MicroLogix 1400 RTD Module

[Jason Ozolins]

When testing out micro, after booting the PLC without any RTDs connected, once connecting an RTD it takes up to 5 minutes before a temperature reading is valid. i.e. if you wait 4 minutes before plugging in an RTD, the temperature will be incorrect for 1 minute. If you plug it in right away, it will be 5 minutes.

Any ideas why?

[Geospark]

When the 1762 RTD module powers up or goes to Run Mode it performs an auto-calibration. The Resistance Temperature Detectors (RTDs) must be connected to the module before the controller reaches Run Mode or else the auto-calibration will not have been carried out properly and the readings will be incorrect.

If you do connect the RTDs after going to Run Mode, then transition the controller to Program Mode and back to Run Mode. This will force another auto-calibration and the readings should now be correct.

Regards,
George

[Jason Ozolins]

Thank you!

[rupej]

Originally Posted by Geospark

When the 1762 RTD module powers up or goes to Run Mode it performs an auto-calibration. The Resistance Temperature Detectors (RTDs) must be connected to the module before the controller reaches Run Mode or else the auto-calibration will not have been carried out properly and the readings will be incorrect.

If you do connect the RTDs after going to Run Mode, then transition the controller to Program Mode and back to Run Mode. This will force another auto-calibration and the readings should now be correct.

Regards,
George

Just curious, do you know what it is calibrating and why?

[Jason Ozolins]

Another question regarding the configuration word. Is it possible in a 1400 to access/write to the configuration word 4, bit 0 via ladder? The manual (screen shot attached) says you can write to it but, I can't figure out how to address it. I'd like to disable/enable the autoconfig in the program.

[Jason Ozolins]

[Geospark]

Sorry for the delay. I hadn't time to look into this for you until today...

Originally Posted by Jason Ozolins

...Is it possible in a 1400 to access/write to the configuration word 4, bit 0 via ladder?...I'd like to disable/enable the autoconfig in the program.

I think the screenshots that you've posted up are from Publication 1762-UM003A-EN-P (1762-IR4) and the word arrangement is from Page 3-7, while the "TIP" is from Page 3-20.

They seem to suggest that this 1762 RTD module will allow you to write to the Configuration words through ladder logic, but as far as I can tell, this does not seem to be the case, normally. I have never attempted to do this before having always just configured the module as normal.

The 1762-IR4 RTD module can be used with the MicroLogix 1100, 1200 and 1400 controllers. When you add this module to the IO Configuration for these controllers, it automatically creates the six Input words addressed to the slot it was added to. Four Input words (Word 0 - 3) are used for the module's four individual Inputs and two Input words (Word 4 & 5) are used for the Status data - General Error, Open Circuit, Over-Range or Under-Range. These Input words are exposed to the user so they can read the Input and Status data from the module and use those addresses in ladder logic.

The module also uses five Configuration words. Four of these words (Word 0 - 3) are for the configuration of the module's four individual Inputs and one word (Word 4) is used just to Enable/Disable the Cyclic Calibration mode. However, while the six Input word addresses are exposed to the user, the five Configuration word addresses are not. These Configuration words are only made available to the user via the module's Adv Config tabular user interface. When you make changes under these tabs they are writing to the Configuration words in the background. On the cal tab you can Enable/Disable the Cyclic Calibration mode. There are no Output addresses created for this module and so a user cannot address the configuration words from ladder logic.

I/O addressing after adding a 1762-IR4 - 4 Channel RTD/Resistance Module to slot 1...


I have added this module to a MicroLogix 1100, 1200 and 1400 controller and they all do not create Output addresses for this module. With no Output addressing, there is no way for a user to write to Configuration Word 4, Bit 0 from ladder logic. You can only Enable/Disable Cyclic Calibration via the Cal tab...


While they say in the above TIP from Page 3-20 that you can change this bit using ladder logic in controllers that support it, they also give this counter TIP on Page 3-2...


What I fail to see is which controllers, be they Allen Bradley or "other", are they referring to as supporting this feature? To my knowledge, the 1762 modules are only used with the above mentioned MicroLogix 1100, 1200 and 1400 controllers; all of which do not allow user access to this bit via ladder logic, normally. (There is that word again...)

Digging deeper...

If you look at the Expansion General Configuration tab for the module you will notice the Input Words and Output Words fields. These values define the number of I/O word addresses that will be created for the module. The Output Words field is 0 and Grayed out. So we cannot manually add Output Words for this module, which is what you would need. It is Grayed out because we have added a predefined module from the list of 1762 modules which automatically assigns the I/O addressing. It does not assign the necessary five Configuration words in the Output Words field, but instead, and because it uses a user interface, it uses five words in the Extra Data Length field beneath...


This allocates, in the background, the necessary five words in the controller's memory for the configuration data manipulated on the user interface tabs.

I have said "normally" a couple of times so far. Here is what I think might abnormally be the case...

What I think they might be referring to is the original option for these modules when they were released to manually add and define them as a generic module. At their release, the then current RSLogix 500 version (v5.20) did not have these modules available for selection under the IO Configuration. They were not available for selection until RSLogix v5.50. So, for RSLogix v5.20 or earlier, you would have to add these modules using the Other -- Requires I/O Card Type ID option at the bottom of the list.

When you add a generic module you need to manually define the Vendor specific information and I/O size, etc. For this module you can add a generic module instead of the predefined module and then enter the same Vendor specifics and I/O size as the predefined module. This will give you the same results as when adding the predefined module from the list. Next you can delete that and add a generic module again, only this time also enter 5 in the Output Words field. Now you will have the same as the predefined module except now the five Configuration Output words will also have been created...


Now you have Output addresses to use in the ladder logic. The relevant bit address being for Cyclic Calibration being O:1.4/0. You should not modify any of the other Word 4 bits or indeed the value of Word 4 itself. Whether these addresses will actually work from the logic and configure the module, or not, I do not know. I do not have an RTD module to hand to test, but it is something you could try, if you wish.

But, there is a problem with the user interface. When I add this module as a generic module, and also add the five Output Words, it does not play well with access to the user interface tabs. I am getting a message when I attempt to open any of the other tabs...


Best guess is that it is referring to the Output Words field and how it is expecting it to be 0 in value. This message suggests to me that my attempt at exposing the five Configuration words is messing with the module's expected I/O size. This could be because my version of RSLogix 500 does have the module in its list and when you configure the Vendor specifics the software auto recognizes the module's predefined configuration and then there is a mismatch of sorts. I would like to try adding this module generically in RSLogix 500 v5.20 or earlier, where the predefined module configuration is not present, but I do not have a version that old installed on anything anymore.

While not being able to access the Configuration tabs is a stumbling block, it might not be a job stopper. If the newly created five Configuration Output words do work, then you could do all your configuring via these words and ignore the tabs. But some of the settings are finicky under the tabs and may be difficult to figure out when you need to enter equivalent values in the relevant Output words. However, there is also a way around this.

When you add this module proper, and you have access to the tabs, you can go to the Generic Extra Data Config tab. Here you will see five values to the right that represent the five Configuration Word values (Word 0 - 4, left to right)...


With the default configuration i.e. a freshly added module, all values should be equal to 0. If you then configure the settings you require, such as enabling a Channel, and go back to that tab, you will now see a value changed. For instance, if you just enable Channel 0, then from the manual you can see that Word 0 Bit 15 is set. This is reflected on the Generic Extra Data Config tab as the left most value equal to -32768...


As we know, for a 16-bit Signed Integer, having just Bit 15 set will equal -32768. If you then say enable all four Channels and disable the Cyclic Calibration mode, you will now see all four Channel Word values equal to -32768 and Word 4 equal to 1...


Then let's say for each Channel we change the Data Format to Engineering Units, set the Units to °F and enable Cyclic Calibration...


So you can see how the values reflect the current configuration.

More to come...

[Geospark]

By using the above "values" method you can add a dummy module and configure it as you would like for the Channels and then take note of the resultant values on the Generic Extra Data Config tab. You can then enter these values into the four Output word addresses, and if they work, they will configure the module with the same settings as if done by the tabs...


You can then delete the dummy module. This should then also provide you with the Word 4 Bit 0 that you are inquiring about.

Whether those manually added Output Words will work or not is pure conjecture on my part as I have not performed any testing. I just know you can do it for other modules for program manipulation of their configuration. Also, if it does work, the disadvantage of not having access to the configuration tabs, and convoluted measures to circumvent this, may far outweigh the advantage of having the Cyclic Calibration bit available to you. The advantage, of course, of having this Bit available via programming is that you if you want periodic calibration, but not at the default 5 minute intervals, then you can program a scheduled calibration say once a day or week, depending on how critical the process is.

Regardless of how much you'd like to have it, you may also read all this and say "What the heck is that all about!"..."Forget it!"...and maybe you'd be right?

As a side note...

The CompactLogix 1769-IR6 RTD module, which may also be used with the MicroLogix 1500 controller, provides a Logix controller with module-defined backing tags of which includes a BOOL tag for the enabling and disabling of the Cyclic Calibration mode feature...


This allows you to program scheduled module calibration in RSLogix 5000/Logix Designer. However, the RSLogix 500 based MicroLogix 1500 would be in the same boat as its siblings i.e. no available Output Bit addressing.

With what I know, that is the best that I can do for you my friend.

Regards,
George

[Geospark]

Originally Posted by rupej

Just curious, do you know what it is calibrating and why?

Jason has posted a little info on this which you can read above. The module, depending on the environment it is used in, may vary a lot in temperature itself over time. This potential internal temperature fluctuation may have an influence on the calculated and resultant temperature input value passed from the module's Analog to Digital Converter (ADC) into the processor. To compensate for this, an internal autocalibration may be used to correct any deviations.

The input module performs autocalibration when a channel is initially enabled. Autocalibration compensates for offset and gain drift of the ADC caused by temperature change within the module. An internal, high-precision, low drift voltage and system ground reference is used for this purpose...

When an autocalibration cycle takes place, the module’s multiplexer is set to system ground potential and an ADC reading is taken. The ADC then sets its internal input to the module’s precision voltage source, and another reading is taken. The ADC uses these numbers to compensate for system offset (zero) and gain (span)...

Basically, the autocalibration mode takes a temperature reading from the field input and a reading from the high-precision reference and compares them. Using the calculated difference it adjusts the offset and gain values for the ADC accordingly. The more often this is performed the less drift is likely.

An important consideration to be made when using autocalibration is the overhead it adds to the module update time...

Effects of Autocalibration on Module Update Time

The module’s autocalibration feature allows it to correct for accuracy errors caused by component temperature drift over the module operating temperature range (0 to 55°C). Autocalibration occurs automatically on a system mode change from Program-to-Run for all configured channels. In addition, the module allows you to configure it to perform an autocalibration cycle every 5 minutes during normal operation or to disable this feature using the Enable/Disable Cyclic Calibration function (default: Enable). With this feature, you can implement a calibration cycle anytime, using your control program to enable and then disable this bit.

(Again, where this bit is supposed to be available I'm not sure?)

If you enable the autocalibration function, the module update time increases when the autocalibration cycle occurs. To limit its impact on module update time, the autocalibration function is divided over several module scans.

Each enabled channel requires a separate 4-step cycle. If cyclic lead compensation is disabled, each enabled channel requires only a separate 3-step autocalibration cycle. The time added to the module update time depends on the filter selected for that channel...

Example: With Lead Wire Compensation and Autocalibration enabled, and a filter of 10Hz selected, the increase in module update time can be measured in seconds. So if high repeatability is required for a process, while maintaining relatively high accuracy, then the interval period of autocalibration can be a very important consideration indeed. For instance, through programming (if possible), one could enable autocalibration whenever the process is idle or at stop, but never when it is in operation.

As with lots of useful features there is nearly always a trade off.

Regards,
George