RUT955 RS232/RS485

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Main Page > RUT Routers > RUT955 > RUT955 Manual > RUT955 WebUI > RUT955 Services section > RUT955 RS232/RS485

The information in this page is updated in accordance with firmware version RUT9XX_R_00.06.07.

Summary

RS232 and RS485 serial interfaces provide a possibility for legacy devices to gain access to IP networks.

This chapter of the user manual provides an overview of the RS232/RS485 page for RUT955 devices.

RS232

Networking rut manual rs232.png

Field Value Description
Enabled yes | no; default: no Turns the RS232 service on or off.
Baud rate 300 | 1200 | 2400 | 4800 | 9600 | 19200 | 38400 | 57600 | 115200; default: 115200 Data rate for serial data transmission (in bits per second (bps)).
Data bits 5 | 6 | 7 | 8; default: 8 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 | NTRIP client; default: Console Specifies the serial connection type. More information on serial types can be found in each of their respective sections of this manual page.
Echo yes | no; default: no Turns RS232 echo on or off. RS232 echo is a loopback test usually used to check whether the RS232 cable is working properly.

RS232 Connector Pinout


RS232 connector type on this device is DCE female. DCE stands for Data Communication Equipment.

Services rs232 connector.PNG

PIN NAME* DESCRIPTION* DIRECTION ON THIS DEVICE
1 DCD Data Carrier Detect Output
2 RXD Receive Data Output
3 TXD Transmit Data Input
4 DTR Data Terminal Ready Input
5 GND Signal Ground -
6 DSR Data Set Ready Output
7 RTS Ready To Send Input
8 CTS Clear To Send Output
9 RI Ring Indicator Output (connected to +5V permanently via a 4.7k resistor)

* The names and descriptions that indicate signal direction (such as TXD, RXD, RTS, CTS, DTR, and DSR) are named from the point of view of the DTE device.

Cables


There are two types of RS232 serial devices: DTE and DCE. DTE typically refers to the serial port on a PC or terminal, while DCE refers to communication devices. Connectors mounted on DTE are likely to be male, and those mounted on DCE are likely to be female.

This device is DCE and has a female connector.


To connect a standard DTE device, use a straight-through Female/Male RS232 cable:

Services rs232 connector female male.PNG

See straight cable pinout below:

Straight cable pinout v2.png


To connect another DCE device to RUT955, a Null-modem (crossed) Male/Male cable should be used:

Male male rs232 cable.jpg

See straight crossed cable pinout below:

Straight crossed rs232 cable pinout v2.png


Maximum cable length is 15 meters or the cable length equal to a capacitance of 2500 pF (for a 19200 baud rate). Using lower capacitance cables can increase the distance. Reducing communication speed can also increase maximum cable length.

RS485

RS-485 is a serial data transmission standard for use in long ranges or noisy environments.

Services rs485.PNG

Field Value Description
Enabled yes | no; default: no Turns the RS485 service on or off.
Baud rate 300 | 1200 | 2400 | 4800 | 9600 | 19200 | 38400 | 57600 | 115200; default: 115200 Data rate for serial data transmission (in bits per second (bps)).
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 Specifies the serial connection type. More information on serial types can be found in each of their respective sections of this manual page.

Maximum data rate vs. transmission line length


The RS-485 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 the 100 kbps data rate can achieve a distance up to 1200 meters. A rough relation between maximum transmission length and data rate can be calculated using this approximation:

Services rs485 formula.PNG

Where Lmax is the maximum transmission length in meters and DR is maximum data rate in bits per second.

Twisted pair is the preferred cable type for RS-485 networks. Twisted pair cables pick up noise and other electromagnetically induced voltages as common mode signals, which are rejected by the differential receivers.

Cable Type


Recommended cable parameters:

PARAMETER VALUE
Cable Type 22-24 AWG, 2 – pair (used for full-duplex networks ) or 1-pair (used for half duplex networks). One addtitional wire for ground connection is needed
Characteristic cable Impedance 120 Ω @ 1MHz
Capacitance (conductor to conductor) 36 pF/m
Propagation Velocity 78% (1.3 ns/ft)

RS485 connector pin-out


Services rs485 port.PNG

NAME DESCRIPTION TYPE
D_P Driver positive signal Differential Output
D_N Driver negative signal Differential Output
R_P Receiver positive signal Differential Input
R_N Receiver negative signal Differential Input
Ground Device ground Differential Output

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 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.PNG

Example 2-wire network electrical connection: to enable a 2-wire RS-485 configuration on a Teltonika router, you need to connect D_P to R_P and D_N to R_N on the device’s RS-485 socket. Termination resistors are placed at each cable end.

Services rs485 2wire.PNG

Termination


When to use (place jumper)

Termination resistor, equal in resistance to cable characteristic impedance, must be connected at each end of the cable to reduce reflection and ringing of the signals when the cable lengths get relatively long. Rise time of the RUT955 RS-485 driver is about 5 ns, so the maximum unterminated cable length is about 12 cm. As transmission line cables will always be longer than 12 cm, termination is mandatory all the time if RUT955 is located at the end of the cable.

When not to use (remove jumper)

If your RS-485 consists of more than two devices and the RUT955 router is located not on the end of the line but, for example, in the middle, RUT955 termination resistor needs to be disabled. In this case, place termination at other devices which are situated at the ends of the line.

How to enable termination

120 Ω termination resistor is included on the RUT955 PCB and can be enabled by shorting contacts (shown in the picture below), placing 2.54mm pitch jumper:

Services rs485 jumper.PNG

But on the RUT955 improved housing design PCB the terminator connections are missing. Since the housing of the RUT955 improved model is not disassembled, the termination resistor must be connected to the outside of the device by connecting a through-hole 100 ohm resistor to the terminal contacts. The connection of the external resistor is shown in the figure:

Networking rut manual services rs485 termination v1.png

Number of devices in an RS-485 Network


One RUT955 RS-485 driver is capable of driving a maximum of 32 receivers, provided that the receiver input impedance is 12 kΩ. If receiver impedances are higher, the maximum number of receivers in the network increases. Any combination of receiver types can be connected together, provided their parallel impedance does not exceed RLoad > 375 Ω.

Modes of different serial types in RS232 and RS485

Console


In this mode the serial interface set up as a Linux console of the device. It can be used for debugging purposes, to get the status of the device or to control it.

Over IP


In Over IP Serial type the router provides a connection to a TCP/IP network for the devices connected via serial interfaces.

Mode: Server


Services rs232 rs485 overip server.PNG

Field Value Description
Protocol TCP; default: TCP Specifies the protocol used in the communication process
Mode Server | Client | Bidirect; default: Server Specifies the 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 Specifies that the first hex zeros should be skipped
TCP port integer [0..65535]; default: none The port number used to connect to the server
Timeout (s) integer; default: none Disconnects clients after the amount of inactivity time (in seconds) specified in this field

Mode: Client


Services rs232 rs485 overip client.PNG

Field Value Description
Protocol TCP; default: TCP The protocol used for data transmission
Mode Server | Client | Bidirect; default: Server Server - waits for incoming connection

Client - initiates the connection

Bidirect – acts as a client by default, but at the same time waits for incoming connections
No leading zeros yes | no; default: no Skips first hex zeros
Server address host | ip; default: no Server address to which the client will connect to
TCP port integer [0..65535]; default: none The port number used to listen for incoming connections
Reconnect intervals (s) integer; default: none Indicates the time period between reconnection attempts

Mode: Bidirect


Networking rut manual rs232 rs485 over ip bidirect.png

Field Value Description
Always reconnect yes | no; default: no When checked, established a new TCP connection aftereach message.
Mode Server | Client | Bidirect; default: Server Serial connection operating mode.
  • Server - this device wait for incoming connection.
  • Client - this device initiates the connection
  • Bidirect – acts as a client by default, but at the same time waits for incoming connections
Server address host | ip; default: no IP address or hostname of the server that this client will connect to.
TCP port integer [0..65535]; default: none Internal port number used to listen for incoming connections.
Reconnect intervals (s) integer; default: none Time period (in seconds) between reconnection attempts in case a connection fails.
Port integer [0..65535]; default: none Server's listening port number.
Timeout (s) integer; default: none Disconnects client after the specified timeout of inactivity
Output OC Output | Relay Output; default: OC Output Output to indicate that application switched from client (default) to server state.
Output state integer [0..1]; default: 0 Output state value after the application reverts to server mode.

Modem


With Modem Serial type, the router imitates a dial-up modem. Connections to TCP/IP networks can be established using AT commands. The connection can be initiated by the device connected via serial interface with an ATD command: ATD <host>:<port>. If Direct connect settings are specified, the connection to the server is always active. Data mode can be entered by issuing the ATD command. Incoming connections are indicated by sending a RING to the serial interface.

Services rs232 rs485 modem v 2.png

Field Value Description
Direct connect host:port | ip:port; default: none Maintains a constant connection to specified host. Leave empty to use an ATD command to initiate the connection
TCP port integer [0..65535]; default: none The port number used to listen for incoming connections. Leave it empty to disable incoming connections
Initiation string string; default: none A command string that will be sent to the modem to initiate it in some special way
No extra CR LF in response yes | no; default: yes Removes extra CR LF before and LF after response code

This is the AT command set* used in Modem mode of the serial interfaces:

COMMAND DESCRIPTION USAGE
A Answers incoming call To answer incoming connection: ATA
D Dial a number To initiate data connection: ATD <host>:<port>
To enter data mode with Direct connect settings: ATD
E Local echo Turn local echo on: ATE1
Turn local echo off: ATE0
H Hang up current call To end data connection: ATH
O Return to data mode To return to data mode from command mode: ATO
Z Reset to default configuration To reset the modem to default configuration: ATZ

* Only these commands are supported in Modem mode.

Modbus gateway


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

Services rs232 rs485 modbus gateway.PNG

Field 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: none The port number used 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 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 permitt 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

NTRIP client


Networked Transport of RTCM via Internet Protocol (Ntrip) is a protocol for streaming differential GPS (DGPS) data over the Internet in accordance with specification published by RTCM.

Networking rut manual rs232 rs485 ntrip client.png

Field Value Description
IP address ip; default: 0.0.0.0 IP address of an NTRIP server.
Port integer [0..65535]; default: none Port number of the NTRIP server.
Mount point string; default: none NTRIP mount point.
Data format NTRIP V2.0 TCP/IP | NTRIP V2.0 RSTP/RTP | NTRIP V1.0 | Automatic detection | NTRIP V2.0 UDP; default: NTRIP V1.0 Specifies the version of NTRIP.
User name string; default: none Username for authentication to NTRIP server.
Password string; default: none Password for authentication to NTRIP server.
Initial NMEA-GGA string; default: none Optional NMEA string that will be used as the default value when initiating a connection to an NTRIP server (this value is only sent to the server if there is no NMEA from router's GPS device).
Acquire NMEA-GGA yes | no; default: no Obtains initial NMEA-GGA string from GPS module of this device. Only works if GPS service is enabled and location fix is obtained at the time of NTRIP service start.