Spectrum Controls 1769sc-HART Modules - Manual (Page 108)

Compact IO™ Isolated HART Analog Output Module
User’s Manual 0300217-03 Rev. A
7-42
discussion of the short frame form. In either format, the single-bit address of the master
is the most significant. Only two masters are allowed for example, a control system and a
hand-held communicator. The most significant bit of the address field differentiates
these two hosts. Primary masters such as the OF4IH use address 1, and secondary
masters such as handhelds use address 0. Please see figure below.
Figure 7-12 (Long Frame Address)
Note: The OF4IH does not support burst mode.
The 1 byte Device Type code is allocated and controlled by the manufacturer. The 3 byte
Device Identifier is similar to a serial number in that each device manufactured with the
same Device Type Code must have a different Device Identifier. The OF4IH
automatically pulls for the device specific codes using the Auto-acquisition process. The
device specific codes that are acquired using this process can be seen in Table 2.
Command
The command byte contains an integer (0 to hex FF or decimal 257) that represents one
of the HART commands. Code 254 is defined as an expansion code and is followed by
another byte allowing more than 256 different commands to be defined if necessary. The
received command code is echoed back by the slave device in its reply.
There are three categories of commands: universal commands, which all HART devices
must implement; common practice commands, which should be used if the particular
function is provided; and device specific commands, which are for functions more or less
unique to a particular slave device.
Byte Count
The byte count portion of the message contains an integer value representing the number
of bytes that form the remainder of this message excluding the checksum. In other
words, the byte count determines the length of the data and status.
Status
Status is included only in reply messages from a slave. It consists of two bytes of bit-
coded information. The first byte indicates communication errors, if any. Otherwise if
communication was good, this byte may indicate the status of the received command
such as a busy device, or a command not recognized. The second status byte indicates
the operational state of the slave device. A properly operating slave device will have
both status bytes set to logical zero. The meaning of the individual status bits can be
found in Appendix D.