Hi everybody,

The first question is : when to use 2 wire and when to use 4 wire loop for the 4-20mA hart communication ?

The second one is : what is the distance limitation for each or in another way what is the maximum resistance for each loop ?

Hello adham;

If you look on the HART Foundation site, you will have acces to many technical documents explaining the HART specifications, such as loop resistance, voltages, resistance....

http://en.hartcomm.org/hcf/org_mbr/documents/documents_spec_list.html

Or you could go the easy way, searching in the HART modules technical information provided by the manufacturer; for example:

Analog input Hart :

https://support.industry.siemens.com/cs/ww/en/view/105037964

In the case of the 6ES7134-6TD00-0CA1 HART analog input, the maximum shielded cable that can connect to a HART instrument should not exceed 800m (point-to-point connection). See below.

Hope this helps,

Daniel Chartier

1)  Distance

As Daniel's attachement shows, the maximum distance is determined by the devices used.

However, a maximum cable spec is not always stated.  Allen Bradleys' HART enabled cards do not list a cable distance maximum.

The theoretical limit for HART is 3000m with perfect cabling and a power supply that meets HART requirements.  The limit is the cable capacitance.

Here's a link to a table from a whitepaper, 00840-0100-4075, Rev AA, by Emerson/Rosemount.

http://s32.postimg.org/vd581fz11/Emerson_HART_cable_guidelines.jpg

Since Emerson invented HART, they're probably in a good position to make the statement.

I've read that the HART specification HCF_SPEC-054 says "at least 1500 m" (5000 ft), if high quality, low capacitance, heavy gauge/large cross section cable AWG 18 (0.75 mm2) is used.

FYI, HART does not publish their specs in electronic form (paper only) so getting your hands on the real spec means you have to buy it a paper copy.

2)  Loop resistance

2 wire loop powered transmitter typically have a 'load line' graph of loop resistance vs power supply voltage.

4 wire devices (analog outputs) typically state how much loop resistance the output can drive.

HART requires a minimum of 230 ohms of loop resistance to operate.
The analog input resistance is what it is.   (positioners or I/P's typically have a 'typical impedance' because it varies with load)
The loop cabling resistance can be calculated from the length used.

3)  2 wire vs 4 wire

I know the PS2 positioner can be wired 2 wire or 4 wire, but I'm not sure there are many other devices that can be.   Whether 2 or 4 wire, the HART signal is only resident on the 4-20mA signal wires, not on the power supply wiring for a 4 wire connection.

You're asking these open ended questions for a reason.  What's the reason?  What are you trying to accomplish?

 

Thanks a lot for both of you for this support, I got my point.

I am no asking for a certain reason but just need to buy Endress & Hausser Radar transmitter and there are 3 options for me:

1) 2 wire Hart

2) 4 wire Hart

3) Profibus communication

I removed the profibus out of equation as the cable is expensive and no need for this high cost, but was wondering is it better to use 2 wire or 4 wire.

however, what I understood from your explanation that 2 wire is better as long as the device doesn't consume much power.

Hello adham;

The choice of 2-wire (instrument loop-powered from the analog input module) or 4-wire (instrument power provided by and external power supply) will very often be dictated by the instrument construction and design. Some manufacturers will provide separate power input and signal input connections (therefore you should use 4-wire configuration), some will expect loop-power (and you must select 2-wire connections).

You must learn to adapt to the caracteristics of the instruments you select (or choose) for an application.

Hope this helps,

Daniel Chartier