3 ms·

Why would anyone bother about controlled impedance or matched trace length on an ISA card of all things? At the speeds ISA runs at you could connect everything

by LIV2 6y ago

Why would anyone bother about controlled impedance or matched trace length on an ISA card of all things? At the speeds ISA runs at you could connect everything with wire coat hangers

metaphor 6y ago
You've summarily conflated a signal's fundamental frequency with the significant harmonic contributions of its edges. Take a moment to review ISA v2.01 spec[1] § 10.4: > To minimize crosstalk between signals and bus reflections, the driver rise/fall rates must be greater than 3ns; only LS or ALS devices are allowed. Faster parts such as F and AS cannot be used to drive the bus. If you thought it odd that normative language like this explicitly calls out tech by name, then you might have some sense of just how concerned Intel was about interoperability at the time...no thanks to wire coat hanger designers. Also consider the language of PC/104 v2.6 spec[2] § 4.7, which is probably one of the last remaining modern platforms with native ISA bus integration: > Whether [ac] termination is needed and where it should be located, is dependent on the specific system configuration and must be determined by the system designer. Now consider for a moment why this language and the paragraphs that precede it exist in this independent standard. This design is a commidity peripheral that will be integrated into arbitrary systems. Length matching literally costs nothing...but even this is apparently a "stretch goal" given the obvious fact that this design completely dismissed the normative max 2.5" stub length requirement. So no, I disagree. For the sake of sensible SI that's too often conveniently dismissed by hobbyists, keep your wire coat hangers in the closet. [1] https://archive.org/details/bitsavers_intelbusSpep89_3342148/page/n65/mode/2up https://archive.org/details/bitsavers_intelbusSpep89_3342148... [2] https://resources.winsystems.com/specs/PC104Spec.pdf https://resources.winsystems.com/specs/PC104Spec.pdf
- LIV2 6y ago
  > Length matching literally costs nothing Well arguably it does cost some time yet is utterly pointless on an ISA card (unless your traces are 40 meters long I guess) there's plenty of pedantic things you could waste time on designing an ISA card

by LIV2 6y ago

Why would anyone bother about controlled impedance or matched trace length on an ISA card of all things? At the speeds ISA runs at you could connect everything with wire coat hangers

metaphor 6y ago

You've summarily conflated a signal's fundamental frequency with the significant harmonic contributions of its edges. Take a moment to review ISA v2.01 spec[1] § 10.4: > To minimize crosstalk between signals and bus reflections, the driver rise/fall rates must be greater than 3ns; only LS or ALS devices are allowed. Faster parts such as F and AS cannot be used to drive the bus. If you thought it odd that normative language like this explicitly calls out tech by name, then you might have some sense of just how concerned Intel was about interoperability at the time...no thanks to wire coat hanger designers. Also consider the language of PC/104 v2.6 spec[2] § 4.7, which is probably one of the last remaining modern platforms with native ISA bus integration: > Whether [ac] termination is needed and where it should be located, is dependent on the specific system configuration and must be determined by the system designer. Now consider for a moment why this language and the paragraphs that precede it exist in this independent standard. This design is a commidity peripheral that will be integrated into arbitrary systems. Length matching literally costs nothing...but even this is apparently a "stretch goal" given the obvious fact that this design completely dismissed the normative max 2.5" stub length requirement. So no, I disagree. For the sake of sensible SI that's too often conveniently dismissed by hobbyists, keep your wire coat hangers in the closet. [1] https://archive.org/details/bitsavers_intelbusSpep89_3342148/page/n65/mode/2up https://archive.org/details/bitsavers_intelbusSpep89_3342148... [2] https://resources.winsystems.com/specs/PC104Spec.pdf https://resources.winsystems.com/specs/PC104Spec.pdf

LIV2 6y ago
> Length matching literally costs nothing Well arguably it does cost some time yet is utterly pointless on an ISA card (unless your traces are 40 meters long I guess) there's plenty of pedantic things you could waste time on designing an ISA card