11 Feb
2006
11 Feb
'06
8:06 p.m.
On Saturday 11 February 2006 07:05 pm, Chuck Guzis wrote:
On 2/11/2006 at 4:46 PM Richard wrote:
The first computer that I actually got hands-on time with (I actually built
the thing) was a Heath H11 system, back around the beginning of 1978 or so.
The "terminal" that we started out with at the time was an LA36, and the
interface used was current-loop. I don't know why that was chosen, at the
time...
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
Speaking of current loop interfaces, just exactly
how is data
transmitted? Is it a current pulse train?
It's exactly what it sounds like. Instead of a voltage driven signalling
as RS-232C, the current loop interface is driven with a current source
(20ma or the older 60ma) for space and the signal is interrupted (no
current) for mark. The big advantage is that so long as the specified
current is maintained, the distance between the sender and receiver can be
substantial. In the old days, the selector magnet on a teletype would be
part of that circuit. IIRC, this is also where the name for the "break"
key came from--it interrupts the circuit.
Cheers,
Chuck