 pm

kartable wrote:
Sat Jul 18, 2020 2:15 pm
Removing the keyboad and mouse does help, you're right. But my installation is supposed to get a PS3 eye for voice recording and a speaker.
To overcome the monitor issue you might need to run all your USB devices off a powered USB hub. Or multiple powered hubs.
kartable wrote:
Sat Jul 18, 2020 2:15 pm
What would it be if I used a 6V 3A power for all this ? I can't find a 5V 4A which would be probably better.
As said; don't even think about a 6V supply. That could destroy your Pi and anything else attached.

A higher current micro-USB 5V PSU may not help due to the polyfuse protection.

Best to try with what you have and a powered USB hub before getting into more complicated solutions.

kartable
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Re: video output with VGA-C connector

Sat Jul 18, 2020 3:21 pm

hippy wrote:
Sat Jul 18, 2020 2:53 pm
Best to try with what you have and a powered USB hub before getting into more complicated solutions.
Thank you for the suggestion, but what kind of powered hub would you see? I have no idea how to connect the usb of screen, ps3eye, speaker and mouse/keyboard through a hub to the pi.

This? https://fr.aliexpress.com/item/33023834 ... hweb201603_

hippy
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Re: video output with VGA-C connector

Sat Jul 18, 2020 5:26 pm

That linked item would probably work but, if the buttons do turn power on and off for the separate ports, that's likely going to prove to be a real pain. That seems designed for charging USB devices rather than connecting USB peripherals.

Any "powered USB hub" without on-off buttons or anything fancy should do the job. Just plug your USB devices into that and it into the Pi.

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Re: video output with VGA-C connector

Sat Jul 18, 2020 5:40 pm

kartable wrote:
Sat Jul 18, 2020 2:15 pm
What would it be if I used a 6V 3A power for all this ? I can't find a 5V 4A which would be probably better.
It's AMPS you need not VOLTS.

5V at anything from 3A to 10A will work.
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