100nF caps were used in "the old days" because of the physical constraints of the package. These days we can push higher values in much smaller packages, so why not make use of ?LarryL wrote: 13 Feb 2024 13:47
- all supporting caps are 1uF - usually I always see 100nF. Any specific reason to go for 1uF?
That was due to parts shortages at the time of some production runs. LS types are preferable.
- The MMU bus isolators & buffers are specced as 74AC373 and 74ACT244 - in my H5 I see LS types, so I think both is OK, depending on availability
This was done because often I found the shifter 16MHz clock caused problems when driving the 1772 on original machines. But because the H5 design was updated, it may not be a problem now. Unfortunately I don't believe anyone has done significant testing to prove this either way.
- the H5 design shows a separate 16MHz OSC for the HD GAL, but there is an option to use the existing 16MHz from the Shifter. Is the latter tested and proven, or shall I go for a separate OSC?
I have personally used the OSC a lot during daily usage for a long time and not noticed any problems. Maybe just wire to the shifter 16MHz feed via a resistor of course, and just have a option for a OSC if something causes a problem...
That would be up to you if you wanted to put both drivers on the same cable. But of course any changes will undoubtedly result in future consequences" at some point . I don't believe anyone has even attempted it on a original machine. Though I would imagine for a ATX it would seem to make sense in having A & B on the same cable.
- for the FDD, the Drive1 signal only goes to the external DIN14, not to the internal IDC. Since an ATX housing has enough space, I would also wire the Drive 1 signal to the IDC. Any reason not to do this? Of course, then internal and external Floppy should not be used in parallel. I could add another jumper to select...
It can be 5V. It was changed on later boards because other items on the bus may use the reset line and be 3.3V devices. So it just made sense to run the reset signal from 3.3V. But as you are not using the 3.3V bus, you can stick to normal 5V levels.
- in the H5 reset circuit, the reset signal is driven from 3V3. Do I need to change things, if I drive this from 5V?
This is also the same thing as your first question of why to use 1uF caps. Because there are dedicated power planes I don't believe that bulk capacitance is necessary. Of course with all the 1uF caps you are effectively having bulk capacitance anyway. I did not see any difference between having them and not on the H4.
- I have not seen any polarized caps (electrolytic, tantalum) in the H5 design (and also not on my H5 board) Is this observation correct, or did I miss something?
I never use tantalum because they tend to catch fire, and I also tried to avoid using electrolytic for long-term reliability.
You could pretty much use any generic transistor, however you would have to run a circuit simulation to see if any of the biasing voltages end up different. I had to do some small changes IIRC when I changed to the BC337 because of that.
- the original H4 audio circuit is using a BC337, only available as THT. Is a BC817-40 (like in the video circuit) the right equivalent?
I don't know what circuit you are referring to because the H5 doesn't have IDE on-board.
- the IDE circuit uses a 2N3906 to drive the reset line. I checked for a SMD equivalent and found BC857B. Good choice?
That would be up to you.
- for Keyboard&Mouse I use the Eiffel schematics. Do you see a need to add a RJ12 socket for e.g. a Mega ST keyboard?
You would have to re-evaluate all the power requirements of all the resistors to see if you could actually do that or not. I personally wouldn't solder a bunch of packages smaller than 0805 anyway.
- for all resistors and capacitors I will go for 0805 footprint (hand soldering possible). Only for the few 75R resistors, JLCPCB has only 0603 in their "basic parts list". Going for smaller footprints for all resistors and caps would save some space, but (at least for me) makes it quite impossible to hand solder. What do you think?
Even so, extended parts sourcing generally costs very little anyway. So no need to limit to basic parts. I normally use more extended parts than basic as the basic range kinda sucks.
