Re: Current loops...

Mark Fraser wrote:

> 20mA and 60mA were both common in TTY loops, with TELEX using
> both plus reversal of polarity (to indicate "off hook" sorta)
> in the "loop" protocol.
>
> Normally, +130v to ground was used for TTY, but for extra range,
> +/- 130vdc could be used. (to overcome long loop resistance and
> still have enough current flow). 20mA was not as immune to
> noise as 60mA....
>
> +130vDC was often wired to places it wasn't desired, like to
> some conductors in a brand new VOICE cable in a central office,
> where an installer was terminating said cable on a dist. frame.,
> pliers on the wire, one hand on the ironwork. Nasty shock.
> But we won't go there..... / mark

Now, here we're talking Western Union real, mechanical, Baudot
(5-bit code) teletypes, like you always used to hear going chug-chug
chug-CHUG-chug-CHUG-chug-chug-chug just before the evening news.
(The big CHUG was when there was a case change, and on model
15, 17 and 19 Teletypes the whole typewriter basket had to shift
up or down about 1/3" in 1/10th of a second. You didn't want to get
your finger caught in that mechanism!) Since there were only 32 codes
possible, the letters case was all full of letters (caps only) and all the
numbers and puctuation was in the other case. So, it had to shift that
basket up and down a LOT.

I even had a model 15 once. Whew, what a mechanical nightmare!
I'm glad those days are over, and hope to never see one of them again,
outside of a museum.

So, the 130 V dc supply and 60 mA current loop were not used on
computers. 15 V and 20 mA were plenty for almost all applications.
And, due to all the crosstalk those DC current pulses caused, any
transmission over long wires was shifted over to modems when that
became possible.

Jon