Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Archive for August, 2011

Cumulus Bus Saga Continues

Posted by retromaster on August 22, 2011

Here is today’s second post: Another update on Cumulus.

First, the bad news: It looks like with the most recent change to CumuluBus, Cumulus bus errors were greatly reduced, but not completely eliminated. This is the case with the long cable setup.

Now the good news: I have tried the short cable setup (using the two closely-located connectors on the IDE cable), and during extensive testing, I have not encountered any bus-related issues.

What does this mean? It means that on some Orics the long cable setup may not work properly. In these cases, Cumulus will most probably still work fine in the short cable setup, though. It will be a somewhat inconvenient, though, as the cable length is not more than 10cms.

Why does this happen? Here is my guess:

The CPLD used in the Cumulus (XC95XL series) is a relatively modern part (at least compared to the Oric, that is). It has pin signal rise times in the range of a few nanoseconds.

In the current Cumulus design, the CPLD is connected to the data bus over a level-shifter IC (also w/ fast rise times) and directly connected to most of the bus control signals. The connection is by means of the 80-wire IDE cable. When the length of this cable exceeds a certain threshold, the cable starts to exhibit “transmission line” effects. Without the necessary termination on both ends of the cable, these effects cause significant signal integrity issues.

Even with the “SLOW” rise time setting on the output drivers, the XC95XL is fast enough to limit the length of the cable to just about 10cms.

I have to admit that I have overlooked this aspect of the design. I thought that since the frequencies on the Oric bus are so low, there would not be problems over a long cable. However, I failed to see that it is the rise time that’s the real governing parameter here, which was a mistake. This is most likely why things go wrong with the long cable setup.

One way of solving this problem could be by adding “slow” (74hc or 4000-series CMOS) buffers on the Cumulus side. Since these parts have much higher rise times, the permitted cable could be much longer. An additional advantage is that this way, possibly, CumuluBus could be eliminated! However, it would take a significant redesign effort, and it would enlarge the main PCB by quite a bit. To tell the truth, I am not really motivated to go through another design cycle, especially considering that Cumulus seems to work fine with the short cable setup.

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Making Copper PCB Stencils for SMT Reflow

Posted by retromaster on August 22, 2011

Over the weekend, I decided to try out making a stencil for solder paste from copper foil. I had thought about this previously, but after seeing an instructable on the same idea, I became more confident that it would work.

The process for making a copper stencil is basically the same as making a PCB. The resist image (i.e. paste layer, inverted) is transferred onto the copper foil using any method of your choice. For my tests, I did PnP Blue toner transfer onto 0.2mm-thick copper foil using a laminator. Afterwards, the back side of the foil is covered to prevent etchant from eating away all the copper. I used clear packing tape for this step. The one crucial thing here is to make sure no air bubbles remain in between the tape and the foil. Finally, etching is performed as usual and after cleaning and removal of the tape, one gets a nice looking stencil as shown in the photos above.

One thing to note is that, if one leaves the tape in place, this method can also be used to make simple flex PCBs. This could be quite useful for repair purposes, as well.

Although in my tests the stencil came out pretty well, the reflow (using a hotplate) did not go all that well, as the photos above show the bridges in between some of the legs. This can be due to several reasons: The paste itself wasn’t at very good condition to begin with, more flux may have been needed, the stencil may have been too thick (too much paste), and surely I could have done a better job when depositing the solder paste. Nevertheless, this is not too bad for a first try, and I am confident that I can tweak and improve the process over time for near perfect results for larger PCBs.

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Cumulus Bus Issues Solved?

Posted by retromaster on August 5, 2011

Here’s a quick update: Apparently, swapping the AHCT variants of the buffer ICs on the CumuluBus with HC ones did the trick! Coupled with some of the changes I did to the CPLD code, Cumulus seems to be working stable with all three of the Atmos’es I have, with no video issues whatsoever. Over the weekend, if I find the time, I’ll be doing some more tests so that we’ll know for sure… and I might also work on the PIC firmware a bit as well, as there might be still some rough edges there.

I may have to do some repair work in the next days as well, since it looks like the Oric 1 I mentioned in the previous post does not work (with or without Cumulus). Fortunately, the ULA is most likely fine since there is video output (vertical B/W bars). Probably the DRAMs have died… Luckily I have stock :).

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