Dear sir,

I need a reliable reference to calculate transaction time for my research work, similar to the method described bysmiffy in the provided link.

Transmission Speed of Data on MODBUS RTU / PROFIBUS / MODBUS TCP. - 42843 - Industry Support Siemens

Mohammad.K_92

Dear sir,

I need a reliable reference to calculate transaction time for my research work, similar to the method described by smiffy in the provided link.

Transmission Speed of Data on MODBUS RTU / PROFIBUS / MODBUS TCP. - 42843 - Industry Support Siemens

Which Siemens devices / services / software are in use?
Which communications protocols?

There is some article published somewhere in this support site, which compares communication protocols (one is Ethernet TCP) and payload sizes.

Wireshark (https://www.wireshark.org/) may deliver what you need as an analysis tool.

Our system is a Modbus RTU and it works with PLC 1500.

We want to perform system loading response calculations, similar to the comment I referenced in the previous link.

In fact, I'm looking for a reference for these calculations.

 The comment is fromsmiffy.

Taking a request for 10 holding registers with you being the master as an example.
For RTU:-
The request packet will be: Slave address (1 byte) + Function code (1 byte) + Offset (2 bytes) + Number of Regs (2 bytes) + CRC (2 bytes) = 8 bytes.
The response packet length will be: Slave address (1 byte) + Function code (1 byte) + Byte Count (1 byte) + Data (20 bytes) + CRC (2 bytes) = 25 bytes.
The number of bits will depend on the serial setup (e.g. parity, data bits etc.).
Assuming 1 start bit, 8 data bits, 1 parity bit and 1 stop bit = 11 bits per byte.
So total number of bits = (25 + 8) * 11 = 363 bits.
Now the bit time must be calculated = 1 / baud rate.
Taking 19200 baud as an example, bit rate = 19200 / 1 = 52 microseconds (5.2E-5 secs)
Giving a total transaction time of 52 microseconds * 363 = 18.876 milliseconds.

Duris_I

Hello, 

Mohammad.K_92

In fact, I'm looking for a reference for these calculations.

 The comment is fromsmiffy.

Taking a request for 10 holding registers with you being the master as an example.
For RTU:-
The request packet will be: Slave address (1 byte) + Function code (1 byte) + Offset (2 bytes) + Number of Regs (2 bytes) + CRC (2 bytes) = 8 bytes.
The response packet length will be: Slave address (1 byte) + Function code (1 byte) + Byte Count (1 byte) + Data (20 bytes) + CRC (2 bytes) = 25 bytes.
The number of bits will depend on the serial setup (e.g. parity, data bits etc.).
Assuming 1 start bit, 8 data bits, 1 parity bit and 1 stop bit = 11 bits per byte.
So total number of bits = (25 + 8) * 11 = 363 bits.
Now the bit time must be calculated = 1 / baud rate.
Taking 19200 baud as an example, bit rate = 19200 / 1 = 52 microseconds (5.2E-5 secs)
Giving a total transaction time of 52 microseconds * 363 = 18.876 milliseconds.

I see in your explanation something wrong from first my look. If I do test with same example (10 registers as WORD longer) and only with speed 9600, my transaction is no longer than 1s. Question is then logically, where you have your +/- 18000ms from and I have cca half slower speed. You can do simple test with some PC app as RTU Slave and do invetigations as I've described.

These calculations have been performed by Siemens experts according to the following link:

Transmission Speed of Data on MODBUS RTU / PROFIBUS / MODBUS TCP. - 42843 - Industry Support Siemens 

Now, I would like to see the reference for these calculations and the method of obtaining it according to the Siemens catalog or any other valid reference.

Duris_I

Hello,

ok, understand. On other side I didn't noticed 18.xxx with decimal point so my mistake with expecting higher value.

Hello,

It's true that I have an issue with the final result obtained, but my concern is focused on the reference for these calculations. In any case, these calculations are necessary to determine the response time, and I simply want a reliable reference like the Siemens catalog that has performed these calculations.