[GreenKeys] 232 COM port to TTY - Mighty-Mite to the rescue!

[Duncan M. Brown]

You can drive a AN/TGC-14 (TT-297, -298, -299) and probably the UGC-38,
-40, -41 series "Mighty-Mites" all most directly from a computer's COM
port; only one component is required!

One of the many unique features of the MITE TTYs is a built-in, solid
state, selector magnet driver.  The driver samples the loop current and
then drives the selector magnets. This allows it to be relatively
insensitive to loop current, and it is spec'ed for a loop current of 20-80
ma in the normal "High Current Range".  There is also a "Low Current Range"
that can be selected with a jumper inside the Line Sensor Module.  The Low
Range is spec'ed at 1-5 ma for the TT-297 and 2.5-10 ma for the TT-298,
-299.  (Can someone with a UGC-40 manual fill us in on its Low Current
spec?)

This in the range of the standard EIA-232 (aka (RS-232) computer COM port
output, so you should be able to drive a MITE TTY directly from a COM port!
I checked it out on my TT-299B with a power supply and only about 3V was
needed to lock it up.  So I wired up a DB-9 connector (pin 3= output, pin
5= gnd) and connected it to the TT-299.  The TT-299 ran closed, but when I
sent characters from the computer, nothing happened at the TTY, even though
I could see the +5, -5 V signal going into it on a scope.  

The MITE selector magnet driver, being electronic, is polarity sensitive. 
So that polarity would not be a problem in the field, the Line Sensor
includes a bridge rectifier. No matter what the polarity is, a "Mark" will
always pass through the bridge to enable the selector magnets.  However,
the computer COM output is bi-polar, in essence an AC signal, and the
bridge rectifier was doing its job and turning my "Spaces" into "Marks"
also!!

The fix for this is just a diode in series with the computer output to make
it uni-polar.  Any diode will work (voltages and currents are very low). 
If your computer only puts out +5, -5 V, then a germanium diode (1N277,
etc) will be better due to the lower drop. ( But I tried a 1N4004 silicon
power diode and it worked OK on mine.)  If your computer puts out 10V or
more, a silicon diode will be fine.  The standard is for the COM port
output (pin 3 on a DB-9) to be low during idle, so this is your marking
condition.  Connect the diode cathode to this pin.

Thats all there is to it!  Maybe not quite as impressive as a M15 banging
out the news, but great for demos, etc, when you don't want to lug the M15
around.

Have fun,

Duncan Brown, K2OEQ
USASA  31J

Chief TTY operator & repairman
AWA Electronic Communication Museum
http://www.antiquewireless.org/