Spectrum Controls 1769sc-HART Modules - Manual (Page 109)

Chapter 7: Enabling and Using HART on the 1769sc-OF4IH
User’s Manual 0300217-03 Rev. A
7-43
Data
This portion of the HART message contains the data, if any, for the command. Not all
commands or responses contain data. For those that do, up to 25 bytes can be included.
Data may be in the form of unsigned integers, floating point numbers, or ASCII character
strings. The number of bytes of data and the data format used for each item are specified
for each HART command.
Checksum
The checksum byte contains the exclusive-or (longitudinal parity) of all the bytes that
precede it in the message starting with the Start Character. This provides a further check
on transmission integrity, beyond the parity check on the 8 bits of each individual byte.
7.5.2 Sending a HART Command to a Field Device via Pass-through
Now that you're familiar with the bits and pieces that make up a HART message, the next
step will be to formulate a message and successfully send the message to the field device
using the pass-through command. The first step is to formulate the message and populate
the source tag Of4ih0PassThruReqTX. This tag is used in the ladder sample shown in
Figure 7-10.
Table 7-18
Tag Nam e Value in Hex Description
HART_PASS_THRU_REQ_TX[0] 00 HART channel
HART_PASS_THRU_REQ_TX[1] 01 Pass-through command designator
HART_PASS_THRU_REQ_TX[2] 82 Start charcter
HART_PASS_THRU_REQ_TX[3] BE Long address byte 0
HART_PASS_THRU_REQ_TX[4] 02 Long address byte 1
HART_PASS_THRU_REQ_TX[5] 0C Long address byte 2
HART_PASS_THRU_REQ_TX[6] 77 Long address byte 3
HART_PASS_THRU_REQ_TX[7] 37 Long address byte 4
HART_PASS_THRU_REQ_TX[8] 23 HART command = 35 decimal
HART_PASS_THRU_REQ_TX[9] 09 Byte count
HART_PASS_THRU_REQ_TX[10] 20 Range units code = 32 decimal
HART_PASS_THRU_REQ_TX[11] 44
HART_PASS_THRU_REQ_TX[12] 16
HART_PASS_THRU_REQ_TX[13] 00
HART_PASS_THRU_REQ_TX[14] 00
HART_PASS_THRU_REQ_TX[15] C3
HART_PASS_THRU_REQ_TX[16] 16
HART_PASS_THRU_REQ_TX[17] 00
HART_PASS_THRU_REQ_TX[18] 00
HART_PASS_THRU_REQ_TX[19] FF Checksum
Upper Range value (This is a
floating point value = 600.0) Note:
The bytes are in reverse order.
Low er Range value (This is a
floating point value = -150.0) Note:
The bytes are in reverse order.
The HART message string, shown in Table 7-18, performs HART command 35 (write
range values). Once the tags are populated with the HART message, the message can be
sent using the ladder in Figure 7-8. The reply for the HART command will be found in
the Of4ih0PassThruQryRX tag. The response message should look like the table shown
below.