1. Sometimes you do have to add the 250 ohm resistor to existing loops.
Lots of existing loops already have 250 ohms or input resistance on the analog input. But that's by no means a given. Some input resistances are 10 ohms, some 50 ohms, some 100 ohms, it's all over the map. Rarely are analog inputs more than 250 ohms.
A 24Vdc power supply will drive 20mA thought 500-600 ohms of loop resistance. You can usually add the 250 ohms in the loop since the copper wire is usually negligible (depends on the distance and wire gauge), and an analog input resistance of 250 and 250 for your HART resistor will not load the loop to where the power supply can't drive the full 20mA.
On the bench, with a 2 wire transmitter with just a power supply, the power supply will 'eat' all the HART signal and you'll never get a HART connection with the resistor.
2. Pactware is freeware software but it has a significant learning curve. Try Vega's or Krohne's version which are still "open".
Pactware requires
- a HART driver which is free that enables HART communications.
- the vendor supplied DTM file for the specific device. Not all HART devices have a DTM.
I've heard that there's a generic HART driver for 10 basic HART commands, but I believe it's licensed driver, not a freeware driver. For use with Siemens devices there's a driver that enables the DTMs (I don't think any other vendor requires a DTM driver).
3. Be warned that the old Mactek serial modems can be flakey when run on USB/RS-232 ports.
Notes from serial port days - port setting change for the Mactek:
For serial HART modems, in Win XP Pro;
change the COM port buffer setting to ‘low’
- Right-click on 'My Computer'
- select Properties >Hardware > Device Manager > Ports > (expand) > Communications Port (COM 1…8)
- double-click COM connection (i.e. COM1)
- Properties > Port settings > Advanced...
- Either uncheck “Use FIFO buffers . . “ or set the value for both the transmit & receive buffers to ‘low (1)’
- Restart Windows for changes to take effect
4.For Mactek USB HART modems, Windows 10 requires the latest version Mactek'.s
SN 314386 and prior only work on Windows 7 and prior
SN 314387 and after work on Windows 10