Spectrum Controls 1734sc-IE2CH - Manual (Page 50)

4-10 Point IO™ 2/4 Channel Analog HART Module
User’s Manual Pub. 0300257-01 Rev. A
pass-through command and the pass-through response data, if available.
There is a handle timeout associated with the final reply message. After the HART
module obtains the requested information from the HART device, it will start a handle
timeout timer. Refer to section 3.5.4for information regarding how to set the handle
timeout. The reply message will be cached in memory during the handle timeout period.
When the handle timeout timer expires the reply message will be discarded, and another
pass through message will be serviced without being rejected. The user defined handle
timeout is in the range of 1 to 255 seconds.
!
Attention
If the HART message being sent or received using the pass-through
command contains double-words or floating point values, the order of
the bytes must be reversed.
Depending on the HART command, the data contained within the HART message may
include floating point numbers or double integers. If a floating point or double integer is
contained within the HART message, the user must be aware that the order of the bytes
that make up the float or double will need to be reversed. The reason for this is related to
how the bytes are stored in the PLC.
The PLC stores the bytes in memory in a format referred to as "little-endian". Little-
endian is an order in which the "little end" (least significant value in the sequence) is
stored first (at the lowest storage address). However, HART devices transmit the byte
data in the reverse order or as you may have guessed "big-endian".
Pass-Through Init
The following table describes the class, service code and instances required to send the
Pass-through Request message.
Table 4-11 (Pass-through Init)
Unconnected Message
Field
Value Definition
Message Type “CIP Generic”
Service Code 0x4E Pass-through Request
Class Name 0x035D HART Object
Instance 1-4 Selects channel 0-3
Object Attribute None, leave this field blank
Request Size
2-257 bytes HART Data & Cmd
Reply Size 4 bytes Number of Data bytes returned
The following tables describe the possible message responses.
Table 4-12 (Request Data)
HART Ladder Passthrough Init - Request packet structure
Offset Field
Value Definition
0 HART Command 0-255 (1 byte) HART Command Number
1 HART Data Size 0-255 (1 byte) Number of Data bytes for selected HART
command
2-256 HART Data bytes 0-255 (1 byte) HART command data