[GreenKeys] Help for ASR33 / Data Dynamics connection...

[Enrico Pozzobon]

Hi everyone,

I bought a Teletype ASR33 for use that in many way via rs232 standard
connection.

My machine is a strange exemplar, because aestetically she's a Teletype
ASR33 but the internal CCU is Data Dynamics 390 electronics configured as
HP2749A for mainframe connection. (previously used with HP-2100 series
machine) AAARGGH!!!

This configuration provide is 20ma current loop with 2 cable (I/O data) and
common ground; then +/-12V supplied by computer (I think necessary also for
supply part of internals boards).

I bought a C.Loop to RS232 converter, I try to connected a PC and the
"receive" loop in active mode and all work fine, my TTY print all characters
sent by PC. 

But "send" loop is a problem... I try to see the signal between pin4 and
ground (loop out) with an oscilloscope and without activate the external
+/-12V supply. In this case I view a signal like the ASCII selected but my
converter not translate any to rs232 side. (scope at 500mv/division)

If I turn on the external PS, I have +12V signal on output line but no
modulation-like sign... 

I try also to use the "active mode" of converter for TX loop without +/-12V
power supply but not work.

Hummm... I know, ASR33 with UCC-6 configured for HP mainframe provide to
bias the teletype's transmit signal to +12V... I've found at deramp.com site
an info for remove this +12V bias from TX line, but is for "normal" ASR33,
not for Data Dynamics cabling!!!! (Is totally different!)

Does anyone have any idea for activate also TX loop on this machine, and/or
how to remove bias voltage from this line leaving integer the current loop
signal?

Sigh! Help... Thank you all for your attention. 

Enrico.    

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