exxos's Atari, Amiga & retro forum

OK, that's definitely not right.

With the system turned off can you measure the resistance between D2 and +5V. It should be around 10K ohms. I'm wondering if the pull-up isn't working. You can also check the resistance relative to ground.

D2 to VCC is 3.3k
D2 to ground is 3.2k.

OK. If undriven the line should float to a mid voltage.

How does this compare to, say, D1?

I'm just trying to think how we can isolate the problem further.

If the resistances on D2 are different from D1 I would be temped to remove all the socketed chips on the data bus except the CPU and ROM and see if the behaviour changes. I guess that would be U300 and U301 and the clock chip if that's socketed. I may suggest that anyway to just rule them out.

I find it hard to believe that even reading random data would have D2 always be zero when all the other bits are changing.

You may note that I'm trying everything which doesn't require desoldering or otherwise messing with the motherboard as that's the last resort as it can break things further.

I'm afraid that the resistances are the same between D1, D2 and D3 and VCC/GND.
Removing U300 and U301 doesn't change the behavior of D2.
As for the clock IC, I assume you are referring to Agnus U101?

As for removing components, I am in agreement, that can often be a perilous and pointless practice.

Kepler wrote: 17 Jun 2024 20:43 I'm afraid that the resistances are the same between D1, D2 and D3 and VCC/GND.
Removing U300 and U301 doesn't change the behavior of D2.
As for the clock IC, I assume you are referring to Agnus U101?

No, U801.

Anyway, it could now be pointing to a stuck gate on the transceiver U105, or latch U106.

Could you try removing the CPU and ROM and have a look at D2? It should, along with all the rest of the data bus be either floating or pulled up to 5V.

I just noticed that U603 is also on the data bus, and there are 3.3K pull-ups on the lines near the co-processor slot.

So, we have three suspect chips if the data bus isn't going high without the CPU and ROM.

Could you check Pin 19 on U105 and U603 and pin 1 on U106. They should always be high at this point. If they're low then they could be allowing the 245s or 373's to be erroneously outputting data.

(Just belt and braces before we go any further. Assume nothing. :-) )

Morning Stephen - Diagrom has started "working"! 😱 The board has recognized Diagrom! I have output on the serial port! It still isn't booting to the Diagrom home screen, instead I see those red and green horizontal lines and the serial output details the absence of chip RAM, but it is being detected.

Yay! We have life!

So, it's likely that the transceiver between the CPU and the RAM isn't working correctly. It may have had the D2 line on the CPU side pulled low but that's changed to not working at all.

@terriblefire do you agree?

If so then you'll need to find a replacement 74F245. The 'F' is important as it needs to be the "Fast" version of the chip.

Are you not just slightly curious as to the reason why Diagrom is now being recognized?