Spectrum Controls 1769sc-HART Modules - Manual (Page 79)

Chapter 7: Enabling and Using HART on the 1769sc-IF4IH0
User's Manual 0300215-03 Rev. A
7-11
Section 7.3
Sending
and Receiving
Messages
Sending messages to and from the module is accomplished using a paging scheme. This
paging scheme uses the module’s input and output words to transfer data between the
controller and the module, 38 bytes at a time (i.e. one page at a time). The paging
scheme is utilized to minimize the number of bytes sent and received at one time from the
module’s input and output image. The maximum message size is 257 bytes.
7.3.1 Module Output Tags Used For Messaging
The IF4IH module utilizes 22 words for sending messages and controlling data flow.
The table below shows the output image for the IF4IH module. For more detail regarding
word 0, refer to Chapter 6.
Table 7-6 (Module Output Table)
Word/Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 HS3 HS2 HS1 HS0 UL3 UH3 UL2 UH2 UL1 UH1 UL0 UH0
1
2
3
4..23
24..45 Reserved
Reserved
Message Request Buffer
Packet Just Scanned
Message Master Control
Message Request Size
Word 2 (Message Master Control)
The message master control initiates the paging process and controls the flow of data to
and from the module. The data flow control is accomplished by using the message
master control with the message slave control to manage which pages are being sent and
what direction the page is going, that is, whether the page is being sent to the module or
read from the module.
Figure 7-5
Note: Setting the Message Master Control word to zero resets the paging logic within
the module and allows the next message to be processed.
Word 3 (Message Request Size)
The message request size is the total number of bytes being sent to the module (not just
the current page).
RR|SS
Message Master/Slave Control (Hex)
Page being sent
(Page = 38 Bytes)
Page last received