Serial Port at the Edge
The edge gateway opens an RS-485 or RS-232 port, built in or through a USB converter, and polls each device on its own address.
Protocol Support
Serial Bus Integration
Read meters, drives, controllers and instruments that speak over RS-485 or RS-232 serial lines. The edge gateway holds the serial port and polls each device, so the cabling never has to reach the internet.
- Standard
- TIA/EIA-485 and TIA/EIA-232 electrical layers, carrying protocols such as Modbus RTU and BACnet MS/TP
- Medium
- Serial cable: shielded twisted pair for RS-485, point-to-point for RS-232
- Topology
- RS-485: multidrop daisy chain with termination at both ends. RS-232: one device per port
- Data Model
- A master polls devices by address, one request at a time; the electrical layer carries the bytes, a protocol on top gives them meaning
What Serial Bus Is
Serial is the oldest and most common way to wire field equipment. RS-485 is a differential multidrop bus where many devices share one twisted pair, and RS-232 is a short point-to-point link. Neither defines what the bytes mean: Modbus RTU, BACnet MS/TP and many vendor protocols ride on top. Because these lines stay on site, an edge gateway with a serial port or a USB converter does the reading, and IoTServa receives the result as normalized tags.
Native Address
Port /dev/ttyUSB0 (RS-485)
9600 baud, 8 data bits, even parity, 1 stop bit
Unit 12, holding register 40001
Protocol: Modbus RTU
Normalized Tag
site.plant.mcc-1.motor-12.run-current
14.2A
How IoTServa Connects Serial Bus
One Trunk, Many Devices
Several devices share an RS-485 trunk. Requests are grouped and timed per line so a slow trunk stays responsive.
Protocol Profiles
Pick the protocol on top, usually Modbus RTU, and apply a device profile for register addresses, scaling and units.
Hold and Forward
If the uplink drops, readings are held on the gateway and sent in order when the connection returns.
Typical Jobs
- Meters and energy analyzers on RS-485
- Drives, soft starters and motor controllers
- Chillers, boilers and pumps with a serial port
- Legacy equipment with no Ethernet
Plan for These
Serial settings must match on both ends: baud rate, parity and stop bits. A wrong setting looks like a dead device.
Wire an RS-485 trunk as a daisy chain with short stubs, terminate both ends, and ground the shield at one end only. Poor wiring causes intermittent errors that look like software faults.
Modbus allows one master per line, so the IoTServa gateway must be the only poller. If another system already polls the line, read through that master instead of adding a second one.
Learn More About Serial Bus
Factories and Plants
Drives, meters and PLCs trended together, with alarms that mean something.
See the SolutionEnergy and Solar
Generation and consumption reconciled in one view.
See the SolutionEstates and Agriculture
Sensors across land where Wi-Fi and cables never reach.
See the SolutionLook Up a Term in the Glossary or check devices on the Hardware Page
Serial Bus Questions
What is the difference between RS-485 and Modbus RTU?
RS-485 is the electrical layer: how voltages on a twisted pair carry bits. Modbus RTU is a protocol that defines requests, addresses and registers. Modbus RTU is very often sent over RS-485, but RS-485 also carries BACnet MS/TP and vendor protocols.
Can IoTServa read RS-485 devices directly from the cloud?
No. A serial line is local to the equipment. An edge gateway on site holds the port and polls the devices, then forwards normalized tags over an outbound connection.
What hardware do I need for a serial port?
A gateway or industrial PC with an RS-485 or RS-232 port, or a USB serial converter. Use twisted pair cable of about 120 ohm impedance for RS-485, with 120 ohm termination at each end of the trunk, and shield grounded at one end.
How many devices fit on one RS-485 line?
The RS-485 standard allows 32 unit loads on a line. Transceivers with a lower load, such as one-eighth unit load, allow more, up to 256 in the best case. A Modbus line has one master and up to 247 device addresses. Fewer devices per line also keeps polling fast.
Does serial bus mean Modbus only?
No. Modbus RTU is the most common case and has its own page, but BACnet MS/TP and some vendor protocols also use serial lines. Send the device manual and the engineering team will confirm what the device speaks.
Bring Your Serial Bus Devices. We Will Read Them.
Tell us the make and model of what is on site. We will scope a pilot against your real equipment.