8-bit acorn hardware • Re: BBC Micro NMI spamming

If the clock is being received cleanly, then there's only a very limited part of the circuit that is specific to the data, so this should be quick to resolve.

Worth checking that your network/termination is in good order before plugging in this machine - with it idle measure D+ and D- on the socket with a meter and they should be somewhere around +1.5V with the D+ a bit (0.2-0.5V) above D-.

Yes, there is around 1.990V and 2.001V on D+/D- and the same on C+/C- (on a working Beeb I get 1.982V and 1.972V respectively) the difference only seems to be 10mV though. I am measuring the voltage at the DIN connector on the back of the machine.

Plugging in the station shouldn't have any effect - if significantly pulled up or down, can only really be the 75159 (or random solder shorts etc).

Yeah, plugging the station in and voltages are now all over the place. C- = 2.042V, C+ = 0.761V, D+ = 1.327V, D- = 1.071V

Removing the 75159 makes no difference to the voltages measured on the pins when connected to the network.
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One possibility for crap on the data is crosstalk from the clock. R47/R48/C18 should be generating a fairly quiet 2.0V rail at the junction of R45/51/34/35 - if this is bouncing up and down or far away from 2.0V then something wrong around R47/R48/C18.

There is around 50mV of noise on the supply at this point, the other Beeb is very similar.

If that's all OK, then really the data-in circuit is just the other half of IC94 and the few resistors around it. R41/R34 and R40/R35 are potential dividers giving you a 1/10 size version of whatever the data pins on the connector are doing, centred around 2.0V. The LM319 is then a differential amplifier of that pair of signals: R36 is positive feedback for hysteresis, which should be keeping the output constant in the presence of minor noise at the input.

So at idle, pins 4,5 of the LM319 should both be close to 2.0V with pin4 slightly more positive than pin 5, but only by about 30mV or so.

I have 1.932V and 1.967V for those pins.

The other LM319 (IC95) shouldn't be involved, and you could unplug it for this stage of debugging.

Yes, I did remove it for the last test, putting it back in and trying the same measurement made a slight difference of around 10mV with around 30mV between the two pins.

I did notice something very strange on the scope... the yellow trace should be flat when there's no data (and there isn't) I suspect you're right about the solder bridge somewhere, but I've looked with the microscope and I can't see any on the underside, so that leaves me with it being under a socket somewhere. Maybe this trace may give a hint at where the short might be. I've checked all the resistor values and they tally with the other Beeb.

BTW: my pin naming convention is looking at the socket from the back, the PCB side, with D+ on the left next to the PSU, followed by D-, GND, C+ and C- on the right (I'm not sure if this is the correct pinout, I used the polarity of the comparator inputs as a guide)

Statistics: Posted by maniacminer — Thu Feb 15, 2024 12:36 am

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