PREVIOUSLY ADDED CAPACITORS WHEN SUPPLY SAG SUSPECTED
A while back I worried that the rails on the PCB were sagging during the heavy output pulses from the eighteen sense amplifier pulse generating gates. The design I originally used, informed by the datasheet for the LDL1117 voltage regulator, was a 4.7 uF capacitor across the regulator 3.3V output, as well as .1uF decoupling capacitors at every chip.
I tacked on a 470 uF capacitor to provide substantial additional buffering, but the spurious retriggering of the timer chips continued to occur. At my last session, I added an additional 46,000 uF across the 470uF and 4.7uF capacitors. The symptoms continued.
POSSIBILITY THAT EXCESSIVELY LARGE CAPACITANCE LEADS TO SUPPLY OSCILLATION
However, it is a known issue with switching regulators that too much buffer capacitance can lead to the regulator oscillating on momentary demands because it is too slow to notice the energy being pulled from the capacitor, then has difficulties recharging the energy into the capacitor.
While the LDL1117 is not a switching device, instead it is a linear regulator, which is why I initially disregarded the risk of excess buffer capacitance. However, it regulates with an op amp and oscillation is certainly possible if I have imbalanced the chip too much.
REMOVED BOTH EXTRA BUFFER CAPACITORS AND RECHECKED 3.3V AND GROUND
I de-soldered the two capacitors from the PCB, reverting to the recommended 4.7uF capacitor across the output of the regulator chip. I then used the same measurements with the oscilloscope that I had yesterday, looking at AC variations on the 3.3V output of the regulator and also between the ground on the PCB and the ground out at an SLT slot on gate B compartment A1 of the 1130 system.
RESULTS OF RECHECKING
The 3.3V rail is much more stable with the extra capacitors out of the circuit. Unfortunately I still have the spurious retriggering.
What is still happening is a big bounce between ground on the regulator of the PCB and the ground pin of an SLT slot in the 1130.
ADDED A SECOND GROUND WIRE FROM THE 1130 TO THE PCB
The PCB is connected to the IBM 1130 ground terminal block by an 18 gauge stranded wire. That is the same wire type used to connect the grounds on all the SLT backplanes from this terminal block. However, I hooked up a second 18 gauge stranded wire between the ground terminal block and the regulator ground connection. No difference in the observed bounce.
GROUND BOUNCE TRACE OF 1130 SLOT PIN MAY LIE DUE TO CONNECTION METHOD
I don't have scope probe tips that slide over the .025" pins on the rear of the SLT backplane (official name is SLT board). I have used jumper wires, male-female type, to fit over the SLT pin and have the scope probe hook grab the other end. In other words, I don't have a great low resistance direct connection to the probe.
GROUND BOUNCE MAY BE DUE TO REGULATOR CONNECTION TO PCB GROUND
The ground plane of the PCB is an inner layer, but the connections are on the top layer. Vias carry the current down to the inner layer. I thought I had plenty to ensure excellent conductivity. Since I have a helper wire from the regulator ground where my new wire attaches all the way to the read timer chip grounds, the retriggering doesn't fully make sense yet.
OBSERVATIONS
When I combine the images of the ground bounce relative to the 1130 SLT logic with the +Storage Read signal coming into the PCB and the pulse retriggering, the wiggles in the control signal seem directly correlated with the ground bounce as seen at the 1130 SLT board. When the data returned has more one bits, the retriggering pulses continue longer and the ground bounces with them.
A cause appears to be the NAND gates sinking the current from the 1130 side to flip on the bits in the B Register. The 1130 side has a capacitor that is charged up to sensitize it, then when our PCB sinks that to ground, the edge causes the flip flop in the B register to turn on.
Four feet of 18 gauge stranded wire should drop about 7 millivolts with that current draw, not hundreds of mV. Two parallel such wires should reduce the max bounce further.
Still not sure of exactly what is causing this thus not certain how to correct it, but getting closer.
I did some rough calculations and though the ground wire has good DC performance it doesn't look so good at MHz frequencies. The ringing seems to be about 7.5 MHz. The wire has about 1.8 μH of self inductance, which presents about 85 Ω at this frequency! This is all very rough and assumes solid core wire (stranded should be better) but you get the idea. Two ideas:
ReplyDelete1) Use braid instead of the wire or something like ribbon cable for the connection to the 1130 with lots of grounds (every other conductor, perhaps).
2) Shorten the ground (may be tricky).
3) Reduce the high frequency component of signals to the 1130 by inserting R/C filters in the signal path. Even just a series damping resistor (say 22 Ω) would help if it doesn't upset the DC performance too much.
None of these are easy to retrofit, unfortunately. I have had trouble before when interfacing new to old logic (albeit not nearly as old as the 1130). Newer logic has *much* faster edges, which require better signal integrity.
I applaud your efforts so far. This sort of thing isn't easy! Good luck!
Merlin
Hi Merlin
ReplyDeleteThank you for the investigation and idea!
The ribbon cables in the 1130 don't have lots of grounds - there is a single ground wire out of 24. They depend on the quality of the ground between the two destinations.
IBM uses 18 (or 20) gauge stranded wire to connect their SLT logic to a common ground. Five connectors with those wires are plugged onto the backplane (SLT board).
I am using two 18 gauge stranded wires to the PCB, although the length is longer than the IBM wiring to the SLT boards because of the placement of my PCB. It should have halved the inductance of a single wire. No change observed versus a single stranded wire.
I had inserted a termination resistor, then an RC network on the input, it didn't stop the spurious retriggering.
It really wouldn't take a very high impedance at the frequency that is ringing to cause an 800 mV swing - about 4 ohms would produce that drop with a draw of 200 mA.
I will work on ways to improve the ground connection or to detune the resonance around 7.5MHz.
Again, I appreciate the ideas and the support. I will conquer this but it is frustrating.
I suspect that some of the inductance is in my PCB, due to the vias where ground connections have to cross between the ground plane layer and other layers where the component is soldered.
ReplyDeleteI am redesigning the PCB with larger traces with several vias for every layer crossing. I don't want to pull the trigger on the fab until I am certain that the problem will be fixed.
Will try overkill on ground connection to the current bodged up board, then if the issue can be eliminated it will be time to spend the money and time.