If you want high-speed communications on a parallel port, you need proper termination. But what is the circuit anyways? Searching around, I found a device with a datasheet, or at least I thought I did…
So, the essentials of proper termination? Provide pull-up resistors for proper open collector signaling. Besides that, you can also provide EF protection diodes and an RF filter (resistor and capacitor to filter out radio frequencies). That’s all the info I could find, nothing about using resistors to terminate the lines at their characteristic impedance.
20201022/DuckDuckGo parallel port bus termination resistors
20201022/https://resources.pcb.cadence.com/blog/termination-resistors-in-pcb-design
20201022/DuckDuckGo ieee 1284 termination impedance
20201022/https://www.vishay.com/docs/60089/vssx1284.pdf
Otherwise, there’s always just the random guess, a resistor around 100 ohms. 120 ohms, 90 ohms, 70 ohms, oh something like that.
OKAY, fine, the final verdict answer. Because IEEE 1284 cables are single-ended with ground wires in between, rather than differential cables, the characteristic impedance is 90 ohms, not 120 ohms or greater as is the case with differential cables. Yeah, it’s not really good data and insight, but that’s the best I can fathom. In addition, the fact that many parallel port devices are not connected with precisely impedance-matched cables.
20201022/DuckDuckGo characteristic impedance db25 cable
20201022/http://laserfx.com/Backstage.LaserFX.com/Standards/ISP-DB25.html
20201022/DuckDuckGo printer cable impedance
20201022/DuckDuckGo what is the impedance of db25 printer cable 1284
20201022/DuckDuckGo what is the impedance of ribbon cable
20201022/http://www.signalintegrity.com/Pubs/news/3_10.htm
20201022/https://electronics.stackexchange.com/questions/487186/what-is-1-27mm-ribbon-cable-impedance-for-the-ground-signal-ground-signal
Oh, wait, I got it. Revisiting one of my links and search results. For twisted pair DB25 cables, 62 ohms, that is the name of the game for single-ended impedance. Differential impedance, that is 100 ohms. But, yes, by all means, target your resistors to lower values, not higher ones.
20201022/DuckDuckGo ieee 1284 ohms
20201022/https://www.hca.hitachi-cable.com/products/hca/products/round/ieee1284/pdfs/bulk-IEEE-1284-cables.pdf
Parallel SCSI Wikipedia article to the rescue!
“Active termination provides a better impedance match than passive termination because most flat ribbon cables have a characteristic impedance of approximately 110 Ω.”
20201022/https://en.wikipedia.org/wiki/Parallel_SCSI#Termination
To remind myself, for single-ended signaling, you terminate both ends by running a resistor between the signal and ground. Inputt and output on both ends, however, should connect directly to the signal wire, not behind the termination resistors.
Yes, 110 ohms is the single-ended impedance, not the differential impedance, no confusion here.
20201022/DuckDuckGo characteristic impedance flat ribbon cable
20201022/https://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2001/2001symposium/flatcablezo.pdf
Also, finally, yes, anecdotal evidence. My printer says that only shielded, twisted pair cable should be used, but it is connected with a flat ribbon cable and it seems to work just fine. Apparently the difference in impedance-matching by 50 ohms and 100 ohms isn’t too bad, but 50 ohms and 1 ohm? That’s terrible.