in reply to: Multidrop mode posted by Deesthik on 23 October 2001 at 18:45:32.
Slightly simplified, this is how it works:
If you choose a polling address other than 0, the output current is parked at a low value (usually, as you say, 4mA), and you can connect many devices in parallel as multidrop , each with a different polling address.
If you set polling address to 0, the device will output the normal 4 to 20 mA signal. BUT, for testing, you can set any fixed current in the 4 to 20 mA range (HART command 40). To go back to normal operation, you send command 40 again, with 0 mA as the commanded current. This doesn't really set the output to 0 mA - it just goes back to the normal live 4 to 20 mA mode.
I hope this is clear! (If not, ask again!)