Hello Guys,
I am working mostly with Siemens PLC’s and am trying to fully understand the wiring of analog input/output modules. From what I’ve read, it seems that modules from other companies (AB, for example) are not really that much different internally. What I have found, is that while the manuals generally describe how to wire up different sensors, they mostly avoid showing any decently useful schematics to give an understanding of what is actually happening with the signals. As a specific example, I am uploading the data sheets to one of the basic AI/O modules for the S7-300 PLC. This module accepts 0-20mA and 0-10V sensor inputs. Judging by a number of earlier forum posts, some of you guys really know this stuff well, and I was hoping someone could answer some questions to help get a better understanding of how these modules work in general (regardless of PLC).
1) As I understand it, based on the Siemens schematic, Mana is the analog common, or the reference voltage for the ADC circuit. When I read this, I understand this to mean that Mana is being compared with the measured voltage. However, on most cards there is a + and – sensor input. Should I assume, then, that the voltage DIFFERENCE between the + and – inputs is being compared to Mana to generate the converted signal, in other words that all of these input cards are actually differential cards and not single-ended? Or can they be wired both ways?
2) Some cards have Mana as an extra terminal, others do not. What should I assume that Mana is connected to on the modules that do NOT have it as a separate terminal, should I assume it is connected to the analog module power common? Or is it connected to the backplane CPU common (these could be different if isolated, right?)?
3) If Mana is the analog common for one AD converter, and the module has isolated channels with multiple ADC’s, then it stands to reason that each channel should have an Mana. Are these generally all connected together internally?
4) If the module is a combined input/output module, are the ADC’s and DAC’s analog commons (Mana’s) generally connected?
5) The manual repeatedly mentions that in certain cases (I believe for non-backplane isolated modules), the ADC common and CPU ground need to be at the same potential. Why is this exactly?
6) In certain cases, the negative of the signal wire has to be connected to Mana. As I understand it, this is to prevent the Common Mode Voltage (noise voltage common to the + and – measurement terminals) from exceeding a maximum relative to some common (ADC analog common?). Could someone explain this in a little more detail?
7) Finally, it is mentioned in the module handbook that for certain modules an equalization current develops between the – signal input and Mana if they are connected together when using 2-wire current output sensors and backplane isolated modules. Could someone explain this?
I know this is a lot of questions, but hopefully this will help others trying to understand this stuff better as well.
Thanks!
I am working mostly with Siemens PLC’s and am trying to fully understand the wiring of analog input/output modules. From what I’ve read, it seems that modules from other companies (AB, for example) are not really that much different internally. What I have found, is that while the manuals generally describe how to wire up different sensors, they mostly avoid showing any decently useful schematics to give an understanding of what is actually happening with the signals. As a specific example, I am uploading the data sheets to one of the basic AI/O modules for the S7-300 PLC. This module accepts 0-20mA and 0-10V sensor inputs. Judging by a number of earlier forum posts, some of you guys really know this stuff well, and I was hoping someone could answer some questions to help get a better understanding of how these modules work in general (regardless of PLC).
1) As I understand it, based on the Siemens schematic, Mana is the analog common, or the reference voltage for the ADC circuit. When I read this, I understand this to mean that Mana is being compared with the measured voltage. However, on most cards there is a + and – sensor input. Should I assume, then, that the voltage DIFFERENCE between the + and – inputs is being compared to Mana to generate the converted signal, in other words that all of these input cards are actually differential cards and not single-ended? Or can they be wired both ways?
2) Some cards have Mana as an extra terminal, others do not. What should I assume that Mana is connected to on the modules that do NOT have it as a separate terminal, should I assume it is connected to the analog module power common? Or is it connected to the backplane CPU common (these could be different if isolated, right?)?
3) If Mana is the analog common for one AD converter, and the module has isolated channels with multiple ADC’s, then it stands to reason that each channel should have an Mana. Are these generally all connected together internally?
4) If the module is a combined input/output module, are the ADC’s and DAC’s analog commons (Mana’s) generally connected?
5) The manual repeatedly mentions that in certain cases (I believe for non-backplane isolated modules), the ADC common and CPU ground need to be at the same potential. Why is this exactly?
6) In certain cases, the negative of the signal wire has to be connected to Mana. As I understand it, this is to prevent the Common Mode Voltage (noise voltage common to the + and – measurement terminals) from exceeding a maximum relative to some common (ADC analog common?). Could someone explain this in a little more detail?
7) Finally, it is mentioned in the module handbook that for certain modules an equalization current develops between the – signal input and Mana if they are connected together when using 2-wire current output sensors and backplane isolated modules. Could someone explain this?
I know this is a lot of questions, but hopefully this will help others trying to understand this stuff better as well.
Thanks!
