Template:Networking rutos manual rs485

Summary

The RS485 function is designed to utilize the serial interface of the device. Serial interfaces provide a possibility for legacy devices to gain access to IP networks. This chapter is an overview of the RS485 section in {{{name}}} devices.

Maximum data rate vs. transmission line length

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The RS485 standart 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:

Where:

• L_max - 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

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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 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:

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:

RS485 Configuration

The RS485 Configuration section is used to configure the parameters of the RS485 serial connection. Parameters used should be selected in accordance with the needs of the user and the type of device connected via the RS485 connector.

The figure below is an example of the RS485 Configuration section and the table below provides information on the fields contained in that section:

field name	value	description
Enabled	yes | no; Default: no	Enables the RS485 service
Baud rate	300 | 1200 | 2400 | 4800 | 9600 | 19200 | 38400 | 57600 | 115200; Default: 115200	Data rate for serial data transmission (in bits per second)
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
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 | NTRIP client; Default: Console	Serial connection type. More information on each serial type can be seen below or by clicking on a link in the value section
Echo	yes | no; Default: no	Toggles RS485 echo ON or OFF. RS485 echo is a loopback test usually used to check whether the RS485 cable is working properly

Serial types

An RS485 serial connection can function in different ways depending on user needs and what devices are connected to the RS485 connector. This section of the chapter is an overview of all different RS485 serial types.

Console

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In Console mode the serial interface is set up as a Linux console that can be used to control the device (much like a regular SSH connection, except established via RS485). It can be used for debugging purposes, to get the status of the device or to control it. Click here to find a guide on how to set up RS485 in console mode.

Over IP

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In Over IP serial type the device provides a connection to a TPC/IP network for the devices connected via the serial interface. The figure below is an example of available configuration fields for the Over IP serial type and the table below provides information on these fields:

field name	value	description
Protocol	TCP; Default: TCP	Protocol used in the communication process
Mode	Server | Client | Bidirect; Default: Server	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	yes | no; Default: no	When checked, indicates that the first hex zeros should be skipped
Client settings: Server address	ip | host; Default: none	IP address or hostname of the server that the client will connect to
Client settings: TCP port	integer [0..65535]; Default: none	Server's port number
Client settings: Reconnect interval (s)	integer; Default: none	Time period (in seconds) between reconnection attempts in case a connection fails
Server settings: TCP port	integer [0..65535]; Default: none	Internal TCP port number used to listen for incoming connections
Server settings: Timeout (s)	integer; Default: none	Disconnects clients after they remain inactive for an amount time (in seconds) specified in this field
Output	OC Output | Relay Output; Default: OC Output	Output to indicate that the application switched from client (default) to server state (this field becomes visible only in Bidirect mode)
Output state	1 | 0; Default: 0	Output state value during which the application reverts to server mode (this field becomes visible only in Bidirect mode)

Modbus gateway

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

The figure below is an example of available configuration fields for the Modbus gateway serial type and the table below provides information on these fields:

field name	value	description
Listening IP	ip; Default: 0.0.0.0	IP address on which the Modbus gateway will wait for incoming connections
Port	integer [0..65535]; Default: 502	Port number used to listen for incoming connections
Slave ID configuration	User defined | Obtained from TCP; Default: User defined	Specifies whether slave IDs should be user defined or automatically obtained from TCP
Slave ID/Permitted slave IDs	integer | range of integers | multiple integers; Default: 1/1-247	Slave ID or 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

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