Subject: Re: serial port control
To: Ted Lemon <>
From: Jukka Marin <>
List: current-users
Date: 02/13/1998 07:25:28
On Thu, Feb 12, 1998 at 02:28:48PM -0800, Ted Lemon wrote:
> Also, I don't think CRTs use switching supplies, do they?

AFAIK, all my CRT's (including the 12-year-old 1081's) do use SMPS.

But, Commodore 64 and the 1541 drive use linear power supplies - you can
tell that from the heat they generate even if you don't have the schematics

> Is it possible that a typical (cheap!) switching supply makes
> assumptions about its supply that aren't valid if the supply produces
> a square wave, and that this could cause the linear supply to behave
> badly even though an ideal linear supply might love a square-wave
> input?

The old linear power supplies hate square wave.  It makes the transformers

> The reason I ask is that I have definitely heard of switching
> supplies being fried by square-wave UPS's.  Could be Urban Legend, I
> suppose.

I have had one SMPS fried by a 1000 VA NetUPS.  It was an IBM with an
electronic power switch, ie. the mains voltage is always connected to
the rectifier bridge and filtering capacitors, but the SMPS system is
turned off using a 5 V signal.  When the machine is OFF, the power
supply draws very little current and the filter capacitors reach the
peak value of the input voltage.  This NetUPS happens to have a square
wave output with high-level peak voltage when the load current is very
low (peaks are well over 400 volts instead of the 325 V they _should_
be).  The IBM SMPS power had all the critical components rated for
400 volts only.. so I heard a loud BANG and saw the magic smoke escaping
the power supply.  Both the power transistors had blown away..

Everything works just fine with this UPS whenever there's some load,
but if there isn't, you're in trouble..  Square wave isn't the problem,
over-voltage spikes are.

Does anyone know the serial protocol used to talk to these NetUPSes?