FSK

[Electromatic]

I recently happened upon a Frequency Shift Key device buzzing like a hive. I traced the few wires between it and a motor control cabinet, and it was generally already disconnected. I think I get the gist of the device but am unsure of whether it's the phone line that activates the relay or vice versa.
Out of curiosity, does anyone have experience with this?

 

[Jraef]

The phone line just carried the signal, you had FSK devices on either end. That was the height of modernity for SCADA systems back in the late 70s, early 80s. The last one I saw still in use was about 10 years ago at a pump station that was built for the military and taken over by a municipality when the base closed. They also had 500HP DC pump drives still in use!

 

[tom baker]

I had about 40 sites with FSK scada controls. There were different frequencies on audio tone, maybe 30. Each frequency could do 2 controls shift up, shift down and carrier. Way to work on but limited speeds and lots of control points required some complicated scanners. Eventually we replaced FSK audio leased lines with two generations of licensed radios, cellular radios, and now dark fiber
I was really good on maintaining and TS FSK scada, phone co new hires struggled with it ask they expected DSL.
But all gone now.

 

[tom baker]

I recently happened upon a Frequency Shift Key device buzzing like a hive. I traced the few wires between it and a motor control cabinet, and it was generally already disconnected. I think I get the gist of the device but am unsure of whether it's the phone line that activates the relay or vice versa.
Out of curiosity, does anyone have experience with this?

Yes. The fm transmitter has a carrier frequency that is always sent, say 2000 hz, if the receiver does not see this frequency, that can open or close a relay
The carrier frequency can shift up, mark, or down, space for different functions, mark and space are telegraph terms
To TS, start at the transmitter. What is the carrier frequency? What inputs cause mark and space?
You need a meter than can measure frequency, see what the TX is sending
Go to the other end and test.
Can be phone line, tx or rx. I used to put tc on phone line at 0db and check other end should be about -16 db loss
I had 25 years working on FSK scada

 

[Electromatic]

I figured that the phone line frequency shifts got translated into opening or closing relays. Do they work the other way, i.e. a change of contact state causes a phone/frequency signal to be sent out?
Thanks for the insight.

 

[tom baker]

I figured that the phone line frequency shifts got translated into opening or closing relays. Do they work the other way, i.e. a change of contact state causes a phone/frequency signal to be sent out?
Thanks for the insight.

Yes but it requires a transmitter on a different frequency monitoring the contacts