USING ACTIVE FET PROBE TO WATCH SIGNAL DURING FAILURE
I had been plagued by the inability to watch what happens when the sporadic failure occurs since the oscilloscope probe and the logic analyzer probes both loaded the circuit enough that the bit never was dropped.
I purchased an active FET probe which introduces much much less loading on a circuit. It was a Tektronix P6201 probe which was sold as new-old stock. I also purchased a used Tektronix 1101 Power Supply which the probe requires for operation. With both shipments finally received, I checked out everything before putting it to use.
TESTING AND CALIBRATING THE 1101 POWER SUPPLY
I downloaded the manuals and began testing the power supply. I certainly didn't want a bad supply to blow up an expensive probe. The box produces +15V, -15V and +5V and delivers it over a unique cable to the probe. I got the voltages set exactly correctly and validated proper behavior of the overcurrent protection.
ACTIVE PROBE VOLTAGE LIMITATIONS ARE WORRYING
The probe uses a field effect transistor to achieve a very high impedance, thus limiting the load on the circuit being measured. However, the FET has a pretty low threshold for maximum voltage. The manual for the probe gave the range of voltages that could safely be measured.
The circuit I will be probing is pulled up to +3V inside the IBM 1130 and then discharges to ground to set a bit into the storage buffer register. A capacitor is involved in the circuit thus there is the possibility of overshoots so I wanted to be quite cautious.
The probe itself can tolerate levels of no more than +/- 0.6V, a 1.2V p to peak and with a DC offset built into the probe can be attached to lines that are +/- 5.6V, no more. It comes with two attenuators - a 10X and an 100X - that increase the max swing and offset. The 100X can connect to a +/- 200V line and record up to 120V swings, while the 10X has a max level of 6V and swing of 12V and a max +/- volt limit of 56.
The probe can be put in AC or DC coupling mode, but even in AC mode the swing is going to be 3V or perhaps a hair more. I needed at least the 10X attenuator to protect the probe.
The advantage is that the attenuators cut loading to the circuit being measured. The probe alone has an resistance of 100K ohm and 3 pF, but the attenuators bring the resistance up to 1M ohm and capacitance down to 1.5 pF.
SET UP THE PROBE WITH 100X ATTENUATOR FOR MAXIMUM SAFETY
I did my first measurements using the 100X attenuator. I hooked it to the backplane pin where my signal arrives to set the flipflop for B register bit 14, then set another probe on the scope to a pin for the detection of a parity error. This would give me the trace on my signal at the time that the bit failed to set and the 1130 stopped with a parity check. Once the Tektronix power supply for the probe finally arrived, I was able to attach it to the 1130 and record some data.
I had the machine set to loop through the memory of the 1130 continually reading the data. In the past, after a few hundred thousand reads I would see bit 14 fail to set when the 1130 MRAM board is trying to set it as a 1 value. This triggers a parity check since the parity generated by the MRAM board is based on all the 1 bits, but the 1130 is missing one so that it finds the parity does not match.
The machine ran flawlessly. Just like with the ordinary scope probe attached. I detached the active FET probe and the machine then took some parity checks. This time it is dropping bit 2, not bit 14, but it is on about the same frequency as before and just as predictable. Just . . . not measurable.
When I force it to record a trace, the pulses look reasonable. The signal drops sharply to ground, then rises back to 3V. There is a small uptick in the middle of the low point, but it might be an artifact because I didn't have the shortest ground path for the probe.
I am going to experiment with the 10X attenuator and make some other changes, hoping that I can get a failure while I am watching. I sure hope that I didn't spend many hundred dollars for the active FET probe and will still fail to observe the problem.
I think it is time to once again look into ground issues of all the compartments. Do you have an isolated ground or did you try removing protective earth from the scope to make it floating? Besides that - if you need somebody to attest that every 1130 came with a free oscilloscope pre-attached I would totally do that.
ReplyDeleteThe oscilloscope was plugged into the mains as was the 1130. No isolation transformer.
DeleteI admit that for convenience of attaching probes, I would attach a ground lead to just one of the four probes and hook it to one of the ground pins on a backplane, but not necessarily the same slot or even same compartment as the key signal I want to observe.
The wires I use to connect the backplane pin to the probe do add some loading themselves. I will first try minimizing the distance and ground the probe with the key signal to the ground pin of the same probe.
Whatever loading my probe adds that stops the failure from occurring, it involves the path through the scope to the one ground connection that was on a different probe.
Time for some experimentation.
Mh, that was not was i meant. The scope itself has its ground probe tied to protective earth. So essentially by having the scope connected to protective earth, and the scope probe ground wire connected to the machine you create a additional path from protective earth to where ever you hooked it up at the machine. An isolation transformer will not change that fact. In theory this will not matter because everything is tied together at the machine anyways, but who knows? Of course one needs to take care, but there is no chance you hook up the scope ground to hazardous voltages while working on the 1130 logic I guess.
ReplyDeleteI did understand your post but expanded on it. I did carefully test the potential between the oscilloscope ground (tied to the electrical ground pin of the receptacles in the workshop) and the 1130 grounds.
DeleteThe 1130 maintains a separate bus bar for earth grounds from the signal ground, but does tie them together in one spot. Some equipment attached to the 1130 similarly ties earth and signal grounds together.
I do have a problem if my Lenovo laptop is plugged into its power brick and then attached via USB cable to an Arduino hooked to the 1130. Things go awry as the Lenovo is doing something odd with the ground. I have to carefully run with only batteries when I use the laptop to control the various enhancements I have added to the 1130 which make use of USB serial connections.
The issue of ground loops is serious particularly with machines that span multiple gates, frames and boxes. I tended to attach only one ground line but that does challenge signal quality. Same with logic analyzer traces, but even more to deal with.
If, as may be the case, the 1130 has some ground issues bonding ground pins on one backplane with others, in spite of the heavy gauge stranded wire joining all of them together, the scope or analyzer probes could be providing a scaffold to partially heal the problem.
I expect to do a myriad of experiments with different ground jumpers and measurements.
You are right of course - the earth ground path remains intact when an isolation transformer is used. I sort of skipped over that fact when thinking about your earlier post.
👍👍👍. You know I had a similar problem with an arduino clone, my Dell notebook and our CHI-2130. Once the notebook is connected to its charger the arduino is getting flaky.
ReplyDelete