Rockwell Automation 1771-TCM,D17716.5.108 TEMPERATURE CONTROL MOD - Manual (Page 25)

Chapter 3
Communicating with Your Module
3-3
2. Read (block-transfer) the system status block from the module on a
regular basis (using a timer to trigger block-transfers) to monitor the
status of the PID loops. The module’s period for updating this
temperature data is approximately one second. The module’s period
for updating the TPO bits it makes available thru block-transfer is
approximately 500ms.
3. Whenever there is no block-transfer in process for the temperature
control module, pass the single-transferred TPO of each loop on to
the output module driving the heating/cooling elements
Single-transfer of I/O data occurs automatically in each I/O scan. That is,
a single unit of I/O data can be exchanged between each I/O module and its
corresponding address in the I/O image table. The size of the unit of I/O
data available for transfer to/from each I/O module is determined by the
I/O addressing mode selected for the I/O chassis. The 1771-TCM module
uses 16 bits of input image and 16 bits of output image. Therefore, the I/O
chassis must be set for either 1-slot or
1 @
2
-slot addressing.
Control Bits
The meaning of the control bits in the high byte of the output image word
and the low byte of the input image word are as follows.
7 6 5 4 3 2 1 0
1 0 0
1 = OK, 0 = Watchdog TimeĆout
1 = Heat
1 = Cool
High byte of output image word
Low byte of input image word
(control bits)
0 0
Your ladder logic must write to the high byte of the output image word of
the 1771-TCM module to tell it whether to send the 8 heat bits or the 8
cool bits. Your ladder logic must then read the low byte of the input image
word of the 1771-TCM module to determine whether it has sent the 8 heat
bits or the 8 cool bits. Based on the value found in the low byte of the
input image word, your ladder logic must move the high byte of the input
image word of the 1771-TCM module to the output image byte of either
the outputs controlling heating or the outputs controlling cooling.
In reading the low byte of the input image word, if your ladder logic
detects bits 7, 6, and 0 to be off (watchdog time-out) it must turn off all
heat and cool outputs. At that point you must cycle power (off then on) to
the 1771-TCM module, and then reconfigure the module. If the watchdog
timer bit stays off, replace the module.
SingleĆTransfer
Programming