Does a MODBUS AI (MAI) make sense?? Because MODBUS is a digital protocol, data is sent in a specific BYTE format, how can it be analog?

The question arose because I saw somewhere a PLC talking to DCS on MAI (I/O type - MAI; Signal type - Serial).

 

Modbus has Holding Registers and Input Registers. You cannot write to a Input Register, it is for inputs only. In this context, an analog input is there for a "Input Register".

 

Of course it makes sense.This would normally give the output of an analog to digital (A/D) converter and is the digital representation of an analog signal at a particular moment in time. The inverse is also widely used, a digital to analog converter provides analog outputs. The number of bits and signing, etc. vary, but the byte order is reasonably consistent. People do forget about the instant in time part and use these numbers as an accurate voltage or current. Looking at a series of these samples will normally dispel that notion.

Regards
cww

 

we have totally digital protocols (FF) that have both AI and DI's. the engineering understanding of A/D and practical are different. i would say that AI just means continuous flow of digitals signals (for dig protocols) while DI gives only one e.g. switch, relay. one would mean something like pressure switch the other pressure tx. analog protocols use a/d converters...

 

If I `ve understood correctly then it means that MODBUS signal format is DIGITAL only i.e. the data that PLC outputs is digital only but there`s a D/A converter which converts this digital signal into analog which travels on the interconnecting cable and at the DCS end an A/D converter converts it back into digital data.

Further as per my understanding, this D/A conversion takes place in MODBUS ANALOG OUTPUT card of PLC & the A/D conversion at the DCS end takes place in the MODBUS ANALOG INPUT card.

Regards,
Tashar

 

You are correct: Modbus Protocol is a "digital, byte oriented protocol". This means that analog data is first converted to a digital representation before transmitting it over the serial data link. There is an Analog to Digital Converter located at the transmitting end that converts each analog input to a 16-bit digital number (in the case of Modbus) that "represents" the physical input. This conversion is completely controlled by the hardware.

For instance you could use a simple converter that would divide the analog input into 256 possible values and you would then have a number that, as close as possible, represents the actual value as one part in 256 (8-bits). More typically, modern day converters have a resolution of one part in 2048 or one part in 4096 or even higher. In any case, the analog input is converted to an integer number that "represents" the actual value.

Standard Modbus uses 16 bits for an analog input. Some versions of Modbus use 32 bits to define an analog value. Some manufactures use the 32 bits to define a floating point number (usually encoded in an IEEE format) but others encode the data as a 32-bit "long integer". It is kinda confusing but thankfully, there are not too many interpretations around and most converters can deal with the different formats.

But the scaling must me known by the receiving end: That means that the receiver must know something about the application. Does one count difference mean one PSI, one degree, one foot per second, one pound, one kg, or whatever. It is just a number. To get meaningful information, you need to know about the application.

This is one of the "features" of Modbus and one of its "problems".

I for one think it is one of its best features!

Jerry Miille

 

It could be done that way. More common is that the A/D or D/A it done on one device or the other and sent as a digital code across the network to the other. The reason being that digital signals are easier to send over long distances without interference. I do agree with the reader who generalized AI to include other numeric quantities as well. A Modbus compatible instrument for example, would would make a register available to the network.

Regards
cww

 

Why would anyone convert D to A, and then A back to D from plc to dcs?

There is only one a/d conversion and it is analog 4-20 to modbus digital signal, and backwards..

What about foundation fieldbus? it doesn't have that "problem".

 

The D/A and A/D is for the benefit of field devices that want an analogue signal. The SCADA/DCS/PLC/etc. computer needs a converted digital value so there has to be an A/D or D/A converter somewhere in the system. The question is only whether to put the A/D and D/A out in the field or in the DCS/PLC rack. The only time you don't have A/D or D/A converters is where the control logic itself is made from analogue components, and these days that is only found in very old installations.

For Modbus (like other field buses), the A/D and D/A can be built right into the transducer/valve/other device. In that case you only need to take the network connection up to the field device and plug it in. If the field device doesn't have network capability, then you add a converter block which acts as a network node and wire the two together.

The only thing that Foundation Fieldbus does differently is that it has the ability to run very simple control loops right in the field device.