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
UPDATE 2024-10-20: According to this datasheet, IEEE 1284 termination
is specified to be mostly ESD protection circuits, except on the
STROBE control line and the eight DATA lines. Those have a series
termination resistor at 33 ohms. This is comparable to the series
termination resistors found inside the original Macintosh computer’s
motherboard DRAM lines.
20241020/DuckDuckGo parallel port termination
20241020/https://www.onsemi.com/pdf/datasheet/pacsz1284-d.pdf
https://media.digikey.com/pdf/Data%20Sheets/ST%20Microelectronics%20PDFS/ST1284-xxA8.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.
Read on →