I'm hoping to get Uncle Bernie's comments on this one.
I have 2 projects I'm working on, one with the A2Pico (A2Retronet) and an SS50 system. Want to eventaully get Fujinet functionality for mikbug & Flex OS. :-) I'm watching the A2Pico thread intently, I'm learning a lot there.
But the other project has me at Wits End! I built a reproduction Frob (Apple 2 board that allows you to develop Atari 2600 VCS code and run it from the Apple II plugged into the VCS). From the VCS's point of view, the Apple II Frob looks like a 2732 ROM cartridge. I built the boards and I've checked for shorts or opens. There appears to be none. I've got matching chips except for the PPC360AN (unobtainium). These appear to be equivalent to a 74ls367 (VCS to Frob address receivers). The Frob to VCS 24 ping ribbon cable is about 2.5ft long. I will build a shorter cable for further testing. The original appears longer. I'm pretty sure it's some kind of race issue. I have 2 working Atari VCS, a Sears VCS and a Heavy 6 (different revisions). The board kinda works in the Heavy 6 VCS, does nothing in the Sears VCS. The view of the 'kinda' working code suggests that the VCS 6507 CPU sees some of the code but must be getting some corruption as the code appears to run then not run then run (rinse-lather-repeat). The TV screen gets the correct image for a moment then goes incorrect then correct again. I've got all the schematics logic analyzer cabling and 3M chip clip and I'm about to start looking at traces. Anypne have any suggestions?
And the one weird detail, I put the VCS end of the cable into a TL866II+ and it reads the ROM image correctly. I should run that in a loop and see if multiple attempts remain the same.
Alas, I lack the clairvoyance / "remote viewing" skills to look deeper into the nature of your problem(s) with that hardware, but I'd think that a 2.5 ft long cable driven by LSTTL and then expecting that anything works is unrealistic.
Your proposal to shorten the cable is on the right track, and you may get a more robust functionality.
I remember, back in the 1990s, both AMD and Lattice sold programming cables for their CPLDs which attached to the parallel printer port, and, alas, they did not really work well. They were too long, but just at the limit where with some printer cards it would work and with others, it won't.
I designed a new programming cable using (IIRC) 74F244 buffers at both the sending and receiving end, a twisted pair flat band cable with controlled impedance, and resistors to do the impedance matching. This cable worked fine up tp 6ft in length, and probably could work when longer, but that was never tried.
I think the same technique could be used for your VCS development cable. But you need those buffers on both ends, to drive the cable and then to receive the signal and restore it to proper logic levels. And be careful with the power supply issue ... unless you power the VCS from the cable, you have two independent power supplies, and so if you use CMOS ICs, they might latch up and release their magic smoke.
Twisted pair flat band cables are heinously expensive but they help to minimize crosstalk. You get some crosstalk mitigation from a standard flat band cable if you use a GND wire between all signal wires, and this also creates a clumsy form of not-so-well inpedance controlled transmission line, so you can try to squash ringing with proper matching resistors on both ends of the cable, but this cheap trick gets nowhere near the signal fidelity of a real twisted pair flat band cable.
- Uncle Bernie
Don't worry I wasn't expecting that much from you (clairvoyance). :-)
Thanks for the comments, that's all I was expecting. I respect your talents.
I don't have much experience with timing issues. But as you've noted the lovely antenna (the ribbon cable) is probably the problem. I'll create the shortest table that is reasonable. I have been thinking about terminating resistors like a scsi cable. It's kind of nice to see my thoughts actually track with yours in this case. I haven't worn my EE hat in a long time. Been doing network and software engineering most of my professional life.
On the twisted pair or the later IDE cables, I thought about both but they both require a redesign of the Frob. It's kind of a repoduction of a museum artifact so the thinking is to keep it as is.
Good new is that the Sears VCS has a socketed 6507, so wiring up the 6502 board is just a tangle of wires (and new antenna). I really need to clean off my bench for this mess.
Besides twisted-pair ribbon cable, like AMP Spectra-Strip, some high speed connections used shielded ribbon cable.
You could try improvising that with a foil wrap, possibly bonding it to ground.
I have a ton of aluminum foil. I'm pretty sure that I need to ground only one end. I can make kludge foil sheild and ground. Wouldn't hurt to test additionally.
in post #5, 'linuxha' wrote:
" I can make kludge foil shield and ground. Wouldn't hurt to test additionally. "
Uncle Bernie comments:
This will not get rid of reflections / ringing and cosstalk between the wires in the flat band cable.
The whole reflection / ringing issue with not properly impedance matched and terminated wires has to do with the rise/fall time of the signals, and it will get worse with faster edges. IIRC, about 11" of wire still is OK with STTL, and this implies that for LSTTL it might work with twice that length. I could look up the actual recommendations in some older TI databooks but finding them would take time.
A kludge foil shield will only help against strong RF interferers from the outside. Which is very, very unlikely to be the root of your problems unless you live near a cell phone tower, a RADAR site, or a TV / Radio transmitter. (I would not want to live there anyways ... could tell you stories about abandoned houses near powerful TV / Radio transmitters ... worthless ... a farmer having fields there but lives further away told me all these people of the abandoned houses died from cancer).
- Uncle Bernie
I've got plenty of solar panels with all sorts of RF noise, an open Apple II (easier to get at) and a 5W low power digital antenna (Ham). Anything stronger is miles away. The wife and I avoided living near the power line. One house was directly under the power lines, with other useful features such as a drop down ceiling and rooms too small for most non-IKEA packaged furniture. The realtor thought he'd be helpful that he'd sell his furniture to move in here and my wife's response was: 'I don't see you living here'. We dropped that realtor.
I'll save the sheilding for later. Might not get to the cable tonight, gotta do other work right now.