ribbon cable TDR test

[Tom Gardner]

On 27/10/19 10:11, Michael Kellett wrote:

Michael Kellett just said that he had a 37-way ribbon cable. That should be
enough ways to let him devote 15 ways to burying each of the eight data lines
between grounded wires.

He'd be mad not to.

Well yes. I presume there aren't sufficient
spare connections, or that the pinout is
pre-ordained.

If the latter, I'd try to find a way of
mutating the pinout.


Thanks for further suggestions.
Seems I didn't make the initial situation quite clear, the source box is a
standard thing already bought by my customer. The 37 way D connector has all 37
pins committed. There are 3 gnd pins, all at one end of the D connector.
The pins are obviously driven by octal bus drivers (spec says max 35mA per pin
and max 70mA per group of 8.)
So I have to take the signals from where they tell me (opposite end of D conn
from the gnd pins !).

I thought something like that was the case, but couldn't be
bothered to go back and check.

I've seen that "conveniently clustered at one end" before
in amateur constructions. A close relative is to cluster all
the decoupling capacitors where the PSU enters the board;
might be worth checking that!

Point out to the client that a single output driving
100ohms through 5V requires a current of up to 50mA until
the reflections have died out. With source termination
that is reduced to 25mA. That's within the spec for
1 pin, but not if all eight outputs change simultaneously.
However, if the source's transition time is slow compared
to the transmission line's length, that will be less of
a concern.

If the obvious simple cable connection doesn't work
/reliably/, then the possibilities are limited and
"sub-optimum"[1].

Educate the client in the costs of noticing/determining/fixing
unreliable behaviour in the field. That may involve formal
documentation to the effect that you cannot be held responsible.
Sometimes you have to decline to do work if it will be too
much trouble in the future.

Consider having a "very fat" connectors which do a least
inappropriate combination of:
- rearrange the grounds more appropriately.
- if there is an obvious clock, preferentially
improve that line's impedance/crosstalk by
surrounding it with gnds.
- insert source termination resistors in the
box
- for non-clock signals, add capacitors after
those resistors to slug any rapid transients
- have receiver termination resistors to prevent
over-voltage slowly knackering the receiver's
input protection diodes (check the rx spec)

If the client doesn't believe you and has a short
attention span, point him to the relevant "Bogotin's
rules of thumb".

[1] code word for lousy