Siemens SIMATIC S7-200 System Manual page 448

S7-200 Programmable Controller System Manual
EM 231 RTD Status Indicators
The RTD module provides the PLC with data words that indicate temperatures or error conditions.
Status bits indicate range error and user supply/module failure. LEDs indicate the status of the
module. Your program should have logic to detect error conditions and respond appropriately for
the application. Table A-34 shows the status indicators provided by the EM 231 RTD module.
Tip
The channel data format is twos complement, 16-bit words. Temperature is presented in 0.1
degree units. (For example, if the measured temperature is 100.2 degrees, the reported data is
1002.) Resistance data are scaled to 27648. For example, 75% of full scale resistance is
reported as 20736.
(225Ω / 300Ω * 27648 = 20736)
Table A-34 EM 231 RTD Status Indicators
Error Condition
No errors
24 V missing
SW detects open wire
Out of range input
3
Diagnostic error
1
Range status bit is bit 3 in module error register byte (SMB9 for Module 1, SMB11 for Module 2, etc.)
2 User Power Bad status bit is bit 2 in module error register byte (such as SMB 9, SMB 11, refer to Appendix D.)
3 Diagnostic errors cause a module configuration error. The User Power Bad status bit may or may not be set before the module
configuration error.
If the PLC has read the data:
All 4 channels in the EM 231 Analog Input RTD 2 Inputs are updated every 405
-
milliseconds.
All channels in the EM 231 Analog Input RTD 4 Inputs are updated every 810 milliseconds.
-
If the PLC does not read the data within one update time, the module reports old data until the
next module update after the PLC read. To keep channel data current, it is recommended that the
PLC program read data at least as often as the module update rate.
Tip
When you are using the RTD module, be sure to disable analog filtering in the PLC. Analog
filtering can prevent error conditions from being detected in a timely manner.
Open wire detection is performed by software internal to the RTD module. Out of range inputs and
detected open wire conditions are signaled by setting the range status bit in the SMB and by
setting the channel data up or down scale per the switch settings. Open wire detection takes a
minimum of three module scan cycles and can take longer, depending on which wire(s) are open.
Open Source+ and/or Source-- wires are detected in the minimum time. Open Sense+ and/or
Sense-- wires can take 5 seconds or more to detect. Open sense lines can randomly present valid
data, with open wire detected intermittently, especially in electrically noisy environments. Electrical
noise can also extend the time it takes to detect the open wire condition. It is recommended that
open wire/out of range indications be latched in the application program after valid data has been
reported.
Tip
If you have an unused channel, you can wire the that channel with a resistor in place of the RTD
to prevent open wire detection from causing the SF LED to blink. The resistor must be the
nominal value of the RTD. For example, use 100 ohms for PT100 RTD .
434
SF LED
Channel Data
Red
Conversion data OFF
32766 OFF
--32768/32767 BLINK
--32768/32767 BLINK
0000 ON
24 V LED
Range Status Bit
Green
ON 0
OFF 0
ON 1
ON 1
OFF 0
24 VDC User Power
1
2
Bad
0
1
0
0
3
note