Posted By
MMS
on 2018-07-26
19:48:41
 |  Expansion port extender - questions, looking for suggestions
I read through some topics on the Expansion port possible problems with the extender.
As told by several sources and more expert people on C64, that the flat cable based extansion cards are crap, as causing a lot of instabilty and read/write problems. That could be corretc, as the signals can influence the data in the next wires running next to it.
On the other hand the problem is not unknown by other systems too. In the old PATA (parallel ATA) connectors only 33MB/s was stable with the 44 pin conectors, but when the number of wires changed to 80 (one ground line between two data lines) the speed and stability increased significantly even up to 133MB/s. (certainly the twisted wire method even better)
So I suppose it was never tried out on Commodores (except on the very expensive IEEE-488 connectors) and that's why it failed on the previous products, as they tried to transfer only the 44 line through this number of wires. And they interfere with each other. Because in the past the trials failed, C64 guys stick to the PCB based expander units (like the Aprotek or the CMD EX3 or EX2+1).
The concept is taht if you have a converter board between the Plus/4 explansiton port pins and a standard hig speed interface connector, than you can buy easily (and hopefully cheap) a professional cable that can do the job for you, and provide significant extansion distance.
Certainly the longer the cable is, the higher voltage is lost on the wires, but TTL can go much lower that the 5V, and still recogniseable (maybe 2.2 if remember well). A second 5V source could be easily given at the other side too, with an USB charger. (certainly from the same source/socket as the machine to prevent zero point mismatch)
Thinking a little, the only real problem against the solution is the too many data lines on our expansion port.
Plus/4 had 50 pins on the expansion port (and not 44 as the C64)
Out of that there are 4 GND lines, 2 +5V, and 3 N.C.
Still 41 data lines... This is a lot o wires to handle.
(C64 has 40 real data lines with two GNDs and two 5Vs).
Even if we manage the 5V on separate lines, and ground as a twisted wire/in between wire, the 41 data lines cannot fit to standard (cheap) cable solutions, like PATA-4 with 80 pin cables let only 40 data lines to be transferred. One is still missing... (BTW the IDE cables with 80 wires also double long than the 44 wired ones (45 cm VS 23cm lenght) )
Questions to the experts:
-Is there any data line NEVER used and most probably will never be used by any external HW cards?
Going through the descriptions I could not find any signal line that is not so important, "External Audio IN" could be used probably by SID card, ROM cards using ROM selector bits, and data/address lines are all important, etc
-Can you suggest any well known and available(=cheap) and good quality standard cable have enough lines to transfer safely 41 pins?
Even DVI-I has only 24 + 5 pins, HDMI has only 18. Two cables would be a little too much, and even from HDMI it would not be enough 
Interestingely the SCSI has a 50 pin version connector, but the price... just the cable and the two connectors would be (minimum) 50 USD.
(just for comparison: IDE cable with 80 wires is just 3USD and the connectors are less than 1.)
FFC cables are a little arkward to handle and too sensitive to handling at the edges, not to mention distance limiation too.
So round cable would be a little better.
Thank you in advance for any idea or suggestions, please highlight if I misinterpreted any documentation.
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