[GreenKeys] TTY-Connect pictures and 160V vs 80V info

[gil smith]

Hi Bob:

Comments below:

At 08:15 AM 7/2/2004, you wrote:
>At 160 volts and 60 ma each loop is going to generate ten watts of
>power. You really have four high voltage loops in the box when both
>sides are set to full duplex. Since two loops are 30 ma and the other
>two are 60 ma that's 15 watts per.  With a 70% efficient power supply
>(pretty good for a little transformer) and the rest of the stuff in the
>box you are close to 50 watts of heat. Depending on the box size and
>what is stacked on top or bottom of it this may be a problem. Sticking
>a nice noisy fan on the back of thing would help a lot.

Yes, a fan (and bigger heatsinks) would be a good idea with 160V 
loops.  With 80V loops it should not be necessary.  Good point about what 
is above/below -- You would want to mount tty-connect at the top of the 
equipment so it does not heat anything else up, and you would want to leave 
a rack space or two above/below it for convection air.


>Another, far less noisy solution would be to mount the loop resistors
>in the cabinet of the TTY. If you go with big old ceramic wire wounds
>then mounting becomes fairly easy since you don't require a heat sink.
>The cabinets are all so darn big that a few more watts won't have much
>of any effect on them.

Could do that I suppose, but if you remove current limiting from 
TTY-Connect, and plug in a tty that does not have resistors, I think the 
selmags will blow.


>One other caution - I have seen more than a few machines that have a
>ground path on the keyboard side. It's not common to see it on the
>magnet side but it can happen. It's probably a real good idea to have
>people ohm out their machines before hooking the board up to them. A
>ground short on either side will smoke the board (also a heat issue
>....) or at least blow a fuse.

All loop circuitry is floating from chassis/earth ground, and is 
opto-isolated from the 232/TU section.  If you have just a single 
half-duplex loop to a tty, and one side of the loop (either side actually) 
is grounded to earth, it will NOT affect operation, will not blow a fuse, 
nor will it damage the board.

But, if you are running two loops in full-dup mode, I suppose certain 
ground combinations could end up causing problems (hafta think about the 
scenarios).  However, I DID presume that the ttys are floating -- is this 
not the case for UNMODIFIED tty gear?


>Given the number of hands these machines have passed through you can
>never be sure of what's in there. Most older setups had the loop
>resistor in series with the transformer. In order to keep the magnets
>in mark you just grounded the "bottom" end of them. A lot of setups
>worked sort of the same way with one side of the keyboard contacts tied
>to ground. The net result would be a machine with one or more magic
>switches on it that kill the board when you close them or even when you
>just plug in the machine.

Yeah, checking machine wiring to know what you have is a good 
idea.  Bringing out a pair of wires for the selmags, and a pair for the 
keyboard contacts is the simplest.

thanks,

gil


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