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How to monitor more sensors than your gateway has terminals

Wiki: Setup, edge and devices

New to gateway setup? Start at How to set up a gateway: from hardware to green. This page picks up once you need more analog sensors on one gateway than its own panel has room for.

When to use this

Your gateway’s own screw-terminal panel has room for a handful of 4-20mA sensors (see wire the gateway’s panel I/O), plenty for one machine. Once you’re monitoring a dozen bearings across a line, or several supply-current clamps across a panel, one gateway’s own terminals run out fast, and a gateway per sensor doesn’t make sense.

The answer isn’t a new gateway per sensor, it’s a small sensor panel that aggregates many 4-20mA loops onto one Modbus TCP connection, which your existing gateway reads the same way it reads any Modbus device. Many sensors, one panel, one gateway.

What you need

  • A Modbus TCP sensor aggregator, a small, commodity DIN-rail module with 8 (or more) isolated 4-20mA inputs and a Modbus TCP port. We recommend the Advantech ADAM-6017 class (or the Moxa ioLogik E1240 class as an alternative), widely available from industrial-automation distributors, roughly $350-400 for an 8-channel unit. Spall does not sell or warranty this hardware. You source and support it like any other panel component.
  • Your 4-20mA sensors, clamp-on current transducers, vibration transmitters, pressure transducers, or any other loop-powered 4-20mA device. Each one lands on one aggregator channel.
  • 24VDC power for the aggregator and the sensors it loop-powers (most loop-powered 4-20mA transmitters draw their power from the same 2-wire loop the aggregator reads, no separate power run to the sensor).
  • A network connection from the aggregator to the same plant/OT network your gateway already reaches its other machines on.

Wire the panel

  1. Mount the aggregator on standard DIN rail alongside your 24VDC supply.
  2. Wire each 4-20mA sensor’s 2-wire loop to one aggregator input channel (signal+ to the channel’s + terminal, loop return to its -). Use shielded twisted-pair cable for any run of more than a few meters, and ground the shield at one end only.
  3. On the aggregator’s own configuration utility (comes with the unit), set each populated channel to the 4-20mA range, this is a one-time setup step per channel, done once when the panel is wired.
  4. Give the aggregator a static IP (or a DHCP reservation) on the same network your gateway reaches.

Add it in the app

  1. From the gateway’s Sources & tags tab (or the Connect-a-Machine wizard), add a new source with protocol Modbus TCP, pointed at the aggregator’s IP.
  2. Apply the Sensor Panel (4-20mA via Modbus Aggregator) machine template, it pre-fills a starter tag set (a current channel and a vibration channel) with the right register addresses and scaling for this class of aggregator. Add one tag per remaining populated channel the same way, following the aggregator’s own manual for the register number of each channel.
  3. Each tag’s scale turns the aggregator’s raw reading into your sensor’s real engineering units (amps, mm/s, PSI, whatever your sensor’s nameplate specifies), the template pre-fills this for the two starter channels. For additional channels, use your sensor’s documented 4-20mA span (e.g. “4-20mA = 0-100A”) the same way the panel I/O guide’s worked scale/offset example walks through for the gateway’s own onboard terminals.
  4. Confirm each tag reads a live, sane value on the gateway’s Status tab before trusting it in a dashboard or alert.

What you get

Every wired sensor shows up as an ordinary tag, same downtime detection, same OEE pipeline, same wallboard tiles as a tag read straight off a machine’s own controller. Nothing downstream treats an aggregated sensor differently. It’s just a signal, sourced from a sensor panel instead of a PLC.

What Spall does not cover

Spall recommends this class of hardware because it’s what we’ve bench-verified our Modbus driver against. We don’t sell, resell, or warranty the aggregator or the sensors themselves. If the panel needs support or replacement, that goes through whoever you bought it from. Your gateway’s configuration (its IP, its register map) doesn’t change unless you swap for a different aggregator model.