Advantages of Foundation fieldbus
SMART Instruments are analogue signal used to represent a physical quantity for example 0-10bar, 0-100degC & can be configured for control or feedback. The reason the range starts at 4mA is to make it easier to distinguish between a genuine fault condition like an open circuit or when there is actually just no process variable. Typically there will only be one device per loop.
Foundation field bus is a digital AC signal which operates under the master slave protocol. There can be multiple devices on a loop known as a network and all have to be Fieldbus comparable and the total network must be within a specified length for use without a repeater
Advantages of FF devices -
Reduced Hardware Requirements
• Smaller System Footprint
• Reduced Device Count
• An Open Network Standard
• Faster Commissioning
• High Signal Integrity
• Greater Signal Fidelity
• Tighter Control
• Real-time Signal Status & Data Quality
• More Powerful Devices
• Enhanced Diagnostics
There can be a huge savings in wiring and I/O costs for a new plant project due to the multidrop and digital capability inherent in FF. Also the inclusion of diagnostic and multivariable information makes commissioning and troubleshooting tasks much easier.
4-20 mA/HART can deliver non-real-time digital information, FOUNDATION fieldbus delivers real-time digital information. So for FF the real-time PV is also digital. Additionally, for the PV, Fieldbus also provides real-time status telling the operators and control strategy what the status of the PV is: Good, Bad (e.g. sensor failure), or Uncertain (e.g. outside calibrated range). This allows operators and the control strategy to clearly tell the difference between an extreme process condition and a sensor failure (because a sensor failure need not be flagged in the PV value by masquerading high > 20 mA or low < 4 mA PV) thus tripping the loop even though there is nothing wrong with the process. Fieldbus also means you can get multiple variables from devices in real-time. Many devices today provide multiple variables, but to put it to good use they have to be real-time; and only FF does that.
Another important point is that 4-20 mA signals go hand-in-hand with on-off signals for discrete devices, and there is no HART for on-off signals. Therefore, there is no HART in on-off valves and other discrete devices which account for maybe half of the I/O count. A major reason for using FF is that FF makes all devices digital including intelligent two-wire on-off valves. That is, multiple real-time I/O, central configuration, and diagnostics etc. for all devices in the plant, not just some. FF also eliminates the need for proprietary protocols for electric actuators / motor operated valves (MOV) and tank gauging systems.
Apart from reducing wiring because multiple devices can connect on the same fieldbus like you mentioned, further wiring savings are also achieved because there are multiple real-time I/O SIGNALS per device over the same two wires. On average devices have about 3 I/O signals each, such as command and feedback for a valve. An electric actuator / motor operated valve (MOV) has many more. In the past, often only 1 I/O per device was connected due to the high cost of 4-20 mA and on-off wiring, I/O cards, and marshalling etc., but with fieldbus all the signals in a device can be used. That is, dramatic wiring reduction and access to full device functionality. With fieldbus you get real-time position feedback on every valve because no additional wires are required. You get the additional real-time measurements in flow and level transmitters because no additional I/O is required.