Spectrum Controls 1769sc-HART Modules - Manual (Page 111)

Chapter 7: Enabling and Using HART on the 1769sc-IF4IH0
User's Manual 0300215-03 Rev. A
7-43
Figure 7-10f
Note: The ladder in Figure 7-10 can be found in the project sample file located on our
website at (www.spectrumcontrols.com)
Section 7.5
HART Protocol
Overview
In order to read and write HART commands to and from the field device reliably using
the IF4IH, you must have a basic knowledge of the HART protocol. This section will
explain in detail the various pieces that make up the HART message and how to
formulate the message and send it to the field device using the module specific Pass-
Through command, which was described earlier in this chapter.
7.5.1 Message Format
HART protocol specifies a message structure as follows:
Figure 7-11 (HART Message Structure)
Preamble Start
Character
Address Command Byte
Count
Status Data Checksum
Note: The HART protocol supports two different formats, long and short frame. Older
HART instruments (up to HART revision 4) used a short frame format. In this format,
the address of the slave device is either 0, for non-multidrop devices using the 4-20mA
current signal, or 1-15 for multidrop devices.