12V needed for ST?

[Badwolf]

Really quick one. I didn't want to pollute another thread.

I've seen it said a lot -- and I just saw RJ (I think) mention it on another thread -- that the 12V line isn't needed for the ST to boot.

I thought this was true too but couldn't get anything out of either of my STs when relying on this.

Turns out my combination of VGA monitor, reduced cost VGA adapter and setting Mono mode does require 12V to get a picture. My RGB cable for colour mode did not require the 12V, as is mantra.

Just so I don't give out duff advice in the future, the quick question is: is this true of normal mono monitors -- ie. it needs 12V -- or is this just a requirement for the reduced cost ST to VGA adapter in mono mode?

Cheers,

BW

[exxos]

Off the top of my head..

12V on the ST/F/M just goes to the RGB port. Either used for scart switching or to power the ST2VGA adapter. Aside from really old floppy drives which use 12V.

12V on the STE is used for the audio. Though I don't think it needs 12V to boot. Ironically I think I powered up a STE without 12V a couple weeks ago, can't remember why. But I think it booted, but I couldn't get a image on my TV IIRC as the 12V was missing. I didn't look into it more though.

As for proper MONO monitors, AFAIK it doesn't use 12V.. but not 100% sure as I never really use mono monitors.

BUT, MONO for the ST2VGA would still nee 12V as it powers the RGB/MONO IC. Some adapters use a physical switch to switch the video signals, but that's just setting up for a world of bad contacts and noise down the line. I vowed never to use signal switches again, like 40 years ago as I hated them with a passion. It's why I use a IC for the switching.. but then you need 12V.. Never any ideal solutions unfortunately.

If your talking about my ST2VGA adapter, then it can likely be powered from 5V.. if you just hook over 12V to 5V and use 5V power only, (feeding 5V into the 12V) Then I would assume my adapter would still function. The voltage is zener diode clamped to 5V (from the 12V 1.2K resistor). So feeding 5V into it should be fine.. But never tried it.

[troed]

For people who still use SCART 12V is also needed to generate the correct signals for TVs to recognize the picture.

(and, if I'm not misremembering, it's also used to generate to correct voltages for serial communication)

[rubber_jonnie]

Aside from what has been said by the others, which is all good info, yes you can boot to desktop using just 5v.

I usually use a VGA adapter for all my testing so the 12v for SCART isn't required, though if I need to use the serial port for diagnostics I hook up the 12v too, as I have 2 bench PSUs.

[Badwolf]

For people who still use SCART 12V is also needed to generate the correct signals for TVs to recognize the picture.

Is that just for the RGB/Composite switching? I think I've used SCART for colour with just 5V.

BUT, MONO for the ST2VGA would still nee 12V as it powers the RGB/MONO IC.

Aha. There's the smoking gun.

OK, it's just my adapter. That's cool, I didn't want to be handing out duff advice if there were something else going on.

I usually use a VGA adapter for all my testing so the 12v for SCART isn't required

Colour mode? Or perhaps you just have a different VGA adapter to me (exxos' reduced cost version)?

Cheers all.

BW

[rubber_jonnie]

I usually use a VGA adapter for all my testing so the 12v for SCART isn't required

Colour mode? Or perhaps you just have a different VGA adapter to me (exxos' reduced cost version)?

Cheers all.

BW

Whilst I did recently get a VGA adaptor from @exxos, I've not tried it with just a 5v setup, but I did all my testing on Brett Rogers STE with just 5v and a VGA adapter and lo res and it was fine in colour.

[czietz]

I hope I can clear up some confusion in this thread.

No 12 V are required for mono output. (Except, of course, if some external adapter is powered from 12 V.)

In STs with a modulator, the chip that generates the composite video signal is powered from 12 V. A typical RGB-SCART cable derives its composite sync from the composite video signal and will therefore not work without 12 V on these STs. On STs without modulator, the composite sync is generated differently (by a transistor and diodes), and does not require 12 V. If you use an RGB color monitor with separate H and V sync, 12 V is not required in any case.

Relevant schematic snippet:

composite.png

In the original 520ST, +/-12 V from the PSU power the serial port driver ICs. In the STf and later, the serial port voltages are generated by an on-board SMPS:

serial.png

[Badwolf]

Thanks all.

So my takeaway is that whilst 5V will let you boot to the desktop, whether you can *see* it depends on quite a few things.

So I think I'll stop recommending it as a troubleshooting shortcut without that nota bene.

Cheers,

BW.

[Darklord]

Perfect timing on this subject. :)

Once before, I powered my STacy with a Pico P/S and simply tied 5v into
the extra floppy drive power lead, and this worked fine.

It's my understanding though (and please correct me if this is wrong) that
the serial port also requires 12v?

If that is so, where would be a good place to feed 12v from the Pico P/S
into the STacy's motherboard?

Thanks.