23 Dec
2016
23 Dec
'16
4:52 p.m.
Hi folks,
Thanks for all the help so far, I've moved on quite a way but I still
haven't managed to get my head around op-amps other than the basics. If I
may trouble your experienced heads with a small circuit diagram:
http://www.binarydinosaurs.co.uk/STCExecutelStartupCircuit.jpg
This is the full circuit of all the things I've been posting recently,
traced and drawn out as completely as I can manage. All the resistors and
diodes have been tested out of circuit and are OK and I've just realised 3
of the resistors are labelled wrongly. R409 is 47k, R415 and R391 are 100k.
IN- at the op-amp is 1.2V from the LM385ZB-1.2 which has pins 2/3 tied
together. IN+ is 1.3V. V+ is 5.3V. The purple trace is RESET for the 8085,
or should be.
Initially the LM385Z had rotted away because of battery leakage so I was
experimenting with diodes in series to try and get the IN- voltage down to
1.2V, and with two IN4148s (0.9V) the whole circuit sprang into life and I
got RESET at the 8085. For two seconds, then it would cycle for two seconds.
This maybe expected behaviour, I don't know.
3 diodes gave me 1.3V which produced nothing at the op-amp output, probably
because there was no difference between IN+ and IN-?
On a whim I managed to solder new legs onto the old LM385Z and it works,
giving 1.2V at IN-, but the output is still only 0.2V.
I don't mind admitting I'm stumped :)
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
23 Dec
23 Dec
7:48 p.m.
On 2016-Dec-23, at 4:52 PM, Adrian Graham wrote:
Thanks for all the help so far, I've moved on quite a way but I still
haven't managed to get my head around op-amps other than the basics. If I
may trouble your experienced heads with a small circuit diagram:
http://www.binarydinosaurs.co.uk/STCExecutelStartupCircuit.jpg
This is the full circuit of all the things I've been posting recently,
traced and drawn out as completely as I can manage. All the resistors and
diodes have been tested out of circuit and are OK and I've just realised 3
of the resistors are labelled wrongly. R409 is 47k, R415 and R391 are 100k.
IN- at the op-amp is 1.2V from the LM385ZB-1.2 which has pins 2/3 tied
together. IN+ is 1.3V. V+ is 5.3V. The purple trace is RESET for the 8085,
or should be.
Initially the LM385Z had rotted away because of battery leakage so I was
experimenting with diodes in series to try and get the IN- voltage down to
1.2V, and with two IN4148s (0.9V) the whole circuit sprang into life and I
got RESET at the 8085. For two seconds, then it would cycle for two seconds.
This maybe expected behaviour, I don't know.
3 diodes gave me 1.3V which produced nothing at the op-amp output, probably
because there was no difference between IN+ and IN-?
On a whim I managed to solder new legs onto the old LM385Z and it works,
giving 1.2V at IN-, but the output is still only 0.2V.
I don't mind admitting I'm stumped :)
The op amp is configured as a schmitt trigger or comparator with hysteresis:
There is no negative feedback so it is operating at full gain and functions like a
comparator.
However there is positive feedback via R412 (*1), this adds hysteresis to the trip
point(s).
(Brief hackneyed, not rigorous, theory of op: As the input differential varies past the
trip point, the output pulls the + input
further above or below the point at which it just tripped, so the inputs now have to
'overcome' a greater differential
to trip as the input differential varies in the opposite direction.)
One input to the (now) comparator is the 1.2V from V407 regulated down from some power
rail.
The other input is the ~ 1/4 voltage divider down from +5V formed by R398 & R406
(netting 1.34V @ 5V).
It appears the idea is that as the +5 supply ramps up at power-on the comparator trip (and
hence release of reset) is delayed till the +5 reaches something around +4.5V.
The ICL7611 is, I expect, a very-low-power (Intersil's niche) op amp. Together with
the CMOS 4081 the circuit appears tailored for low-power operation.
Is it supplied by the battery?
It may require the battery presence for stable supply at time of power-up to get reliable
reset operation from this power-on-reset circuit.
The diode-resistor pairs at the 4081 AND gate inputs turn the AND gates into
'asymmetric edge delay gates': the resistor together with some capacitance delays
the switching of the gate for an edge of one direction, while the diode shorts the delay
for an edge of the other direction.
If there is no cap at the input, they must be relying on the gate capacitance of the CMOS
inputs, making for a pretty short delay.
*1: Are you sure that's a 220? resistor? It's awfully small compared to the
impedance of the voltage divider feeding it.
8:34 p.m.
On Fri, Dec 23, 2016 at 7:48 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
The op amp is configured as a schmitt trigger or comparator with hysteresis:
There is no negative feedback so it is operating at full gain and functions like
a comparator.
However there is positive feedback via R412 (*1), this adds hysteresis to the
trip point(s).
(Brief hackneyed, not rigorous, theory of op: As the input differential varies
past the trip point, the output pulls the + input
further above or below the point at which it just tripped, so the inputs now have
to 'overcome' a greater differential
to trip as the input differential varies in the opposite direction.)
A recent EEVblog video has some whiteboard explanation of using
positive feedback for hysteresis if anyone is curious...
EEVblog #941 - Schmitt Trigger Tutorial
https://youtu.be/Ht48vv0rQYk?t=13m43s
24 Dec
24 Dec
4:09 a.m.
On 24/12/2016 04:34, "Glen Slick" <glen.slick at gmail.com> wrote:
On Fri, Dec 23, 2016 at 7:48 PM, Brent Hilpert
<hilpert at cs.ubc.ca> wrote:
Ah cool, I was watching some of Dave's tutorials yesterday but in one of
them he lost me completely towards the end, talking about Virtual Ground on
op-amps.
Cheers,
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
The op amp is configured as a schmitt trigger or comparator with hysteresis:
There is no negative feedback so it is operating at full gain and
functions like a comparator.
However there is positive feedback via R412 (*1), this adds
hysteresis to the trip point(s).
(Brief hackneyed, not rigorous, theory of op: As the input
differential varies past the trip point, the output pulls the + input
further above or below the point at which it just tripped, so the
inputs now have to 'overcome' a greater differential
to trip as the input differential varies in the opposite direction.)
A recent EEVblog video has some whiteboard explanation of using
positive feedback for hysteresis if anyone is curious...
EEVblog #941 - Schmitt Trigger Tutorial
https://youtu.be/Ht48vv0rQYk?t=13m43s 
5:38 a.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Adrian
Graham
Sent: 24 December 2016 12:09
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at classiccmp.org>
Subject: Re: ICL7611 op-amp (was: Motorola MC14081B)
On 24/12/2016 04:34, "Glen Slick" <glen.slick at gmail.com> wrote:
> On Fri, Dec 23, 2016 at 7:48 PM, Brent Hilpert <hilpert at cs.ubc.ca>
wrote:
>
> The op amp is configured as a schmitt trigger or comparator with
hysteresis:
There is no negative feedback so it is operating at full gain
and functions like a comparator.
However there is positive feedback via R412 (*1), this adds
hysteresis to the trip point(s).
(Brief hackneyed, not rigorous, theory of op: As the input
differential varies past the trip point, the output pulls the + input
further above or below the point at which it just tripped, so
the inputs now have to 'overcome' a greater differential
to trip as the input differential varies in the opposite
direction.)
A recent EEVblog video has some whiteboard explanation of using
positive feedback for hysteresis if anyone is curious...
EEVblog #941 - Schmitt Trigger Tutorial
https://youtu.be/Ht48vv0rQYk?t=13m43s
Cheers,
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
4:07 a.m.
On 24/12/2016 03:48, "Brent Hilpert" <hilpert at cs.ubc.ca> wrote:
OK, that explanation helps a bit. My brain still has trouble 'slowing down'
how these components work, like when I first dropped from 3GL programming to
assembly. Fortunately there are a lot of tutorials out there.
On a whim I
managed to solder new legs onto the old LM385Z and it works,
giving 1.2V at IN-, but the output is still only 0.2V.
I don't mind admitting I'm stumped :)
The op amp is configured as a schmitt trigger or comparator with hysteresis:
There is no negative feedback so it is operating at full gain and functions
like a comparator.
However there is positive feedback via R412 (*1), this adds hysteresis to the
trip point(s).
(Brief hackneyed, not rigorous, theory of op: As the input differential varies
past the trip point, the output pulls the + input
further above or below the point at which it just tripped, so the inputs now
have to 'overcome' a greater differential
to trip as the input differential varies in the opposite direction.) One input to the (now) comparator is the 1.2V from
V407 regulated down from
some power rail.
The other input is the ~ 1/4 voltage divider down from +5V formed by R398 &
R406 (netting 1.34V @ 5V).
It appears the idea is that as the +5 supply ramps up at power-on the
comparator trip (and hence release of reset) is delayed till the +5 reaches
something around +4.5V.
That's what I figured yesterday which is why I was surprised that
reinstating the LM385Z didn't make it all spring into life.
The ICL7611 is, I expect, a very-low-power
(Intersil's niche) op amp. Together
with the CMOS 4081 the circuit appears tailored for low-power operation.
Is it supplied by the battery?
It may require the battery presence for stable supply at time of power-up to
get reliable reset operation from this power-on-reset circuit.
That's a very good point so now I'm going to have to scrabble around to find
the battery to find out what voltage it was since I removed it ages ago and
may have recycled it. I don't recall any markings on it though, it looked
like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
pictures of it however.
The 5V input at R395 does head off towards the battery location before it
hits the resistor so I'll trace that out too.
The diode-resistor pairs at the 4081 AND gate inputs
turn the AND gates into
'asymmetric edge delay gates': the resistor together with some capacitance
delays the switching of the gate for an edge of one direction, while the diode
shorts the delay for an edge of the other direction.
Gotcha, I wondered why the diode was there.
If there is no cap at the input, they must be relying
on the gate capacitance
of the CMOS inputs, making for a pretty short delay.
*1: Are you sure that's a 220? resistor? It's awfully small compared to the
impedance of the voltage divider feeding it.
My bad, it's a 1.4Mohm and tests ok.
Thanks!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
11:04 a.m.
On 2016-Dec-24, at 4:07 AM, Adrian Graham wrote:
On 24/12/2016 03:48, "Brent Hilpert"
<hilpert at cs.ubc.ca> wrote:
The circuit makes sense if the battery is connected to the Vcc line of the ICL7611, 4081,
etc.,
D1 would be backflow prevention into the +12V line .
R416 would be current limiting providing ~ 8mA trickle charge of the NiCds.
Speaking of such equipment, our museum has one of these buried somewhere, a Nortel
Displayphone:
http://classiccmp.org/dunfield/disphone/index.htm
I don't think it has all the apps of the Executel,
it's phone + on-screen phone directory & phone apps + RS232 terminal with 300bps
modem.
I hope to work on it and document it sometime.
The ICL7611 is, I expect, a very-low-power
(Intersil's niche) op amp. Together
with the CMOS 4081 the circuit appears tailored for low-power operation.
Is it supplied by the battery?
It may require the battery presence for stable supply at time of power-up to
get reliable reset operation from this power-on-reset circuit.
That's a very good point so now I'm going to have to scrabble around to find
the battery to find out what voltage it was since I removed it ages ago and
may have recycled it. I don't recall any markings on it though, it looked
like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
pictures of it however.
The 5V input at R395 does head off towards the battery location before it
hits the resistor so I'll trace that out too.
1:50 p.m.
On 24/12/2016 19:04, "Brent Hilpert" <hilpert at cs.ubc.ca> wrote:
On 2016-Dec-24, at 4:07 AM, Adrian Graham wrote:
Looking at the back of the board the battery WAS originally connected to the
7611 and 4081 at manufacturing but then that connection was physically cut
out and a jumper wire added to the line that supplies the MM58174AN RTC
that's next to them, which is derived from the 12V DRAM supply.
Pity, I was kind of hoping that if I added the/a battery back in then the
7611 would be happy at power up but it looks like it's not involved at all.
Could I add a bit of adjustability by replacing R397 with a 100K pot?
*embarrassed face* The resistor formerly known as R416 is actually R424,
seen here top right in a hybrid front/back mix of the board around the
battery which includes the CMOS chips:
http://www.binarydinosaurs.co.uk/STCExecutelBatterySectionHybrid.jpg
You can see the cutout under the upper left hand battery terminal and the
blue jumper wire running to the 4081. The two green resistors upper left in
the cluster of 5 are R398 and R406. All the red caps are decouplers.
On 24/12/2016 03:48, "Brent Hilpert"
<hilpert at cs.ubc.ca> wrote:
The circuit makes sense if the battery is connected to the Vcc line of the
ICL7611, 4081, etc.,
D1 would be backflow prevention into the +12V line .
R416 would be current limiting providing ~ 8mA trickle charge of the NiCds. The ICL7611 is, I expect, a very-low-power
(Intersil's niche) op amp.
Together
with the CMOS 4081 the circuit appears tailored for low-power operation.
Is it supplied by the battery?
It may require the battery presence for stable supply at time of power-up to
get reliable reset operation from this power-on-reset circuit.
That's a very good point so now I'm going to have to scrabble around to find
the battery to find out what voltage it was since I removed it ages ago and
may have recycled it. I don't recall any markings on it though, it looked
like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
pictures of it however.
The 5V input at R395 does head off towards the battery location before it
hits the resistor so I'll trace that out too. 
Speaking of such equipment, our museum has one of
these buried somewhere, a
Nortel Displayphone:
http://classiccmp.org/dunfield/disphone/index.htm
I don't think it has all the apps of the Executel,
it's phone + on-screen phone directory & phone apps + RS232 terminal with
300bps modem.
I hope to work on it and document it sometime.
Nice!
Hope you have a great xmas :D
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
2872
days inactive
2872
days old
7 comments
4 participants
participants (4)
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dave.g4ugm＠gmail.com -
glen.slick＠gmail.com -
hilpert＠cs.ubc.ca -
witchy＠binarydinosaurs.co.uk