HART Multidrop vs WirelessHART: Which to Use
When you need to add several measurement points and do not want a home-run cable for each, HART offers two very different answers: put many transmitters on one wire with multidrop, or skip the wire entirely with WirelessHART. They solve the same problem in opposite ways, and the right choice depends on what wiring you already have and how fast the points update. This page compares them criterion by criterion for the engineer deciding how to add points economically.
Multidrop vs WirelessHART in one line: Choose HART multidrop when a suitable twisted pair already reaches the area and the measurements are slow and non-critical, since it puts many digital-only devices on one existing wire cheaply. Choose WirelessHART when pulling any cable is expensive or impractical, since its battery-powered mesh adds points with no wiring at all. The decision turns mostly on whether wire already exists and how fresh each reading must be.
The Decision in One Paragraph
Both multidrop and WirelessHART exist to add many measurement points without a dedicated loop per point, but they attack the cost differently. Multidrop reuses one existing pair of wires for many digital-only transmitters; WirelessHART removes wire from the equation with a battery-powered radio mesh. So the decision is largely about your starting conditions: if a usable twisted pair already runs to where the points are, multidrop leverages it, and if getting any wire there is the expensive part, WirelessHART sidesteps the whole problem. Both give up the fast, continuous analog value, so neither suits control loops.
Criteria Side by Side
The two approaches diverge on wiring, power, and scalability, and a direct comparison makes the trade clear.
| Criterion | HART Multidrop | WirelessHART |
|---|---|---|
| Wiring | One shared twisted pair for all devices | No signal wiring; radio mesh to a gateway |
| Power | Loop-powered over the shared pair | Battery-powered field devices |
| Analog output | Given up; current parked at a fixed value | None; data is digital over the radio |
| Update rate | Slow; devices polled one at a time on 1200 bps | Set by the reporting rate and superframe |
| Adding a point | Wire it to the existing pair, assign an address | Mount it and let it join the mesh |
| Best when | A suitable pair already reaches the area | Pulling any cable is costly or impractical |
Read down the table and the pattern is that multidrop is a wiring-reuse play and WirelessHART is a wiring-avoidance play. Multidrop keeps you in the familiar loop-powered, wired world but forces all its devices to share one slow pair and one power budget. WirelessHART frees you from wire entirely at the cost of managing batteries and a radio mesh. The multidrop mode page and the WirelessHART overview each cover their side in more depth.
When Each Wins
Multidrop wins when the infrastructure is already there and the measurements are undemanding. If a twisted pair already runs to a skid or an area and you need to add a handful of slow, non-critical readings - a few temperatures or static pressures that update every so often is fine - multidrop lets you hang them on the existing pair without new cable or new power runs. It is the economical choice precisely when it can reuse what exists for measurements that do not need to be fast.
WirelessHART wins when wire is the enemy. Retrofitting monitoring onto an existing plant, reaching a remote or hard-to-cable location, adding points across a road or a hazardous area where cable installation is expensive - these are where not pulling wire is worth managing batteries for. The self-healing mesh also scales more gracefully as you add points than a single shared pair does, so a growing set of retrofit points usually favors wireless. The mesh-topology page covers why density helps a wireless deployment.
For either choice, the data lands the same way for monitoring: through a gateway or interface into a system that trends and alarms it. A platform such as Merobix reads multidrop devices through a HART interface and WirelessHART devices through the gateway uplink, so the choice between the two is a field-infrastructure decision, not a monitoring one. Both routes end with the points visible as tags an operator can watch, which means you can pick the field approach on its own merits.
Frequently Asked Questions
Is WirelessHART always better than HART multidrop?
No. WirelessHART wins when pulling cable is expensive or impractical, but if a suitable twisted pair already reaches the area and the measurements are slow and non-critical, multidrop reuses that existing wire more cheaply and avoids managing batteries and a radio mesh. The right choice depends on your starting wiring and how fresh each reading must be, not on one being universally superior.
Can I use multidrop or WirelessHART for a control loop?
Neither is suited to a fast control loop. Both give up the continuous analog value - multidrop parks its current and reports digitally on a slow shared pair, and WirelessHART reports at a scheduled rate over the radio. They are meant for monitoring slow or non-critical measurements, so a control loop that needs a fast, continuous value should keep its own point-to-point analog loop.
Which scales better for adding many points?
WirelessHART generally scales more gracefully for a growing retrofit, because each new device joins the self-healing mesh without new wiring, whereas multidrop crowds more devices onto one slow shared pair and one power budget. Multidrop is economical when reusing an existing pair for a few slow points; a large or expanding set of new points usually favors the wireless mesh.
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