What is Pin 9 on a VGA cable?

[TVeblen]

I see a lot of VGA cables being sold on Newegg that do not have all 15
pins. It looks like Pin 9 is missing. Wikipedia says that Pin 9 is
KEY/PWR (+5V). I don't know what that means.

Does it make any difference whether you have all 15 pins or not?

 

[Franc Zabkar]

I see a lot of VGA cables being sold on Newegg that do not have all 15
pins. It looks like Pin 9 is missing. Wikipedia says that Pin 9 is
KEY/PWR (+5V). I don't know what that means.

Does it make any difference whether you have all 15 pins or not?

Pin 9 powers the EDID EEPROM chip in the monitor. If the EDID cannot
be read, then PnP won't be able to determine the monitor's specs. If
the EEPROM is powered by the monitor, then pin 9 may not be necessary.
I'm not certain, though. What is certain, however, is that you will
need pins 12 and 15 (data and clock) for PnP. If these are absent,
then you will still be able to use the monitor, but you will need to
set it up manually with the correct INF file.

http://en.wikipedia.org/wiki/Extended_display_identification_data

http://pinouts.ru/Video/VGA15_pinout.shtml

- Franc Zabkar

 

[Sjouke Burry]

I see a lot of VGA cables being sold on Newegg that do not have all 15
pins. It looks like Pin 9 is missing. Wikipedia says that Pin 9 is
KEY/PWR (+5V). I don't know what that means.

Does it make any difference whether you have all 15 pins or not?

Be glad that it is missing.
I blew 5-6 ground traces in an expensive computer.
The vga display had 5 volt without current limit on pin 9,
and the computer had it connected to ground.
As luck would have it, I found the traces,and could re-wire them.
Thee is no known benefit for the presence of pin 9.

 

[TVeblen]

Pin 9 powers the EDID EEPROM chip in the monitor. If the EDID cannot
be read, then PnP won't be able to determine the monitor's specs. If
the EEPROM is powered by the monitor, then pin 9 may not be necessary.
I'm not certain, though. What is certain, however, is that you will
need pins 12 and 15 (data and clock) for PnP. If these are absent,
then you will still be able to use the monitor, but you will need to
set it up manually with the correct INF file.

http://en.wikipedia.org/wiki/Extended_display_identification_data

http://pinouts.ru/Video/VGA15_pinout.shtml

- Franc Zabkar

This was correct Franc.

I received my new cable yesterday. I replaced the 6 foot "True 15 pin"
VGA cable that came with the Dell 2007FP monitor with the new 10 foot
"14 pin" VGA cable and booted. There was a momentary screen flicker as
Windows (XP-SP3) loaded, but the desktop came up and all was good.

But - In device manager the monitor was no longer recognized as the
2007FP, but as "Generic Plug & Play Monitor", and the Dell driver was
replaced by a Windows Driver.

This is one of my old secondary computers I use in my office
occasionally, I just needed to move it to a more convenient location, so
I'm thinking I'm not going to bother too much with it.

But I'm wondering if I could re-install the Dell Driver, or if without
receiving the EEPROM data it would not make any difference?

Do I have any pressing reason why I need a "true" 15-pin VGA cable?
It is pretty annoying how hard it is to find one of these in the cable's
specs on Newegg.

 

[Paul]

This was correct Franc.

I received my new cable yesterday. I replaced the 6 foot "True 15 pin"
VGA cable that came with the Dell 2007FP monitor with the new 10 foot
"14 pin" VGA cable and booted. There was a momentary screen flicker as
Windows (XP-SP3) loaded, but the desktop came up and all was good.

But - In device manager the monitor was no longer recognized as the
2007FP, but as "Generic Plug & Play Monitor", and the Dell driver was
replaced by a Windows Driver.

This is one of my old secondary computers I use in my office
occasionally, I just needed to move it to a more convenient location, so
I'm thinking I'm not going to bother too much with it.

But I'm wondering if I could re-install the Dell Driver, or if without
receiving the EEPROM data it would not make any difference?

Do I have any pressing reason why I need a "true" 15-pin VGA cable?
It is pretty annoying how hard it is to find one of these in the cable's
specs on Newegg.

Why not test with Monitor Asset Manager ?

This program claims to access the hardware EEPROM data directly, if available.
(I.e. It won't take a cached copy from somewhere else in the OS.)
If you can see the block of data coming from your monitor, then the
interface is working fine. If nothing comes across, that would be
an entirely different matter. Note that the menu on the left has
entries for direct access and for copies from the registry.

http://www.entechtaiwan.com/util/moninfo.shtm

I have the same problem as you - a monitor with what appears to be a
working EEPROM, yet reported as a generic monitor within Windows.
I installed the 6KB "monitor driver" from NEC for my monitor, and
that is how I work around it. Not every monitor has a monitor driver
package, as some companies feel the Plug and Play method is bulletproof.
The monitor driver package is probably shipped, because of the
color management file included in it, rather than being created
expressly as a means of fixing PNP problems. The color management file
is something that would be useful to a Photoshop user, so the screen
colors match other media devices.

Paul

 

[Franc Zabkar]

Do I have any pressing reason why I need a "true" 15-pin VGA cable?

I extended my existing 15-pin cable (by adding a KVM box) with a cable
from an old pre-EDID VGA monitor. I used the INF supplied with the
monitor and all is well.

- Franc Zabkar

 

[TVeblen]

Why not test with Monitor Asset Manager ?

This program claims to access the hardware EEPROM data directly, if
available.
(I.e. It won't take a cached copy from somewhere else in the OS.)
If you can see the block of data coming from your monitor, then the
interface is working fine. If nothing comes across, that would be
an entirely different matter. Note that the menu on the left has
entries for direct access and for copies from the registry.

http://www.entechtaiwan.com/util/moninfo.shtm

I have the same problem as you - a monitor with what appears to be a
working EEPROM, yet reported as a generic monitor within Windows.
I installed the 6KB "monitor driver" from NEC for my monitor, and
that is how I work around it. Not every monitor has a monitor driver
package, as some companies feel the Plug and Play method is bulletproof.
The monitor driver package is probably shipped, because of the
color management file included in it, rather than being created
expressly as a means of fixing PNP problems. The color management file
is something that would be useful to a Photoshop user, so the screen
colors match other media devices.

Paul

I'll give that utility a go Paul, thanks. It should shed some light.

I find it interesting that the monitor (that had been hooked up to the
subject computer for quite a while) was no longer recognized, as it had
been, just because a new cable was used without pin #9. I'm now
wondering if just Dell uses this "non functioning" pin to transmit an
ID, or do all digital monitors do so?

I will play with it as time permits and post back in case anyone is
interested.

 

[Paul]

I'll give that utility a go Paul, thanks. It should shed some light.

I find it interesting that the monitor (that had been hooked up to the
subject computer for quite a while) was no longer recognized, as it had
been, just because a new cable was used without pin #9. I'm now
wondering if just Dell uses this "non functioning" pin to transmit an
ID, or do all digital monitors do so?

I will play with it as time permits and post back in case anyone is
interested.

As Franc explained, Pin 9 is a (redundant) power source.

The monitor has its own power source, and can use that to deliver 5V
to the EEPROM inside the monitor. The intention of pin 9, was to cover
cases where the monitor was turned off, and had no internal power. If
pin 9 existed on the cable, then power could come from the computer.

My first VGA cable, had pin 9 removed, so someone must have known that
it isn't a good idea to ship power on data cables like that.

All that the missing pin 9 should do, is prevent the computer from
reading the monitor, when the monitor is completely without power. If
the monitor is sitting in standby (monitor LED indicates "no signal"),
there should still be power flowing to the EEPROM, and the serial clock
and data should be working at that point. Missing the pin 9 thing
should not prevent it from working. I leave my monitor in standby all
the time, and the monitor lights up the backlight, as soon as
the computer starts, and there is a signal on the VGA cable
on the R-G-B-H-V lines.

If the monitor had protection internally on its +5V, and a fuse or
something had opened up, that might be a reason you suddenly have
a dependence on pin 9 as a source of power. (That would imply that
the monitor's internal power source, is no longer connected to
the EEPROM.) You'd have to check with the manufacturer, to see
if there was protection on it. That's the only thing I can think
of, to account for your symptoms. Normally, the power sources
would be wired-OR, and either one power source or the other, would
power the EEPROM. For unregulated power rails, you can use
simple diodes, to isolate the potential power sources, and
select the power source with the highest voltage.

-->
Pin 9 ---------------- diode -----+
|
Internal Power ------- diode -----+-----> +5 EEPROM
--> | |
I2C clk data
| |
| |
to_video_card

Paul

 

[TVeblen]

As Franc explained, Pin 9 is a (redundant) power source.

The monitor has its own power source, and can use that to deliver 5V
to the EEPROM inside the monitor. The intention of pin 9, was to cover
cases where the monitor was turned off, and had no internal power. If
pin 9 existed on the cable, then power could come from the computer.

My first VGA cable, had pin 9 removed, so someone must have known that
it isn't a good idea to ship power on data cables like that.

All that the missing pin 9 should do, is prevent the computer from
reading the monitor, when the monitor is completely without power. If
the monitor is sitting in standby (monitor LED indicates "no signal"),
there should still be power flowing to the EEPROM, and the serial clock
and data should be working at that point. Missing the pin 9 thing
should not prevent it from working. I leave my monitor in standby all
the time, and the monitor lights up the backlight, as soon as
the computer starts, and there is a signal on the VGA cable
on the R-G-B-H-V lines.

If the monitor had protection internally on its +5V, and a fuse or
something had opened up, that might be a reason you suddenly have
a dependence on pin 9 as a source of power. (That would imply that
the monitor's internal power source, is no longer connected to
the EEPROM.) You'd have to check with the manufacturer, to see
if there was protection on it. That's the only thing I can think
of, to account for your symptoms. Normally, the power sources
would be wired-OR, and either one power source or the other, would
power the EEPROM. For unregulated power rails, you can use
simple diodes, to isolate the potential power sources, and
select the power source with the highest voltage.

-->
Pin 9 ---------------- diode -----+
|
Internal Power ------- diode -----+-----> +5 EEPROM
--> | |
I2C clk data
| |
| |
to_video_card

Paul

Today I attempted to install the INF file "driver" from the monitor's
CD. No go. (Monitor properties > Driver > Update Driver). "Windows
cannot update this driver".

So if pin 9 has nothing to do with the symptom (monitor no longer
recognized as 2007FP after replacing the VGA cable when it was before),
does that mean that the cable is potentially defective?

Next step with be that monitor utility.

 

[Paul]

Today I attempted to install the INF file "driver" from the monitor's
CD. No go. (Monitor properties > Driver > Update Driver). "Windows
cannot update this driver".

So if pin 9 has nothing to do with the symptom (monitor no longer
recognized as 2007FP after replacing the VGA cable when it was before),
does that mean that the cable is potentially defective?

Next step with be that monitor utility.

This is a copy of the INF for my monitor. I don't really
see how this installation is dependent on hardware detection.
I suspect it is just adding stuff to the registry. I think I
right-clicked on this file, and did an "install" from the popup.

******* nl1765.inf *******
[SourceDisksFiles]
nl1765.icm=1

; Manufacturers
;-------------------------------------------------
[Manufacturer]
%NMD%=NMD

; Manufacturer sections
;-------------------------------------------------
[NMD]
%NEC-LCD1765%=NEC-LCD1765.Install, Monitor\NEC6621

; Install Sections
;-------------------------------------------------
[NEC-LCD1765.Install]
DelReg=DEL_CURRENT_REG
AddReg=NEC-LCD1765.AddReg, 1280, DPMS
CopyFiles=NEC-LCD1765.CopyFiles

; AddReg & DelReg sections
;-------------------------------------------------
[DEL_CURRENT_REG]
HKR,MODES
HKR,,MaxResolution
HKR,,DPMS
HKR,,ICMProfile

; Pre-defined AddReg sections
[1280]
HKR,,MaxResolution,,"1280,1024"

[DPMS]
HKR,,DPMS,,1

; Model AddReg sections
[NEC-LCD1765.AddReg]
HKR,"MODES\1280,1024",Mode1,,"31.0-83.0,56.0-75.0,+,+"
HKR,,ICMProfile,0,"nl1765.icm"

; CopyFiles section
;-------------------------------------------------
[NEC-LCD1765.CopyFiles]
nl1765.ICM

; User visible strings
;-------------------------------------------------
[Strings]
MonitorClassName="Monitor"
NMD="NEC-Mitsubishi"
NMDDiskLabel="NEC-Mitsubishi Monitor Installation Disk"
NEC-LCD1765="NEC LCD1765"
*******

The most amazing thing about my monitor ? It still works.
I expected inverter or backlight problems before now,
but so far, no complaints. I'd say the contrast ratio has
shifted a bit over the years, and it's a bit harder to read.

Paul

 

[Paul]

That's weird !

This diagram has pin 9 missing:

http://pinouts.ru/Video/VGA15_pinout.shtml

When you scrolled down that web page, did you see this ?

http://pinouts.ru/Video/VGA15_pinout.shtml

"VGA DDC2 pinout details

VGA pin new function
9 Optional +5V output from graphics card
12 Bidirectional data line (SDA)
15 Data clock (SCLK)"

That explains what pin 9 would be doing, if it was
present, and the video card supported it. Pin 9 delivers
power, so the EDID can be read. And DDC2 is bidirectional,
so it is capable of more than plain EDID reading.

More info on DDC can be found here.

http://www.vesa.org/vesa-standards/standards-faq/

"DDC

Q: Why is +5 volts on pin 9 mandatory for system units and/or graphic cards.

A: It is becoming common for system management software to interrogate peripheral
devices for product type and serial number as a form of asset management. This
is usually scheduled for nighttime but if the monitor is powered off then no
data can be collected. Providing +5 volts allows the DDC circuit in the monitor
to remain active even when the monitor itself is powered off.
Ref.: Section 4.1.4.1"

HTH,
Paul