Spectrum Controls 1756sc-OF8H Series A - Manual (Page 108)

94 ControlLogix ™ Analog HART Modules
Figure 7.10e (HART Pass-Through Request and Query Process)
Note: The ladder in figure 7.10 can be found in the project sample file
located on our website at (www.spectrumcontrols.com)
HART Protocol HART Protocol HART Protocol HART Protocol HART Protocol
O v O v O v O v O v e r e r e r e r e r v i e v i e v i e v i e v i e w w w w w
In order to read and write HART commands to and from the field device
reliably using the IF8H or OF8H, 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.
Message Format
HART protocol specifies a message structure as follows:
Figure 7.11 (HART Message Structure)
PREAMBLE START CHARCTER ADDRESS COMMAND BYTE COUNT STATUS DATA CHECKSUM
Each item of the message structure shown above is explained as follows.
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.
HART revision 5 introduced the long frame format. In this format, the
address of a slave device is a worldwide, unique 38-bit number derived