How to connect a KFD2-HMM-16 HART multiplexer to a PLC or SCADA over Modbus

Many plants already have a HART multiplexer in the marshalling cabinet. A Pepperl+Fuchs KFD2-HMM-16 system sits across dozens of 4–20 mA loops and can talk HART to every transmitter on them. It was usually installed for one job: letting asset-management software such as PACTware, FieldCare or AMS configure and diagnose the instruments.

What the multiplexer doesn't do on its own is give that data to the control system. The PLC or SCADA still sees only the 4–20 mA signal, while the secondary variables, units and device status sit unused one cable away. This guide explains how to put a HART multiplexer master with Modbus in front of a KFD2 system so the PLC can read it, step by step.

How a KFD2-HMM-16 system is built

• KFD2-HMM-16 modules clip onto a DIN rail. The first is the master; further modules are joined to it by a 14-pin power-rail ribbon.
• Termination boards connect to each module by a 26-pin ribbon. The field loops pass through them, and each board serves up to 16 transmitters.
• An RS-485 link from the master module goes to whatever acts as the HART master. Traditionally that's an asset-management PC with a serial converter.

The multiplexer scans its loops in the background and keeps a cache of each transmitter's latest values. A HART master asks the multiplexer for that data over RS-485, and can also pass HART commands through it to an individual transmitter.

Why a PLC can't read the multiplexer directly

The KFD2's RS-485 port speaks HART framed for a multiplexer, not Modbus. Almost no PLC, SCADA package or DCS can be its master without extra software, and the asset-management PC that usually is the master isn't designed to pass live values on to a control system.

The practical fix is a gateway that becomes the multiplexer's HART master on one side and a Modbus server on the other. The PLC then reads the transmitters' data as ordinary Modbus registers, without anyone touching the field wiring.

What you gain

• Up to four process variables per transmitter (PV, SV, TV, QV) instead of one analogue value, for example flow, total, temperature and density from a single meter.
• Units and loop current alongside each value, so the control system knows what it's reading and can spot a drifting analogue input card.
• Device status, so a transmitter that reports a malfunction or loses communication is flagged in the PLC instead of silently holding its last reading.
• No rewiring. The 4–20 mA loops carry on exactly as before. The gateway only adds a digital path in parallel.

Step by step: KFD2-HMM-16 to Modbus TCP

The steps use the HG1v4 Intelligent HART Gateway, which supports the KFD2-HMM-16 in its Multiplexor operating mode and is set up entirely from a web browser.

Step 1: Wire the RS-485 link

Connect the gateway's RS-485 terminals (TX+ and TX-) to the KFD2 master module's serial input. In multiplexer mode the gateway's own HART loop terminals aren't used: all HART traffic goes through the multiplexer. Make sure the KFD2 modules are powered and the 14-pin ribbon between them is seated.

If an asset-management PC was the multiplexer's master until now, the gateway takes over that RS-485 link. The PC tool can still reach the transmitters through the gateway (see "Keep your asset-management tools" below).

Step 2: Create a multiplexer configuration

Open the gateway's web interface, go to Configuration and click New Multiplexor. This sets the operating mode and locks the serial port to the multiplexer link. The operating mode is chosen when a configuration is created, so start from New Multiplexor rather than editing an existing direct-loop configuration.

Step 3: Scan and import the transmitters

Click Scan Devices. The gateway reads the list of transmitters the KFD2 already has in its cache. If the multiplexer hasn't scanned its loops yet, or you've added instruments since, use Rebuild & Scan, which asks the KFD2 to physically re-scan every loop. That can take several minutes on a large system.

Tick the transmitters you want and click Import. The gateway addresses each one by its unique HART address rather than its poll address, so transmitters on different loops that are all at poll address 0 don't clash. The HG1v4 handles up to 16 field transmitters per gateway. For a larger KFD2 system, choose the instruments the control system actually needs, or use more than one gateway.

Step 4: Save, Build and Deploy

Click Save, Build, then Deploy. Back on the Home page, the first tab is the multiplexer itself and each following tab is a field transmitter showing its live values. The gateway takes the process variables and loop current from the multiplexer's cache, so it doesn't need to poll each transmitter for them separately.

Step 5: Read the registers from the PLC or SCADA

In multiplexer mode the gateway's serial port is the link to the KFD2, so the control system connects over Modbus TCP on the gateway's Ethernet port (port 502 by default). Each transmitter gets a fixed block of registers. With the recommended Minimal map every device has the same 72-register layout starting at register 1000, so adding a transmitter never shifts the others. The Modbus Map page lists every register, its data type and the function code to read it with.

• Float word order: HART values are 32-bit floats across two registers. Set the gateway's float swap mode to match the PLC, or the values will read as nonsense.
• NaN on communication error: turn it on, so a transmitter the multiplexer can no longer reach reads as "not a number" rather than a frozen value.
• Timeouts: allow a few seconds with retries, and read the status registers with the values.

Keep your asset-management tools

Adding a Modbus path doesn't mean giving up PACTware, FieldCare or AMS. The HG1v4 offers a virtual serial port over the network: install the free HG1VspManager on the engineering PC and Windows gets a COM port that the tool opens as usual, reaching the transmitters through the gateway and the multiplexer. While the tool is working, the gateway pauses its own polling so the tool has the loops to itself. Modbus clients keep the last values in the meantime, and polling resumes 10 seconds after the tool goes quiet.

Routine jobs don't even need the PC tool. The gateway's Commissioning page can read and change tags, ranges and units of the transmitters behind the multiplexer directly from the browser.

Common problems

• Scan Devices returns an empty list: check the RS-485 wiring and polarity, check the KFD2 is powered and its ribbon cables are seated, then try Rebuild & Scan.
• A transmitter shows COMM in its status: the multiplexer isn't getting a valid answer from it. Check the loop wiring on the termination board, the loop resistance (at least 250 Ω for HART) and the transmitter's poll address, then re-scan.
• Values look byte-swapped: change the float swap mode on the gateway or the data type in the PLC.
• Values pause for a while: someone is connected through the virtual serial port. Polling resumes 10 seconds after the tool goes quiet.
• Updates are slower than on a direct loop: that's normal. The multiplexer scans its loops in turn at HART's 1,200 bit/s, so a large system takes longer to refresh than a single loop.

Beyond Modbus

The same transmitter data can also go to the cloud. The HG1v4 publishes readings to any MQTT broker as they change, including AWS IoT Core and Azure IoT Hub, and sends email or webhook alerts when a transmitter fails, asks for maintenance or goes outside limits you set. It includes free secure remote access (no subscription, no per-device fees), so an engineer can check the multiplexer's instruments without a site visit.

Remote access is provided at no additional charge and requires no subscription. It is opt-in per device and requires the gateway to have outbound internet access. Service terms may change for future products or releases.

Get started

New to HART integration? Start with how to connect HART devices to a PLC or SCADA over Modbus, which compares HART input cards, multiplexers and gateways.

The HG1v4 Intelligent HART Gateway is a DIN-rail HART to Modbus gateway and KFD2-HMM-16 multiplexer master, set up entirely from a web browser and designed and built in the UK. Already running an HG1 or HG1+ on a multiplexer? The HG1v4 is a drop-in replacement: same footprint, same power, same wiring.

£399 excl. VAT · 12-month warranty · read the user manual · buying for a company? Request a quote or ask an engineer