RS-485 Interfaces

This page lists RS-485 devices that connect to a PC, either by converting to RS-232, or via USB and other interface. RS-485 devices are use for both BACnet and Modbus networks. See for hardware details and installation methods for RS-485 devices.


RS-485 to Ethernet Interfaces

Moxa MGate MB3170

  • Moxa MGate MB3170 Advanced serial-to-Ethernet Modbus gateway, UL Listed, CE, FCC Class A,
  • RS-485 terminal block, RJ45 10/100BaseTX.
  • Dual Ethernet ports allow users to string multiple Modbus gateways together using standard RJ45 Ethernet cables.

Moxa NPort 5130

  • Tested with WattNode Modbus
  • Moxa NPort 5130 DB-9, UL, CE, FCC Class A, includes jumper-selectable 1K or 150K pull-up/pull-down resistors. Moxa also makes a DB9M to terminal block adapter (which avoids the need for soldering to a mating DB9F connector), called the MiniDB9FTB, and the pin-out of this is the same as for the Moxa UPort 1100 USB/RS485 adapter.
  • Neteon Technologies, Inc [1]
  • Moxa Mini DB9F-to-TB

Contemporary Control Systems, Inc.

  • Tested with WattNode BACnet
  • The model BASRTP-B BAS Portable Router routes messages between BACnet/IP and BACnet MS/TP networks
  • Web server for convenient commissioning, re-configuring and troubleshooting
  • Powered via USB port

Lantronix UDS1100

Control Solutions, Inc.

They sell the Babel Buster line of converters for converting among Modbus RTU, Modbus TCP, BACnet, LonWorks, SNMP, etc.

  • Babel Buster SPX – Modbus TCP – RTU, SNMP

eze System, Inc.

They sell an Ethernet to Modbus adapter/controller as well as an EzeIO Metering kit that bundles a CCS Modbus WattNode meter and CT’s with their device, which also offers GSM connectivity.

Miscellaneous Brands/Models

Virtual Ethernet COM Port Drivers

There are several programs available that make virtual COM ports. These can be used to get a Modbus signal converted back from Ethernet into a COM port so that Modbus software could access the packets as Modbus RTU.

RS-485 USB Interfaces

The following interfaces provide RS-485 through a USB connected device. These generally create a virtual COM port under Windows.

B&B Electronics: USPTL4

  • Tested with WattNode Modbus
  • Non-isolated
  • 15kV ESD
  • 4.7k internal bias resistors
  • Note: Set all four DIP switches to “ON” position to disable echo and select 2-wire RS485 interface
  • WattNode Modbus meter’s A-/B+ can be wired to either TDA/TDB or RDA/RDB.

SerialComm: Model USB-485B

  • Non-isolated

SerialGear: USB-COM-I

  • Isolated

SerialGear: USB-COMi-M

  • Tested with WattNode Modbus
  • Not isolated
  • 15kV ESD
  • Built-in 120 ohm termination resistor
  • Built-in 750 ohm bias resistors
  • Connect WattNode Modbus terminal A- to adapter terminal 1 (next to the corner); terminal B+ to terminal 2; and terminal C to terminal 6 (next to the USB connector).
    The DIP switches should be S1=ON, S2=OFF, S3=OFF and S4=OFF for Half Duplex, 2-wire operation without Echo.
    For reliable operation in the presence of industrial EMI sources and where long bus lines are used, it is recommended to use bus biasing and termination. To enable these, open the metal case and install jumper 1-2 (120 ohm Tx termination), jumper 3-4 (750 ohm Tx+ pull-up) and jumper 5-6 (750 ohm Tx- pull-down). The remaining jumpers may be left in the storage position as shipped from the factory.

Sealevel: SeaLINK+485I

  • Fully isolated

Sealevel: SeaLINK+485

  • Same as SeaLINK+485I but without isolation

Quatech: QSU2-400

ICP DAS: I-7561

  • 3000V isolation

ICP DAS: I-7563

  • 3000V isolation
  • Three RS485 ports


  • Tested with WattNode Modbus
  • Optically isolated
  • 15kV ESD
  • 4.7k internal bias resistors
  • Set all four DIP switches to “ON” position.
  • Note: Does NOT work with terminated networks unless two bias resistors are replaced with different values, requiring soldering!

ByteRunners’ Serial Gear Clones,+RS-422,+RS-485+Serial+Adapters+(Single+Port)

  • USB-COMi-M and USB-COMi-SI-M (isolated)

US Converters’ USB to RS-485/422 Mini

  • 600W surge protection, 15KV static electricity protection
  • Terminal block

US Converters’ USB to RS-485/422 Converter

  • 600W surge protection, 15KV static electricity protection

Moxa: UPort 1130

UPort 1130

  • Software selectable support for both two-wire and four-wire RS-485
  • Not isolated
  • 15KV ESD
  • Comes with mini-DB9F-to-terminal block adapter

RS-485 to RS-232 Converters

RS-232 to RS-485 conversion is tricky and not recommended for new applications. A terminated RS-485 bus requires 60mA of drive current, but port-powered RS-232 adapters cannot provide this much current, so unless the adapter is powered with an external supply, these will not work with terminated or heavily loaded buses. Some vendors of this type are listed below.

  • [B&B 485SD9TB, $79] Works for bench testing but is not adequate for production use. Does not support termination. Poor drive.

RS485 to WLAN Interfaces

See Also

Keywords: RS-485 adapter