How to Commission HART Multidrop Mode
HART multidrop lets you hang several transmitters on a single pair of wires and read them all digitally, which is invaluable on a remote skid where cable is scarce. But it trades away the 4-20 mA analog value, and it fails in confusing ways if two devices share a polling address or the current is not parked correctly. This page is the commissioning procedure in execution order: set the addresses, disable analog current, wire the loop, and prove every device answers before you leave.
Commission HART Multidrop in one line: To commission HART multidrop mode, give each transmitter a unique non-zero polling address, set each device's loop current to fixed (parked, so it no longer represents the process value), wire all devices in parallel across one pair with a single loop resistor and supply, then poll each address from the host to confirm all devices answer. In multidrop the 4-20 mA signal is abandoned; every reading is digital over HART.
What You Need
You need a HART master to configure each device - a handheld communicator or a laptop with a HART modem - the transmitter datasheets to confirm each supports multidrop, a single loop power supply, and one loop resistor sized for the HART physical layer. You also need a clear addressing plan: which device gets which polling address, written down, because the whole scheme depends on every address being unique.
Confirm before you start that the host system can actually read multidrop. Many analog input cards read only the 4-20 mA value from a single device and cannot poll multiple digital addresses; multidrop needs a HART-aware master or multiplexer. If the receiving side cannot do digital-only HART, multidrop is the wrong choice and you should reconsider. The trade-offs are laid out in what HART multidrop mode is.
Set a Unique Polling Address on Each Device
Configure each transmitter one at a time, on the bench or with only that device connected, so there is no address ambiguity while you work. In each device set the polling address to a unique non-zero value; in classic HART, address zero is the single-drop analog default and moving a device to any non-zero address is what enrolls it in the multidrop group. Two devices sharing an address is the number-one multidrop failure: the host sees garbled or colliding replies and cannot talk to either cleanly.
Record the tag, serial number, and assigned address for every device as you go. This map is what you use later to confirm the right transmitter answered at the right address, and it is what the next technician needs when a device is replaced. Configuring devices in isolation and building the address map deliberately prevents the most common commissioning rework, which is discovering an address clash only after everything is wired into the field junction box.
Park the Loop Current to a Fixed Value
In multidrop the analog current no longer carries the process value, and several devices cannot all swing the loop current independently anyway, so each device's current output must be fixed (parked) at a low constant level. Set this in each transmitter's configuration - the classic multidrop convention parks the current at a fixed minimum so the several devices together draw a small, predictable total. If you skip this and leave a device in normal analog mode, it will fight the loop and disrupt communication for everyone on the pair.
Confirm the total parked current of all devices, plus the loop resistor and supply voltage, still satisfies the HART physical layer and the supply's budget. This is a per-device datasheet number multiplied by device count, so it is site-specific; work it out for your actual devices rather than assuming a limit. Getting the parked current right is what turns a set of independent transmitters into a stable shared loop.
Wire the Loop and Poll Every Address
Wire all the devices in parallel across the single twisted pair, with one power supply and one loop resistor for the whole segment - not a resistor per device. Multidrop is a parallel connection: every transmitter sees the same two conductors and the same HART carrier. Keep polarity consistent and observe the same shielding and grounding discipline you would on any instrument loop so the HART signal is not swamped by noise.
From the host, poll each configured address in turn and confirm the expected device (matched by tag and serial from your address map) answers with a live process value. A successful poll of every address, with the right device on each, is the proof that commissioning worked. If an address does not answer or the wrong device replies, you have a duplicate address or a wiring fault to resolve before you consider the job done.
Verifying the Result
Walk the full address list and confirm each poll returns the correct tag, a sensible primary variable, and no communication errors over several read cycles. Cross-check each reading against a local indication or a known process condition so you are confident the digital value is trustworthy, not just present. Then confirm the host's scan of all devices completes within the time your monitoring needs, because polling many devices in series is slower than reading one analog point.
Finally, cycle power on the loop and re-poll to confirm every device rejoins at its assigned address without manual intervention. A device that comes back at a default address after a power cycle was never saved correctly and will drop out of the scan in service. Only when every address answers reliably across a power cycle is the multidrop segment commissioned.
Common Mistakes
The classic error is two devices left on the same polling address, which the host cannot reliably separate - always assign and record unique addresses. The second is leaving one device in normal analog mode so it drives the loop current and corrupts communication for the whole segment; every device must have its current parked. The third is expecting the 4-20 mA value to still work - in multidrop it does not, and any control loop that needs the analog signal cannot use a multidropped transmitter.
Also common is discovering too late that the receiving hardware cannot poll multiple digital addresses, so confirm host capability first. And do not exceed the segment's electrical budget by adding one device too many; the parked-current-times-device-count math is real and a shared supply has a finite limit. Sizing the segment before you wire it avoids the worst kind of rework.
Frequently Asked Questions
Do I lose the 4-20 mA reading when I switch a HART device to multidrop?
Yes. In multidrop the loop current is parked at a fixed low value and no longer represents the process variable, so the analog reading is gone and every value is read digitally over HART. This is the core trade-off: multidrop saves wire by sharing one pair among several transmitters, but any control loop or input card that depends on the live 4-20 mA signal cannot use a multidropped device. If you need the analog value, keep that device single-drop at address zero.
What happens if two multidrop devices have the same polling address?
The host cannot cleanly address either one. When it polls that address, both devices may respond and their replies collide, producing communication errors, garbled data, or a device that appears to drop in and out. This is the most common multidrop commissioning fault. Fix it by configuring each device in isolation, assigning a unique non-zero address to every transmitter, and keeping a written map of tag-to-address so a future replacement does not reintroduce a clash.
How many transmitters can I put on one HART multidrop loop?
The practical limit is set by the segment's electrical budget and the host's polling requirements, not a single universal number, so treat it as site-specific. Each parked device draws a small fixed current, and the sum of those currents plus the loop resistor and supply voltage must still satisfy the HART physical layer. Polling many devices in series also slows the update rate. Work the current budget from your actual device datasheets and confirm the resulting scan time meets your monitoring needs before you commit to a device count.
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