Usefulness of Conversion from Modbus to HART.

S

Thread Starter

Shailesh Patel

My client wants to have all transmitters with HART protocol. But some special transmitters like weighing transmitter from Sartorius (there is no other option) do not have HART. So the solution is to buy transmitter with 4-20 mA and Modbus on RS-485. Then buy third party’s modus+4-20 mA to HART converter (suitable for transmitter).

Now this converter does not have Device description (DD) file. Converter vendor asked to use Generic DD file from DCS vendor.
Now my quotation is?

1) Since the transmitter is not as per HART standard, is this system useful to client?

2) What type of data available if I use generic DD file for transmitter (any other transmitters).

3) What kind of data required from transmitter (HART) to connect with asset management system.
Is there any technical reason so I can convince client not to have weighing HART transmitter .

Thanks in advance.
Shailesh Patel
 
>1) Since the transmitter is not as per HART standard, is this system useful to client?

I'll take your word that there's a converter that converts 4-20mA to a primary HART variable.

The HART data (in this case, datum) is typically read from an analog input that is inherently a current loop; unless it is a HART multiplexed system, where the current through the loop is fixed and HART is the only process data available.

If the connection of the 4-20mA-to-HART converter is to an analog input, then nothing is gained, because no data beyond the primary process variable. Presumably, the converter converts the analog into HART, but to what end? The analog input will convert the analog to a digital number, too. What's gained with a HART conversion? Why the complexity for no added benefit?

If the 4-20mA-to-HART converter is to be wired into a HART multiplexed network, I'd inquire first as to whether the converter is designed for that service. The advantage of such might be keeping devices on a HART multiplexed network. Whether the benefits outweigh the costs, is another point.

The installations of multiplexed HART networks was really for the diagnostics on continuous processes. Minus diagnostic data, why HART?

>2) What type of data available if I use generic DD file for transmitter (any other transmitters).

Assuming the converter will communicate via universal commands, and if it meets the HART Foundations' guidelines, then the HART Communication Application Guide gives the list of universal commands on page 16:

UNIVERSAL

All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).
- Read manufacturer and device type
- Read primary variable (PV) and units
- Read current output and percent of range
- Read up to four predefined dynamic variables
- Read or write eight character tag, 16 character descriptor, date
- Read or write 32 character message
- Read device range values, units, and damping time constant
- Read or write final assembly number
- Write polling address

>3) What kind of data required from transmitter (HART) to connect with asset management system?

An analog output transmitter just has the process variable, no other information.

You should ask the Modbus-to-HART converter vendor whether there's any data beyond process variables that is available via HART.
 
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