in reply to: HART signal acceptance posted by Fred Zhang on 03 November 2005 at 16:33:46.
HART protocol rules say that a receiver always detects a voltage signal. (That's why there always needs to be resistance somwhere in the loop). But a HART signal sender may generate the HART signal as either current (relying on the loop resistance to convert it to voltage) or directly as a voltage. A 4-20mA measurement device does the former, just adding the HART signal current to the "dc" measurement signal. A secondary master usually generates a voltage HART signal. A DCS system might do either: on the measurement side, a DCS will generate a voltage signal across the 250-ohm load and the high impedance of the 4-20mA transmitter; on the valve control output, a DCS will usually generate the HART as a current modulation added to the "dc" control current, which converts to voltage across the few hundred ohms resistance of the valve/actuator device. A valve/actuator device will generate the HART signal as a voltage (across its own internal resistance).
Even during initial configuration when a field measurement device is first connected, it will be drawing the basic 4mA current (or generating it, if it is an active output device). The DCS (master) can send a voltage signal, and the field device (slave) can reply by adding the HART signal on top of the 4mA (plus and minus 0.5mA).
Any field device which does not provide a 4-20mA signal at all, and only uses HART, would still be able to send a current signal, but it would have to be bipolar. It is more likely to generate a voltage directly, with capacitive coupling.
Does this make it clearer? !!