Modbus RTU vs Modbus TCP: choosing between RS-485 and Ethernet I/O

Most remote I/O projects start with the same decision: put the modules on an RS-485 bus and poll them with Modbus RTU, or connect them to Ethernet and use Modbus TCP. This article compares Modbus RTU vs Modbus TCP from the installer's point of view: cabling, topology, scan time and addressing, and how to combine both with a gateway. It ends with the ICP DAS series that fit each option.

The same protocol on two different networks

Both variants use the same data model: coils and discrete inputs for bits, input and holding registers for 16-bit values. A register map works the same way on either network; only the transport changes.

Modbus RTU runs on a serial line, normally two-wire RS-485. One master polls the slaves one at a time, and each slave has a unit address on the bus. Modbus TCP carries the same requests over TCP/IP, normally on port 502. Each device has its own IP address and can usually answer more than one client, for example a PLC and a SCADA server.

Modbus RTU vs Modbus TCP at a glance

Cabling and topology in practice

RS-485 is a bus. Run the cable from module to module with short stubs, terminate both ends with 120 ohm and keep a common signal reference. At moderate baud rates a segment covers 1200 m, which suits long runs through a plant with a few signals at each location. Beyond 32 unit loads or 1200 m, add a repeater such as the I-7510, an isolated RS-485 repeater with automatic direction control.

Ethernet is normally a star with one switch port per module. The ET-7200 and ET-2200 series have a built-in two-port switch, so modules can be daisy-chained along a machine like an RS-485 bus. Both list a LAN bypass function, which on ICP DAS devices keeps the rest of the chain communicating if one unit loses power. PET versions can be powered from an IEEE 802.3af PoE switch.

Speed: a worked example

Polled serial buses are often slower than expected. Take 20 analog modules, each read with one request for 8 holding registers:

• The request is 8 bytes and the response 5 bytes plus 16 data bytes, so one poll moves 29 bytes.
• With 8 data bits, no parity and 1 stop bit, each byte is 10 bits on the wire: 290 bits per poll.
• At 9600 bps that takes about 30 ms, plus two silent intervals of 3.5 characters, about 7 ms. Roughly 38 ms per module, or about 0.75 s for the bus.
• At 115200 bps the transfer takes about 2.5 ms, and the Modbus specification recommends a fixed 1.75 ms silent interval above 19200 bps. Roughly 6 ms per module, or about 0.12 s for the bus.

Module response time and retries come on top. For temperatures, levels and energy values the slower figure is usually fine. For fast digital signals Ethernet removes the bottleneck, since the transfer time at 100 Mbit/s is negligible and modules can be polled in parallel. Synchronised I/O at millisecond cycle times is a job for EtherCAT.

Mixed systems: gateways and data concentrators

Many plants have working RS-485 buses and a newer Ethernet network for SCADA. A Modbus TCP to RTU gateway connects the two without replacing the serial modules. The TCP client sends requests to the gateway's IP address, and the unit ID in each request selects the serial slave.

• tGW-700 series: compact Modbus TCP to RTU/ASCII gateways with one to three serial ports, most with PoE. Up to 32 TCP masters can share a serial port, and a read cache answers repeated requests from memory to reduce load on the bus.
• GW-2500iM series: the same conversion with a two-port Ethernet switch, isolated RS-422/485 ports and security functions such as Modbus TCP/TLS, HTTPS, SNMPv3 and an IP filter.
• MDC-700 series: data concentrators that poll the serial devices themselves with up to 250 command definitions, collect the values in a data pool of 9600 registers per data type, and serve up to 8 Modbus TCP hosts.

A gateway passes requests through, so the serial bus still sets the response time. A concentrator decouples the two sides, which helps when several SCADA clients read one bus.

Which ICP DAS series fit

RS-485 and Modbus RTU

• M-7000: Modbus RTU and DCON, 1200 to 115200 bps. Many models share their hardware with an I-7000 module, for example M-7017R and I-7017R.
• tM series: compact modules with DCON, Modbus RTU and Modbus ASCII.
• I-7000: ICP DAS DCON protocol only. It suits extensions of existing DCON systems; for a Modbus master, choose the M-7000 equivalent.

Ethernet and Modbus TCP

• ET-7000: one Ethernet port, Modbus TCP and Modbus UDP, built-in web server.
• ET-7200: Modbus TCP and UDP with a two-port switch and LAN bypass.
• ET-2200: two-port switch with LAN bypass, plus MQTT and SNMP V2c, useful when the same I/O also feeds an IIoT platform.
• PET versions: PoE-powered versions of ET-7000, ET-7200 and ET-2200 modules.

Send our technical sales team your signal list, cable distances and master device, and we will propose a matching set of modules and gateways.