Why is 4-20 mA Current Loop Called "TTY Interface"?
Also can anyone give me some more examples of such live "zero" standards, except, 3-15 psi?

Thanks in advance.

Best Regards,
Khalil Sultan.

 

I think you are mistaken. TTY loops are either 20 or 60 ma, digital, mark-space. They are a refinement of the original Morse Telegraph.

4-20 loops are analog instrumentation loops.

As for the 4 ma offset an example portrays the reason very well. In the analog world there is always noise and therefore a signal to noise ratio. If we assume a noise level of 400 ua we would have a 10 to 1 or 20 db S/N at 4 ma. If we used say 0 to 10 ma and the same noise level the S/N would be zero at 400 ua, minus 6 db at 200 ua and approaching infinity at zero signal level. There are other reasons too but if you don't see a problem with an infinite S/N then you are probably in the wrong business.

Fred Townsend

 

> Why is 4-20 mA Current Loop Called "TTY Interface"?

Its not. 4-20 mA is a standard for transmission of analog process information such as from a pressure sensor to a PLC.

TTY interface is a serial data interface (a very old one) like RS232 which uses a current loop (which I think might actually be 20mA) for noise immunity.

I have no idea about the second part of your question.

 

Because TeleTYpes once used a current loop for serial transmissions over long distances at low speed (not a problem for a tty). I don't think that generalization is very accurate unless the data is digital.

Regards

cww

 

The 4-20 mA TTY signal is not the same as a 4-20 mA analogue instrumentation signal. The TTY signal is a digital off/on (4 or 20 mA) serial communications signal. This is like RS-232, except it uses changes in current instead of
changes in voltage to indicate logic levels.

The "TTY" stands for "teletype" because it was used for teletype systems. A TTY signal can be driven longer distances than an RS-232 signal, and is less prone to electrical interference.

TTY is rarely seen anymore. The Siemens S5 PLC series uses a TTY signal for the programming port, but you are unlikely to see it anywhere else.

************************
Michael Griffin
London, Ont. Canada
************************

 

You are confusing signal types. There is a process 4-20 ma(0-100%) signal and there is digital signal known as "20ma TTY current loop" used to send serial data. This type of digital signal is a bit dated but still used. This web site might be of interest on the subject:

http://www.repairfaq.org/filipg/LINK/PORTS/F_The_Serial_Port1.html

The 4-20 ma loop is pretty much the process signal standard but there are still some 10-50ma and 1-5ma loops around and for real old pneumatic systems you might find 6-30psi.

Bill Mostia
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Partner
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[email protected](b) [email protected](h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

 

Dear all,
If I understood you correctly, then you have mixed up a digital communications interface (namely current loop RS 232) with a means of
transmitting analog signals over an electrical interface(4-20 mA loop).
When RS 232 needs to go beyond design (nominally 15m) a current loop is used which extends it to 300m or more. The name tty comes from teletype terminal which was the defacto standard for server-client based computing in
the seventies and early eighties. The 4 and 20 mA signals represent mark and space when data is transmitted digitally.
Other live zero signals 'were' 10-50mA, 15-60 psi etc.

thanks

Tomy

 

> 200 ua and approaching infinity at zero signal level. There are other reasons too but if you
don't see a problem with an infinite S/N then you are probably in the wrong business.<

Hi All

Thanks Fred for an enlightening response, however I believe you have missed the point.

0-20ma loops are used quite frequently in industry however 4 - 20 ma loops are far more common as when the wire breaks the 4 ma disappears
and this can be measured and reacted to appropriately.

Bidirectional setpoints to devices such as variable speed drives are most frequently +/- 20ma or +/-10v, using a wire break signal for such
an application, (with an offset of 4 ma or 2 v) is also done, but quite uncommon. Using a direction changing input and the magnitude of such a speed in a 4 - 20ma signal would probably ,make the nervousness (close to 0 speed) of some speed controller circuits a lot easier to deal with, but I have never seen it done, probably due to the complexity of changing direction from + to -4 ma.

Sorry but I don’t think your signal to noise ratio theory on a current signal holds a great deal of water.

Regards
Donald P

 

>TTY interface is a serial data interface (a very old one) like RS232
which uses a current loop (which I think might actually be 20mA) for
noise immunity.
>
>I have no idea about the second part of your question.<

Hi All

I have no idea if or why an analog signal of 20ma or otherwise would be referred to as TTY, my TTY signal is as described ANONYMOUSLY below, for
data transmission and entails the reversal of the current direction for signal changes, and was used in the teletype industry, or telex which I
think is the same thing, for transmitting typed text of various descriptions. The desirability of 20ma signals is due to its superior noise immunity when compared to voltage level based signals. TTY was very slow and in the early days could not go faster then 4800 baud, I did see it running at 9600 baud during my career but is now a little used standard in process control for a multitude of reasons not the least of which is the advancements of the electronics industry and I
suppose better means of achieving common mode rejection of induced voltage signals.

Regards
Donald P

 

Probably called "TTY" as there once was an electrical communications interface based upon the 4-20 ma current. I recall an entire series of printers with current loop interfaces to their host computers. DEC was big in this.