FIELDBUS OR WIRELESS HART INPLACE OF MODBUS

G

Thread Starter

Grace

Dear Experts,

I have a question. The customer would like to have a transmitter with MODBUS output (User configurable update, 2.4 Ghz DSSS, Wi Hart.) and will be connected to a wireless gateway. The available transmitter has only Foundation fieldbus or Hart/ wireless HART output. Can I use the FOUNDATION FIELDBUS? or use wireless HART instead.

Regards,
Grace
 
1) Wireless HART gateways talk Modbus. In wireless HART, HART is protocol used over the airwaves between the field transmitter and the 'gateway', the box that field data comes into at the control room end. Wireless HART gateways routinely use Modbus/TCP (ethernet) slave registers for providing access to the collected field data.

A plant SCADA, HMI, PLC or DCS with a Modbus master (might also be an OPC server with a Modbus driver) reads the data from the wireless HART gateway. Although the field device itself is not Modbus, that usually isn't the requirement. The requirement is to have Modbus access to the data at the control room end.

ISA 100 wireless takes the same approach. Data from the field transmitters is available from Modbus TCP slave registers to a Modbus master in HMI, SCADA, PLC or DCS.

Both wireless HART and ISA 100 run in the 2.4GHz bands.
Both wireless HART and ISA 100 have battery powered field transmitters that do not need external power.

2) Many wireless providers use 2.4GHz wireless radios but do not provide field transmitters, they provide the radio link and a gateway that talks Modbus. The idea is that a field 'node' radio takes an analog 4-20mA output signal from anybody's field transmitter into its analog input and sends the value to the gateway. In the case of Banner Engineering, the gateway radio will either make the value back into a 4-20mA signal or provide it in Modbus RTU (RS-485) or Modbus TCP (ethernet) slave register.

One drawback to this option is that although some vendors have battery powered radios, the battery power is not intended to power the field transmitter. The assumption is that the field transmitter is a 4 wire externally powered device.

Restricting the field transmitter to Modbus output is very limiting as to the choices, if any. Using an analog transmitter with option 2 opens up the choices dramatically. If the wireless HART or ISA100 have the right kind of field transmitter, then either meets the Modbus requirement, at considerably higher cost than option 2.

As far as I know, Foundation Fieldbus has no conversion to Modbus in any fashion.
 
L

Lynn August Linse

Some vendors do run raw Modbus/RTU encapsulated in ZigBee or other wireless.

One serious 'weakness' in any such design for a sleeping, battery-powered sensor is that Modbus requires the Master to poll the slave ... who might sleep 99.9% of the time! So you really want a data-push solution and not a Master-Slave poll solution. Wireless HART & other battery-powered sensor solutions will have the sensor push the data unsolicited when they wake.

The most direct solution if you have an EXISTING WirelessHART device is find a Modbus/TCP to WirelessHART gateway. These will get around the data-push issue because the Modbus poll returns the cached 'last seen data', and obviously can't query the sleeping sensor directly.

A simple internet search for 'Modbus wireless hart' turns up quite a few existing products.
 
J
The Emerson model 1420 WirelessHART gateway can be used in this application

WirelessHART is used between the gateway and the wireless transmitters.

The WirelessHART gateway makes process variables available as Modbus/RTU, Modbus/TCP, and OPC suitable for DCS, PLC, HMI, and other software. All data including setup/configuration and diagnostics is made available using the HART-IP protocol suitable for intelligent device management software:
http://www.eddl.org/DeviceManagement/

Cheers,
Jonas
 
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