K8031: Aquisition stops

Hi,

When I just started a mesurement session with the K8031 it seems that the acquisition sometimes stops (the graph on the screen gets “frozen”) while the scope screen still says “running”. Clicking again on e.g. “trigger off” restarts the acuisition.

I think that while “running” in the “trigger off” mode acquisition should always occur.

Apparently this phenomenon has no relation to the time the K8031 is powered up, only to the time the software is started up to perform some measurements.

Also, this “sometimes stopping” gets less and less while continuing measuring with the K8031, until completely gone.

The phenomenon is the same in the “normal” or “slow” parallelport communication speeds.

I have also experimented with the parallel port setting in the PC’s bios but that one seems to have no infuence on the matter.

Anyone an idea?
Thanks in advance. :smiley:

The reason to this problem may be some sort of LPT communication problem. Maybe the LPT port output of the PC can’t supply enough voltage (or current).
Please check the voltage of C44 when the scope is running. Should be about 2.9V. The scope seems to work even if the voltage is about 2.2V.

Hi Vel255, thanks for your reply.

Here are some measurements done in the parallel port interface (sadly, during these mesurements no stops occurred):

Voltage on the emitter of T1:

Als long as T1 is not “on” the voltage is high (4.75V), the drops and spikes are caused by the switching on (and off) of T1.

Voltage on C44:

Apparently, the voltage is always above 2.5V, most of the time above 3V.

Voltage on C45:

The drops in the voltage seem to come from the voltage drop on E T1. This drop is vasible on C45 via D9…D11 and R50…R52. To remove this drop, D9…D11 should be removed (did not try it), leaving of course less power to load C44, which can be in turn a problem.

All voltage were mesaured using the K8031 itself, so keep in mind any “horizontal” synchronisation of measured voltages.

Thanks in advance!

It seems that the voltage on C44 is OK. Anyhow the problem may be due to the lack of power from the LPT port of the PC.
Yes, you may get higher voltage to the C45 by removing D9…D11.
But as you noted, this may reduce the “main” supply to the transistor T1.

Replacing all the diodes D8…D15 with Schottky type (e.g. BAT42, BAT43…) may help too…

Hi Vel255, thanks for your quick reply. I will do a test with Shottky diodes. That will make the voltage on C44 somewhat higher.

Anyway, I have noticed also that, when I measure (with the K8031 itself) at pin7 of optocoupler IC19 (the base of the output transistor, which is not connected to anything else) that I get also a “stop” of the data acquisition, I even hear some relays clicking, exactly as in the “real” phenomenon.
If I keep the probe on the pin, then if the probe is on x1, the data acuisition can not be restarted, if is is on x10 then sometimes restart is possible. I did interchange IC19 and IC20 but the phenomenon stays.
Does this indicate something (e.g. a weak current coming out of the photo diode into the base of the output transistor?).

Thanks in advance. :smiley:

I made similar test as you have done: Touching the base of the IC19 optocoupler effects the ‘click’ sound but the acquisition does not stop.
The base of the IC19 gets some current via the probe tip because whole the input section of the scope is raised to the voltage of about 2.5V. The current is flowing to the base of IC19 via the oscilloscope’s input 1M resistor chain.

Anyhow this may indicate that the drive current to the optocoupler IC19 is rather low as you expected.

You can test this by decreasing the value of R47. You may first try in parallel about 200 ohms resistor to see if any help…

Hi Val255, thanks for the explanation. That explains the disturbance: that current makes the output transistor of IC19 conduct (a little) while it should not. Sorry I did not check thoroughly the input circuit. I was already wondering why the 5V regulators only outputted about 3V when I measured them with the K8031 itself. :oops:

[quote=“VEL255”]Anyhow this may indicate that the drive current to the optocoupler IC19 is rather low as you expected.
You can test this by decreasing the value of R47. You may first try in parallel about 200 ohms resistor to see if any help…[/quote]
Thanks for the suggestion. I will do the test. :slight_smile:

Hi Vel255,
I discovered that the optocoupler IC19 is also driven while the voltage on C44 is in its “low” part (2.75 Volt, see the middle oscillogram in one of the previous posts):

Pin3 of IC 19 (drive out of the parallel port):

[color=darkblue]Keep in mind that the “high” voltage is so low because of the presence of D13, the low voltage on C44 and a conducting T1.[/color]

Pin 2 of IC 19 (just below the led current limiting resistor R47):

As you can see the voltage accross the 100R is only 0.8V during the low period on pin 3, resulting in a led current of 8 mA. Is this enough?

I did not expect to see drive for IC19 while C44 was still charging up and the voltage on it is still very low. :?:

Anyway, I will also experiment with a lower value for R47 to make the drive current higher.

Thanks a lot in advance! :smiley:

It seems that the LED current of 8 mA is just OK (or nearly OK). The current transfer ratio (CTR) of the 6N136 is typically 24% at the room temperature. The collector resistor R36 is 2K2. There is needed about 1.8 mA collector current to pull the collector voltage down enough to give logical low to the IC16.
The calculated output current is 24% of 8 mA = 1.92 mA.
Should work if the optocoupler’s CTR is typical, even if it is minimum 19%. Anyhow this 8 mA LED current is very near the low limit…

Hi Vel255, thanks for your reply.

I made the current trough the led of IC 19 bigger: placed a resistor of 100R in parallel with the existing 100R (R47). The current is now approx. 14 mA. The result is still the same, no improvement in the “stopping” behaviour. :frowning:

Question: should the setting of the parallel port in the bios (spp, epp, ecp, ecp+epp) influence the output capabilities or timing capabilities of the port? I can not see any difference in my PC. Thanks in advance. :smiley:

Another question: I can not understand the function of T1: why is it necessary to switch off the “supply” coming from the parallel port? Thanks in advance. :smiley:

I think too there is no change in the “strength” of the parallel port if the settings are changed.

The transistor is there to reduce the LPT port current when there is no communication to the scope. Especially important when the function generator K8016/PCG10 is connected to the same LPT port.

I think there is no hardware problem in your K8031.
The problem is the Windows XP that tries to find a LPT printer and disturbs the oscilloscope data communication periodically.

To cure this problem the only thing you have to do is to check the check box “Disable LPT Plug-and-Play” on the PC-Lab2000SE startup screen. Click OK and reboot is requested to do.

Quote from the startup help file:

[quote]Running PC-Lab2000SE software under Windows XP
If you are running the LPT connected oscilloscope or the function generator software under Windows XP, then you have to disable the Plug and Play detection of the LPT port.
The Plug and Play detection system of Windows XP periodically checks the presence of attached devices to the parallel port. This operation interferes with the operation of the PC oscilloscope and the function generator and may cause trace jumps on the oscilloscope screen and may turn off the function generator.[/quote]

[quote]If you are running the LPT connected oscilloscope or the function generator software under Windows XP, then you have to disable the Plug and Play detection of the LPT port.
To cure this problem the only thing you have to do is to check the check box “Disable LPT Plug-and-Play” on the PC-Lab2000SE startup screen. Click OK and reboot is requested to do.[/quote]
Hi Vel255,
I hope this is indeed the cause of the problem. Plug and play was “on”. I am so sorry I did not read (thoroughly) the “ReadMe.txt” file explaining the problem. :frowning:
If it is indeed the solution to the problem then I will be very embarassed. :oops:
I will send you the result tomorrow morning.

Hi Vel255,

This was indeed the cause of the problem. Everything works like a charm now!
Sorry for the (extensive) inconvenience I caused. :oops:
Thanks very much for the (also extensive) support. :smiley:

No problem! This may be useful for other visitors to this forum.
Nice that your K8031 is now working fine! :slight_smile: