Every process indicator covered so far in this series reads one signal from one loop. That’s the right tool for most panels. But some panels don’t have a single 4–20 mA loop to read — they have a PLC, a network of other devices, and a need to show a value that already exists somewhere else on that network. That’s a different problem, and it calls for a different device: a Modbus display like the AKYTEC SMI2-M.
What Makes a Modbus Display Different
A standalone indicator — the ITP15, ITP17, or ITP11 — is wired directly into an analog signal loop. It reads a current or voltage and shows a value derived from that one physical connection. It doesn’t know or care what else is happening on the panel.
A Modbus display works differently. Instead of reading an analog loop, the AKYTEC SMI2-M connects to an RS-485 network and communicates using the Modbus RTU or ASCII protocol — the same communication standard most industrial PLCs, controllers, and I/O modules already speak. Rather than measuring a physical signal itself, it displays a value that another device on the network already has.
This distinction matters more than it sounds. An analog indicator shows what’s on its own wire. A Modbus display shows whatever value it’s told to show, from whatever device is feeding it data — which opens up display scenarios a standalone indicator simply can’t do.
How the SMI2-M Operates on a Modbus Network
The SMI2-M supports three distinct operating modes on its RS-485 connection:
Master mode — the SMI2-M actively polls another device on the network and displays one of its parameters. This is useful for devices that can only operate as a Modbus slave and have no display of their own, such as a standalone temperature controller (TRM-series). The SMI2-M’s own internal logic controls its color and blinking behavior in this mode.
Slave mode — the SMI2-M receives a value pushed to it by a Modbus master, typically a PLC or programmable relay. Color and blink behavior can be driven either by commands from that master device or by the SMI2-M’s own internal threshold logic.
Spy mode — the SMI2-M listens passively to traffic already moving on an existing Modbus bus with its own master and slave devices, picking out a specific request or response (by device address, function code, and register address) without joining the network as an active participant. This is the mode that matters most for retrofits: it lets a display be added to a network that’s already running, without touching the existing master/slave configuration.
Multiple SMI2-M units can share a single RS-485 bus, which is the other structural difference from analog indicators — a bus with several displays needs far less cabling than the equivalent number of analog loops run individually to a panel.
Data Types and Display Flexibility
Because it isn’t tied to a single scaled analog range, the SMI2-M can display a wider range of data formats than an analog indicator: INT, DINT, WORD, DWORD, REAL, STRING, a bit-mask “portrait” display, and a time value in an “xxx:yy” format. Leading zeros, decimal places, and linear scaling of the incoming value are all configurable, and the display supports blinking and “creeping” (scrolling) modes for values that don’t fit cleanly in four digits. Like the ITP17, it uses a three-color display with user-defined thresholds, so it can double as a status indicator on top of showing the raw value.
Physical Specification
The SMI2-M shares the same practical mounting logic as the rest of the AKYTEC indicator range: it fits a standard 22.5 mm panel borehole, carries an IP65 rating on the front face, and uses removable screw terminals for quick device replacement without rewiring. It requires a 24 V DC power supply — unlike the loop-powered ITP11, it isn’t powered by the data connection itself, since RS-485 doesn’t carry power the way a 4–20 mA loop does. Configuration is done through a MicroUSB port using AKYTEC’s SMI2 Configurator software, and the unit is rated for -40°C to +70°C operation.
Basic Bus Configuration Terms Worth Knowing
Setting up any Modbus RS-485 device — the SMI2-M included — involves a handful of parameters that show up on every device’s configuration screen, whether it’s a display, a PLC, or a controller:
- Baud rate. The serial link’s data speed, most commonly 9600 or 19200 bps on Modbus networks. Every device sharing a bus must be set to the same baud rate, along with matching parity and stop-bit settings, or communication simply won’t establish.
- Device address. Each Modbus device on a bus needs a unique address (1–247) so the master knows which unit it’s talking to. This is typically set during configuration via the SMI2 Configurator software or a similar tool on other akYtec devices.
- Register mapping. The value a display shows corresponds to a specific Modbus register — commonly referenced using holding-register notation (the 4xxxx range) — on the source device. Configuring the SMI2-M means telling it which register, on which device address, holds the value to display.
- Bus length and termination. RS-485 networks running longer cable lengths (roughly over 100 m) generally need termination resistors at each end of the bus to prevent signal reflection and read errors — a detail worth checking on any retrofit where displays are added to an existing, already-long bus run.
- Polling load. In Master mode, each additional device the SMI2-M polls adds to the total cycle time before it comes back around to the first one again — more devices on a bus, or higher register counts per poll, means a slower effective update rate on the display. For most panel-indication use cases (an operator glancing at a value, not a fast-changing waveform) this isn’t a practical concern, but it’s worth accounting for on a bus that’s already busy with other master/slave traffic.
None of this is SMI2-M-specific — it’s the same handful of settings every Modbus RS-485 device shares — but it’s the layer of configuration that sits between “the display is wired in” and “the display is actually showing the right value.”
Choosing Between a Modbus Display and a Standalone Indicator
The decision usually comes down to one question: does the value you need to show already exist somewhere on a network, or does it need to be measured directly?
- If a transmitter feeds an analog signal straight to the panel and nothing else needs that value, a standalone indicator is simpler, cheaper, and has fewer configuration steps. There’s no network to set up and no protocol to match.
- If the value already lives inside a PLC, a programmable relay, or another Modbus-capable device — and the panel just needs to surface it locally without pulling a separate analog wire — a Modbus display avoids duplicating the measurement entirely.
- If several values from different devices need to appear across a bank of displays on one panel, a shared RS-485 bus with multiple SMI2-M units is dramatically less wiring than running that many individual analog loops.
- If a display needs to be added to a network that’s already operating, without altering the existing master/slave setup, Spy mode is a capability no standalone indicator offers.
Neither format replaces the other — they solve different wiring problems. A panel with a handful of independent transmitters is usually simpler with standalone indicators. A panel that’s already built around a PLC and an RS-485 backbone gets more value from a networked display that taps into data that’s already there.
Specifying for a GCC Panel
For help deciding between networked and standalone indication on a project across the UAE, Qatar, or the wider GCC, Paklink’s instrument and control team can confirm the right approach for your panel’s architecture — see the full industrial process display range or get a quote.
Frequently Asked Questions
Does the SMI2-M need its own power supply?
Yes. Unlike a loop-powered indicator such as the ITP11, the SMI2-M requires a 24 V DC power supply separate from its RS-485 data connection — the communication cable doesn’t carry power.
Can the SMI2-M work with any PLC?
Any PLC or controller with an RS-485 port that supports the Modbus RTU or ASCII protocol can communicate with the SMI2-M, regardless of manufacturer — Modbus is an open, vendor-neutral protocol.
How many SMI2-M units can go on one RS-485 bus?
Multiple units can share a single bus, each with its own unique device address (1–247). The practical limit depends on the bus’s overall polling load and cable length rather than a fixed device count.
Can a display be added to a Modbus network that’s already running, without disrupting it?
Yes — this is what Spy mode is for. It lets the SMI2-M listen to existing traffic on a bus and pick out a specific value without joining as an active master or slave, so the existing configuration doesn’t need to change.
Does the SMI2-M replace a SCADA screen or HMI?
No. It’s a local, panel-level display for one or a few specific values — the same role a standalone indicator plays, just sourced from the network instead of a direct analog signal. It doesn’t provide the broader visualization, alarming, or historical trending a full SCADA/HMI system does.
