be80be:
retrolefty the diagram I posted on the first page is basically the same thing you posted just it's not a one chip package. It's from TI appnote.The this is not that hard of a deal 4 mA is the low 20 is the high you can send t data stream out like that that's easy to read. This was used back before rs232 to send data the first IBM pc had 4 to 20 on the card that had the com port on it and people rented phone lines to send data over the copper wire this way.
MIDI is a digital forum of this
I think you are terrible confused or mistaken about the industrial standard 4-20ma current loop. It's not a digital transport link using just 4ma or just 20 ma as it's two signalling levels. Rather it's a true continuous analog measurement link where a 4ma state means the measurement is at 0% value of the measurement range, 12ma is 50% value and 20ma is the 100% range value. The actual current flowing in the loop can be at any value between 4 to 20ma at any given time depending on what the sensor measurement is.
You are confusing that with the old teletype serial communications standard that used a 0 and 20ma DC current loop to send digital serial data. That is a totally different animal that has no connection to the analog 4-20ma current loop standard used in the process control industry.
Lefty