in reply to: reg Broadcast Request posted by Mani on 25 January 2008 at 04:57:35.
You are right, in HART only one device is allowed to talk at a time, otherwise collision would destroy the message. But in fact, only one device will respond to a broadcast message. The reason for this is that the broadcast address (38 zeros) may only be used with commands which provide some other means of telling the slaves which one should respond. In the HART rev.6 standard commands, that means commands 11 and 21, where the Tag or Long Tag is included in the data field, and command 73 (Find Device) where one field device has been armed by (for example) pushing a button.
To discover the Unique IDs of all connected slave devices in multidrop, the master must try SHORT FORM command 0 with each possible polling address in turn (0 to 15, or 0 to 63 with rev.6 devices).