1. Explain how a HART device works 2. What are the two transmission modes for Mo

Question

1. Explain how a HART device works 2. What are the two transmission modes for Modbus? 3. If there is fault to a slave, can it report to the master immediately? 4. Do slaves reply to a broadcast request from master? Why? 5. How many masters can a Mobus have? How many slaves can it have? (Look in the Modbus specification!) 6. What are in a Modbus frame? 7. Why does the master have to wait after it sends out a broadcast request? 8. How many bits in each byte in RTU mode and ASCII mode? What are these bits? 9. What are t3.5 and t1.5? How do you calculate them? 10. Decode the following RTU Modbus messages. Are they from the master or the slave? I l. Create a Modbus message in RTU mode for the Master to request the following information from slave 2: Read coils 10-23. Suppose coils with even labels are all 1 and the ones with odd numbers are all 0. What is the message in RTU mode that the slave should send? 12. Create a Modbus message in RTU mode for the Master to request the following information from Slave 3: Read holding registers #2-5. Suppose the holding registers have the following values: 125. 250, 10, and 50 respectively. What is the message in RTU mode that the slave should send?

Explanation / Answer

1)Answer:

HART:

HART is an acronym for Highway Addressable Remote Transducer. It involves a digital signal superimposed onto a 4-20mA analog signal in order to encrypt instrument information beyond the standard analog signal. What this means is that the standard 4-20mA signal oscillates up and down from the average 4-20mA value by a small amount.

The actual analog signal from the device is represented by the blue curve. The average of the signal is represented by the red curve, and this is the standard 4-20mA analog signal measured by the PLC. A HART-compatible I/O card interprets the oscillation as a series of 0s and 1s, which is the basis for communication in the digital world. The system works like this: A PLC will transmit a message to the device in oscillations as 0s and 1s, and the device will receive, interpret, and transmit back a response corresponding to the message that was received.

This is known as a master/slave relationship and is useful for functionality beyond the standard 4-20mA measurement, such as:

2)Answer:

Transmission modes for modbus are

->RTU

->ASCII

->TCP

RTU:

Each message data byte consists of two 4-bit hexadecimal characters. Devices and applications transmit each message in a continuous stream of characters

ASCII:

Each message data byte consists of two ASCII characters.

TCP:

The Modbus data message is encapsulated in a TCP frame.

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