2 Jul
2006
2 Jul
'06
6:02 a.m.
Subject: Re: Vector Imagery
From: "Ethan Dicks" <ethan.dicks at gmail.com>
Date: Sun, 02 Jul 2006 18:49:18 +1200
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
On 7/2/06, Chuck Guzis <cclist at sydex.com> wrote:
I've built that first simple ladder DAC (to hang a 1950s Triplett
meter off of a parallel port for just random analog fun)... the
problem is that it's difficult to tune the individual resistors to
prevent the needle (or voltage) from going visibly *down* when
transitioning the DAC from, say, 0x3F to 0x40 or 0x7F to 0x80. The
cumulative errors of the low order bits can get to the point that you
don't get a constant output increment by incrementing the digital
value. The needle did deflect from 0 to full, but writing a ramping
program showed very clearly that it wasn't a continuous progression.
First, the 1950s triplett meter is likely 20,000ohms/volt at will load
any ladder using resistors larger than 1000ohms in an R2R ladder.
The 5v range looks like a 100K resistor across the probes.
Second, The output port is TTL? CMOS? IF TTL the output swing is
not to 5V to with even light loads. IF CMOS output voltage will sag
if the load is too great.
For 8bit R2R ladders I use R=10k and 74HC244(or other high current
output CMOS) as a buffer. The load is usually very high impedence
(VTVM, FETVM, DVM, OPamp voltage follower).
Allison
What I really would like to find is a formula for
calculating R2R
ladder values so that I could get a 0V-5V output from an 8-bit
parallel port. Constructing it is easy, once one knows what resistors
to pick.
-ethan
2 Jul
2 Jul
8:33 p.m.
On 7/3/06, Allison <ajp166 at bellatlantic.net> wrote:
>
>Subject: Re: Vector Imagery
> From: "Ethan Dicks" <ethan.dicks at gmail.com>
> Date: Sun, 02 Jul 2006 18:49:18 +1200
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
>I've built that first simple ladder DAC (to
hang a 1950s Triplett
>meter off of a parallel port for just random analog fun)...
First, the 1950s triplett meter is likely
20,000ohms/volt at will load
any ladder using resistors larger than 1000ohms in an R2R ladder.
The 5v range looks like a 100K resistor across the probes.
I've already cracked open the meter and tweaked the internal load
resistor. The simple 8-resistor ladder I built _works_, with the
exception of a noticable dip in voltage between 2^5-1 and 2^5, and
2^6-1 and 2^6, and 2^7-1 and 2^7.
Second, The output port is TTL? CMOS? IF TTL the
output swing is
not to 5V to with even light loads. IF CMOS output voltage will sag
if the load is too great.
To be honest, I have no idea - it's an integrated port on a modern
machine. If I had to guess, I'd say it's a CMOS port, but I don't
know that for a fact.
For 8bit R2R ladders I use R=10k and 74HC244(or other
high current
output CMOS) as a buffer. The load is usually very high impedence
(VTVM, FETVM, DVM, OPamp voltage follower).
OK... that's a place to start. I know I can make it work with an
8-bit buffer, but once I get a powered chip external to the PC in the
mix, I might as well wait for the next plane and use an 8-bit or a
12-bit DAC and OPamp for a "real" D-A circuit.
-ethan
3 Jul
3 Jul
2:10 p.m.
I've already cracked open the meter and tweaked the internal load
resistor. The simple 8-resistor ladder I built _works_, with the
exception of a noticable dip in voltage between 2^5-1 and 2^5, and
2^6-1 and 2^6, and 2^7-1 and 2^7.
What resistor vavlues are you using, and what tolerance?
Second, The output port is TTL? CMOS? IF TTL the
output swing is
not to 5V to with even light loads. IF CMOS output voltage will sag
if the load is too great.
To be honest, I have no idea - it's an integrated port on a modern
machine. If I had to guess, I'd say it's a CMOS port, but I don't
know that for a fact.
7:14 p.m.
Then connect up your 'DAC', write FF again,
and measure the voltages
again. If they're significantly lower, your DAC is loading the port too
much, use higher value resistors. You might also try writing 0 to a
particular port line, measuring the voltage there with the others at all
0's and all 1's. Then do the same thing with the line you're monitoring
set to 1. Repeat those tests for all 8 port lines. If you see the voltage
on the line being monitored change when you change other bits, again I
think the resistors are too low.
Go simpler: Use a ZN428 from Ferranti ;o)
11:19 p.m.
On 7/4/06, Alexandre Souza <alexandre-listas at e-secure.com.br> wrote:
I think you missed the part where if I ordered one, it might show up
by 1 November.
I have to use what I have on hand.
-ethan
P.S. - Tony, thanks for the info on port loading, etc. I'm trying to
avoid a multi-cable setup so that when I turn things over to the next
guy, it's as uncomplicated to attach as possible. Also, while I've
used PS/2 ports, etc., in the past, I'm not sure how much longer
they'll be standard equipment. We already have servers here with no
parallel ports. I've rigged up a USB-to-banana-jack cable for +5V for
my breadboard (that's where my simple "DAC" is at the moment), but the
only free USB ports are on the front, leaving me the only option of
parallel cable in the back for data, and a long cable all the way
around the machine to the front for power. Not a robust end-user
situation, I'd say, but OK for puttering around.
Then connect
up your 'DAC', write FF again, and measure...
Go simpler: Use a ZN428 from Ferranti ;o)
6746
days inactive
6748
days old
5 comments
4 participants
participants (4)
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ajp166＠bellatlantic.net -
alexandre-listas＠e-secure.com.br -
ard＠p850ug1.demon.co.uk -
ethan.dicks＠gmail.com