C1P Superboard power supply issue

[davisgw]

Hi,

I've spent this summer populating a partially complete Superboard II and an unpopulated 610 board
and further adding a floppy interface with a data separator. I've powered it with a PC ATX power supply
and spent LOTS of time debugging my mods for 6264 8K x 8 memory to the 610 board. But as I neared
the completion I was getting inconsistent success, only to discover that just adding or removing about
6 inches to the wire length to power the Superboard II affected whether it would even boot.

Not sure whether my ATX switching supply or the wire length is the issue, has anyone else noticed
power supply wiring needs to be as short as possible? I have confirmed this with my bench supply. Also,
has anyone else successfully powered the Superboard II with an ATX power supply?

By the way I have two Floppy drive equipped Superboards and the one that is mostly unmodified also
experiences the same issues, so it is my baseline for my debug of the other one....

Glenn

[harrywalter99]

You may be having problems with RFI for whatever reason. It you happen to have a ferrite ring in your junk box, a turn or two of the lead in from your PSU should prove the point. I have a ATX power supply setup as a bench supply. I haven't had any issues when I have used it.

[mcsele]

Another thought: how thick (what gauge) is the wire you are using? Thin wire (e.g. 20/22ga hookup wire) has a relatively high resistance that might be problematic when drawing AMPS through it. Check the actual voltage right at the chips on one of the boards with a DMM ... if below, say, 4.9V that could be the issue.

Had that issue with a homebuilt EPROM programmer once: it worked fine but when actually programming the chip draws a lot of current, voltage sagged, and it all crapped out!

And it goes without saying that the boards should have numerous decoupling caps (0.1uF) on them.

Good Luck debugging!

[davisgw]

Thanks for the replies...I've made some progress.

I shortened the wires from the ATX (I'm pretty sure it's 18 gauge wire) and it seems to have helped. I also discovered that the floppy drive
I was testing with has a write issue so I had to recreate some of the diskettes that failed to boot up. I did all this on my mostly unmodified
Superboard with 610 expansion and floppy interface.

Now to the problem with the Superboard and the 610 board that I modified for 6264 RAM chips. I've run hours of memory tests to verify
the memory works, and it boots with HEXDOS and OS65DV3, but fails to boot DOS/65 every time. I've made a piggyback memory card that
gets most of it's chip select signals from the 610 board, with some minor added logic for the 6264's. Obviously I have a flaw in the (addressing?)
logic but I am not making much progress figuring out what it could be, since I lack a multichannel digital analyser (I have a 2-channel analog
scope). I will repost this question on another appropriate forum....

Any ideas?

[BillO]

The technology the Superboard was built with had poor noise immunity. Add to that it has a rather limp implementation of supply decoupling. Do you have a scope with which you could check the noise on the 5V supply coming from that switching PS?

Also, measure the voltage getting to the chips at various points around the board. You need a minimum of 4.8V but that might be too low if the PS noise is significant.

[davisgw]

Hello again. I think I have resolved my power supply issues, but it took a couple unexpected turns. The failing system was a Superboard with a 610 board expansion with 32K of 2114 RAMs, all were 2114-550 as listed in the parts list. My second system is a Superboard with 2114-300 RAMs and a 610 board that I have added 6264 RAMs to get the total of 32K. This second system was less of a load on the ATX supply and the voltage was about 4.85V on the board, and it ran error free. The system populated completely with 2114 RAMs would pull the voltage to 4.80V which slowed down the memory and caused failures when booting up OS65D. Then I replaced the 8K of Superboard RAM with 2114-2 RAMs, but there were still occasional failures.

So after my attempts to increase the 5V output from the ATX supply failed to solve the issue, I took the weakest of my ATX supplies and bundled all of the 5V and ground wires to reduce the voltage drop in the wiring. That resulted in 4.90V on the board and solid success booting OS65D. I plan to make the same modification to another ATX supply also.