Template:Networking rutos manual rs485

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The information in this page is updated in accordance with firmware version .

[[{{{name}}} RS485|RS485]] service was moved to [[{{{name}}} Serial Utilities|Serial Utilities]] page from FW version {{{series}}}_R_00.07.03.


The RS485 page is used to configure the operating parameter of RS485 serial connector.

This manual page provides an overview of the RS485 page in {{{name}}} devices.

General information

Maximum data rate vs. transmission line length

The RS485 standard can be used for network lengths up to 1200 meters, but the maximum usable data rate decreases as the transmission length increases. A device operating at the maximum data transfer rate (10 Mbps) is limited to a transmission length of about 12 meters, while a distance up to 1200 meters can be achieved at 100 Kbps. A rough relation between maximum transmission length and data rate can be calculated using this approximation:

Services rs485 formula.PNG


  • Lmax - maximum cable length in meters.
  • DR - maximum data rate in bits per second.

Twisted pair is the preferred cable type for RS485 networks. Twisted pair cables pick up noise and other electromagnetically induced voltages as common mode signals, which are rejected by the differential receivers.

2-Wire and 4-Wire Networks

Below is an example of a 4-wire network electrical connection. There are 3 devices shown in the example. One of the devices is the "master" and other two are "slaves". Termination resistors (120 Ω each) are placed at each cable end. Four-wire networks consists of one master with its transmitter connected to each of the slaves' receivers on one twisted pair. The slave transmitters are all connected to the master receiver on a second twisted pair:

Services rs485 4wire v2.PNG

Example 2-wire network electrical connection: to enable a 2-wire RS485 configuration you need to connect D_P to R_P and D_N to R_N on the device’s RS485 socket. Termination resistors are placed at each cable end (120 Ω each):

Services rs485 2wire v2.PNG

RS485 Configuration

The RS485 Configuration section is used to set up the main operating parameters and the serial type of the RS485 connector.


Field Value Description
Enabled off | on; default: off Turns the RS485 service on or off.
Baud rate {{{baud_rates}}}; default: {{{baud_rate}}} Data rate for serial data transmission (in bits per second (bps)).
Data bits 5 | 6 | 7 | 8; default: 7 Number of data bits for each character.
Parity None | Odd | Even; default: None In serial transmission, parity is a method of detecting errors. An extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.
  • None (N) - no parity method is used.
  • Odd (O) - the parity bit is set so that the number of "logical ones (1s)" has to be odd.
  • Even (E) - the parity bit is set so that the number of "logical ones (1s)" has to be even.
Stop bits 1 | 2; default: 1 Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. Two stop bits are required if slow electromechanical devices are used.
Flow control None | RTS/CTS | Xon/Xoff; default: None In many circumstances a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a "handshaking" method, usually distinguished between hardware and software handshaking.
  • RTS/CTS - hardware handshaking. RTS and CTS are turned OFF and ON from alternate ends to control data flow, for instance when a buffer is almost full.
  • Xon/Xoff - software handshaking. The Xon and Xoff characters are sent by the receiver to the sender to control when the sender will send data, i.e., these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the Xoff character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an Xon character to tell the sender to resume transmission.
Serial type Console | Over IP | Modem | Modbus gateway; default: Console Specifies the serial connection type.
Look to the sections below for information on different RS485 serial type options.
Full Duplex off | on; default: off Turns RS485 Full Duplex mode on or off.


Console mode requires no further configuration than the settings above and is used as a direct-access method to the device's shell interface. For this purpose you may want use such applications as PuTTY on Windows and microcom, minicom, picocom or similar applications on Linux.

Networking rutos manual usb tools printer server usb to serial console.png

Over IP

The Over IP serial type is used to manage serial connections over a TCP/IP network.

Networking rutos manual rs485 over ip v1.png

Field Value Description
Protocol TCP | UDP; default: TCP Protocol used in the communication process.
Raw mode off | on; default; default: off When enabled, all data will be transmitted transparently.
Mode Server | Client | Bidirect; default: Server This device's role in the connection:
  • Server - the device waits for incoming connections.
  • Client - the device initiates the connection.
  • Bidirect - acts as client by default but waits for incoming connections at the same time.
No leading zeros off | on; default: off When checked, indicates that the first hex zeros should be skipped.
Server settings: Port integer [0..65535]; default: none Internal port number used to listen for incoming connections.
Server settings | TCP: Timeout (s) integer [0..32767]; default: 300 Specifies an inactivity time limit (in seconds) after which an inactive clients is disconnected.
Bidirect: Output Specifies which output to manage.
Bidirect: Output state 0 | 1; default: 0 Default output state value, when the application is started.
Server settings | UDP: Number of clients 1-10; default: 1 Specifies how many UDP clients will be supported simultaneously (predefined clients does not count towards this limit).
Server settings | UDP: Predefined client 1 address ip4; default: none Specifies IP address for predefined connection 1.
Server settings | UDP: Predefined port 1 port; default: none Specifies port number for predefined connection 1.
Server settings | UDP: Predefined client 2 address ip4; default: none Specifies IP address for predefined connection 2.
Server settings | UDP: Predefined port 2 port; default: none Specifies port number for predefined connection 2.
Client settings: Server Address ip | host; default: none IP address or hostname of the server that this client will connect to.
Client settings: Port integer [0..65535]; default: none Server's listening port number.
Client settings: Reconnect interval (s) integer; default: none Time period (in seconds) between reconnection attempts in case a connection fails.
Serial device read time integer [0..1000]; default: none Specifies custom read time for the serial device.
Full Duplex 0 | 1; default: 0 Enables RS485 Full-Duplex.
Server settings | TCP: Max clients integer [1..32]; default: 32 Specifies how many clients are allowed to connect simultaneously.
TCP: Always reconnect off | on; default: off When enabled, a new TCP connection will be made after sending every data package.

Modbus gateway

The Modbus gateway serial type allows redirecting TCP data coming to a specified port to an RTU specified by the Slave ID. The Slave ID can be specified by the user or be obtained directly from the Modbus header.

Networking rutos manual rs485 rs485 configuration modbus gateway.png

Field Value Description
Listening IP ip; default: IP address to listen for incoming connections. The default value ( means that this device will listen for incoming connections on any interface or IP address.
Port integer [0..65535]; default: 502 Port number to listen for incoming connections.
Slave ID configuration type User defined | Obtained from TCP;
default: User defined
Specifies whether slave IDs are user defined or automatically obtained from TCP.
Slave ID | Permitted slave IDs integer | range of integers;
default: 1 or 1-247
Specifies the slave ID of range of permitted slave IDs. The way this field is named and its function depends on the value of the Slave ID configuration field.
A range of IDs can be specified by placing a hyphen (-) between two integer numbers. For example, if you permit slave IDs in the range of 10 to 20, you would specify it as: 10-20
You can also specify multiple values that are not connected in a range using commas (,). For example, to specify 6, 50 and 100 as permitted slave IDs, you would have to use: 6,50,100
Open port automatically off | on; default: on Automatically adds a traffic rule in the firewall configuration to open the required port for serial communication.
Caution: use with care if listening IP is left as the default value ( Leaving it as such will leave the device open for remote connections on the specified port.
Full duplex off | on; default: off Turns RS485 full duplex on or off.

IP Filter

The IP Filter section is used for configuring which network is allowed to communicate with the device. You may add a new instance by selecting the Interface and pressing Add.

Networking rutos manual console ip filter.png

Then enter the IP address and save.

Networking rutos manual console ip filter instance.png

[[Category:{{{name}}} Services section]]