WiFi Modbus TCP-to-RTU Gateway + I/Os

Wx100-ModbusGW is a 2-in-1 Modbus device. For the price of a single low-cost unit, you effectively receive 2 devices:

1. A wireless Modbus-TCP to Modbus-RTU converter gateway.
2. A Modbus-TCP digital/analog I/O boards.

Best of all, both the gateway and the I/O functions are available simultaneously to the SCADA host or the IIoT client.

Built on the Wx100 super PLC platform, this device is supplied as a standalone, ready-made gateway device suitable for any SCADA or IIoT applications – no programming required!

Wx100-ModbusGW  enables any legacy RS485 connected Modbus-RTU devices to instantly become Wireless Modbus-TCP server . All settings such as the serial port baud rate, WiFi SSID etc can be accomplished using the built-in keypad+OLED display, or from a PC or a smartphone.

1. A Modbus-TCP client, such as a SCADA host, can send Modbus-TCP command packets to the gateway via LAN or the Internet. The gateway will automatically convert the Modbus-TCP command packets it receives into Modbus RTU protocol and send out to its RS485 bus. Likewise, when the Modbus RTU device send a response packet, the gateway will convert the RTU response packet it receives into Modbus-TCP response packet and send out via the network router. Hence any Modbus-RTU device connected in parallel to the gateway’s RS485 port can be accessed from a single Wx100-ModbusGW device
2. Wx100-ModbusGW also provides industrial voltage level digital and analog I/Os on a 20-pin headers on its side that provides 8 digital inputs, 6 analog inputs and 6 digital outputs that can be directly read and written by the SCADA host. The I/O signals can be tapped using a simple 20p ribbon cable. Alternatively, an optional terminal board with screw terminals is available for purchase to facilitate quick wiring.

 

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WiFi & Modbus-TCP
WiFi Frequency: Supported Types: WiFi Speed: Number of incoming connections: Supported Modbus functions:	2.4GHz 802.11b, g, n  @HT20 Up to 72Mbps >= 5 Function 01,02,03,04,05,06,15,16
RS485 Serial Port
Supported Baud Rates:Data & Stop Bits: Parity:	2400,4800,9600,19.2K,38.4K, 62.5K,100K,115.2K and 230.4Kbps 7 or 8 data bits; 1 or 2 stop bits; Odd, Even or None
I/Os
Input OFF voltage: Input ON voltage: Output OFF voltage: Output ON voltage: Output ON max current:	<= 3VDC >= +5VDC Vp – 2V (Vp is the Power supply voltage) < 0.1V <=0.5A
Power Consumption
Voltage (Recommended) Voltage (Min. & Max) Current:	12 to 24VDC 9V & 30VDC 50mA @24VDC

 

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The Wx100-ModbusGW is flexible and can be installed on the front cover of any industrial enclosure. It can also be installed readily on a DIN rail or using just screws, nuts and PCB standoffs. Please refer to Appendix A for detailed illustration on the various options for installing the Wx100-ModbusGW.

4.1 ModbusTCP Gateway Wiring

If you are only using Wx100-ModbusGW as a gateway and not using its I/O function, then wiring is really simple. You only need to connect a regulated, 12 to 24V DC (maximum 30V) power supply to the +/- power supply terminals and a two-wire RS485 cable to the +/- terminal of RS485 connector, as shown below:

If desired you can also cover up the unused I/O header pins using the silicone dust cover supplied with the product after installation of the Wx100-ModbusGW to protect the pins against accidental short circuiting.

4.2 Wiring to The I/O Pins

Wx100-ModbusGW features 14 industrial voltage level digital and analog I/Os (see I/O specifications in Section 3) on a 20pin, 0.1″ pitch IDC header that can be read/written directly by the Modbus client.

For absolute lowest cost you can simply make a matching ribbon cable with a 0.1″ pitch IDC connector and tap the signals directly from the wires. The I/O signals are located on the header pins as shown below

(Note you can ignore pin 19 and 20 as these two are alternative pins to supply power to the Wx100-ModbusGW module and they are not needed if you have already wired the power directly to the modules’ power supply screw terminals).

 

4.3 Wiring Using Optional Wx-TERM8

However, for more professional installation we recommend purchasing an optional Wx-TERM8 board which includes a connecting ribbon cable, a PCB that provides  LED indicators for digital i/os and screw terminals to simplify wiring to your field sensors or actuators, as shown below:

 

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The Wx100-ModbusGW connects to the TCP/IP network via Wi-Fi. Hence the first thing that you need to do is to configure the device using the known SSID of the Wi-Fi network in order to connect to it to perform its function.

The Wx100-ModbusGW has a built-in keypad and an OLED display which makes it very convenient to configure the Wi-Fi parameters without any other tools. However, It is also handy if you prefer to configure it use your Smartphone or PC with the help of the built-in web App.

Once you have setup the WiFi, the only other thing you need to do is to setup the RS485 communication parameters to make it work successfully as a gateway to connect RS485 based Modbus RTU devices wirelessly  to the TCP/IP.

Most  Modbus RTU devices can only communicate using a predefined set of “serial parameters”. These include the bit rate (aka BAUD Rate), the number of data bits (7 or 8 bits), the number of stop bits (1 or 2 bits) and the “parity” (none, odd or even).  All connected Modbus RTU devices must be configured to the same set of serial parameters in order to share the same “RS485 bus”.  Please refer to the device manual of each connected RTU device on how to change their serial communication parameters.

Once the set of serial parameters have been chosen, the Wx100-ModbusGW must be configured to operate on the same set of parameters. You can do this quite easily using its built-in keypad and display, allowing you to change the parameters at anytime, as follow:

a) Setup RS485 Using Keypad

To change a parameter, press the number associated with the parameter (e.g. press 1 to change the “Baud Rate”) or press the “Up/Down” arrow key on the keypad to move the highlight bar to the parameter, then press the green “Enter” key and it will cycle through all the supported baud rates (from 2400bps to 230.4Kbps).

If you press the “4” or “5” key the menu will display the second page, allowing you to change the “Parity” as well as RTU time-out (the amount of time the gateway will wait for a response from the RTU before declaring time-out error to the Modbus TCP client).

 

b) Setup RS485 Using PC or SmartPhone

You can also set up the RS485 using your PC or Smartphone web browser and point to its IP address and load the following web page:

http://[IP address]:9080/netconfig.htm

(please remember to always enter the port number “:9080” following the IP address)

The lower section of the above web page contains all the available selections for each parameter. For example if you click on the “Baud Rate” choice box you can select from one of the commonly supported baud rate:

Once all the serial parameters have been defined, click “Save RS485 Settings” button to save the parameters. The changes to RS485 settings are applied immediately and reboot is not required.

c) Changing Gateway ID

The Wx100-ModbusGW possesses its own 8-bit Modbus ID that is important such that it is what determines whether a Modbus TCP command packet it receives is destined for itself (i.e. to be used for I/O read and write), or to be translated into Modbus RTU packet (i.e. to function as a gateway) as explained in Section 7 – Using Wx100-ModbusGW

The default ID for the Gateway is 255 and we recommend not changing it unless absolutely necessary. In Modbus TCP specifications, both ID=0 and ID=255 is defaulted for the gateway itself and hence must not be assigned to any slave RTU.

If for any reason you need to change the gateway ID, you can do it using either the keypad as described in a) or via the Web App as described in b).

Note:

1. If the new ID you just changed happen to also be the same ID as one of the RTU devices connected to the RS485 port, then any Modbus TCP packet will no longer be routed to that slave device. In other words, the Modbus TCP client (e.g. SCADA Master) will lose communication with the RTU device but instead any read/write command will be processed by the gateway itself.
2. The standard RTU broadcast command (using Modbus ID = 00) is not supported by the gateway.

 

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The Wx100-ModbusGW functions immediately as a Modbus-TCP to Modbus-RTU gateway and as a Modbus-TCP I/O board once the PLC is booted up and successfully connect to the WiFi network. The 8-bit Modbus ID embedded inside the Modbus TCP command packet will determine where the packet goes to eventually, as illustrated in the following diagram:

a) Using The I/O Board Function

All the ModbusTCP client need to do is to connect to port 502 (standard ModbusTCP port) of the gateway’s IP address and and start sending Modbus TCP protocol packet.

The Wx100-ModbusGW will automatically process all incoming Modbus-TCP protocol packet it receives. The gateway first examines the 8-bit Modbus ID embedded in the TCP packet. If the Modbus ID matches the Wx100-ModbusGW own ID (default is 255 decimal, or FF Hex), then it will treat the command packet as destined for its own I/O and internal registers and will respond accordingly. The client can read the device digital inputs and outputs using Modbus function 1,2,3 or 4 and write to the device digital outputs using Modbus function 5,6 or 16. This is how the client can control the I/O of the Wx100-ModbusGW device using Modbus-TCP commands.

 

b) Using the Modbus Gateway Function

On the other hand, if the 8-bit Modbus ID in the Modbus TCP packet that the Wx100-ModbusGW receives does not match its own ID, then it will regard the packet as being destined for Modbus RTU devices connected to its two-wire RS485 port. The device now effectively function as a gateway and it will immediately translate the ModbusTCP protocol packet into Modbus RTU packet and send out to the RS485 port.

If a Modbus RTU device that is attached to the gateway’s RS485 bus has a matching ID to the RTU command packet, it will process the RTU command packet and send back a RTU response packet. The gateway upon receiving the RTU response packet will in turn translate it into Modbus TCP protocol packet and return to the Modbus TCP client. This completes the command/response Modbus TCP protocol exchange between the Modbus TCP client and the Modbus RTU device. The process occur automatically without any further user intervention.

 

c) Displaying Modbus Gateway Statistics

Wx100-ModbusGW provide a simple statistics for its work as a Modbus-TCP to Modbus-RTU gateway.  By pressing the MENU key on the keypad and then select “1. Run MBTCP Gateway” the OLED screen will display real time info about the number of Modbus TCP packets it receives and the number of RTU errors it encounters as shown in the following pictures:

Notes:

In normal operation, it is not necessary to display the real time statistics in order for the Wx100-ModbusGW to function as a gateway. It is provided more as a troubleshooting tool during commissioning and during maintenance.

In fact running the display continuously will consume some CPU performance as the CPU must spend time refreshing the slow graphical display, resulting in slower performance and perhaps slightly higher error rate than when it is not running the display.

To exit the display screen, press the “hamburger” MENU key.

 

d) Troubleshooting Gateway Error

In normal operation, if all the RTUs accessed by the Modbus TCP client are connected to the RS485 port and are working properly, then there should be very low error count (typically < 0.01%). However, if the display is showing a lot of errors  then something is not right and should be rectified to ensure that the gateway performance is not compromised.

The Modbus TCP client side software should be able to report no response from a particular slave ID # and you can then investigate the RTU device with that slave ID to check for configuration errors.

The most likely causes of communication error with the RTU slave are:

1. The client is trying to access a Modbus RTU slave that is not connected to the RS485 bus.
2. The RTU slave’s RS485 serial port settings is incorrect and not the same as the rest of the connected devices.
3. More than one RTU device connected to the same RS485 bus has the same ID, resulting in conflicting response when both devices try to respond to the same command.
4.  The RS485 wire to the affected RTU slave is broken.
5.  The RTU slave RS485 hardware is faulty.
6. The RTU slave is located too far away from the gateway. Although RS485 specifications allow the bus length to be up to 1000m (4000 ft), long distance is challenging for communication at high baud rate and it may also be necessary to provide proper termination of the RS485 bus as well as deploying pull-up and pull-down resistors on the RS485 bus.
7.  The RS485 serial port setting of the Wx100-ModbusGW is not configured correctly (Note that this should affect communication with all the RTU slave devices and not just a single RTU)

 

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Notes:

1.  All I/Os are mapped to the Holding Registers as well as Input and Output Registers. Up to 16 digital input and 16 digital outputs can be read using Modbus function 03 or 04 (they are treated the same by the device). For example, the Holding Register address #0 contains the logic states of the first 16 digital inputs. The Holding Register address #16 contains the logic states of the first 16 digital outputs.
2. Likewise, you can write to Holding Register address #16 to change all the digital outputs 1-16.  Both Modbus function 06 (write to a single channel) and function 16 (write to multiple channels) are supported.
3. The digital inputs and outputs can also be accessed as individual bits instead of as 16-bit words. The last two columns in Table 8.1  provide their respective Modbus bit addresses. So your Modbus TCP client can access any I/O bit using these addresses. Modbus function 01, 02, for bit read and function 15 for bit write are supported.
4. If your SCADA program can only specify Modbus address in “Modicon” convention, you simply add 4-0001 to the zero-offset binary address in Table 8.1. For bit addresses, simply add 1-0001 to the I/Os’ zero-offset binary bit addresses shown in Table 8.1.
5. The table shows mapping of up to 80 digital inputs, 80 digital outputs and 80 analog inputs. These are expanded I/Os that are available with purchase of optional expansion I/O modules. Please contact the manufacturer for more information if you require more local I/Os than is available on the basic Wx100-ModbusGW.

 

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