Examples |
All examples below assume the following string table:
1 "Rack no: %3o has %2d modules.\r\n" example 1 2 "\x02\x03%.2C%.2C%#R" example 2, Modbus slave 2, function 3 3 "AK HM %.14s %.3d" example 3 4 "\x02\x03\xfa%.250C%#R" example 4, Modbus response 5 "\x02\x83%c%#R" example 5, Modbus exception
Example 1. COMPRINT of Text with Integers | ||
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Format: 1 Two parameters: integer, integer |
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Sample output: |
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"Rack no: 5 has 2 modules."
Example 2. COMPRINT of Modbus RTU Query | ||
In this example binary is printed rather than ASCII, so our format string has hex escapes. The desired Modbus RTU query is Read Holding Registers, Modbus function number 3 and intended for slave 2:
+-------+-------+-------+-------+-------+-------+-------+-------+ | 02 | 03 | Ref Hi| Ref Lo| Num Hi| Num Lo| CRC Lo| CRC Hi| +-------+-------+-------+-------+-------+-------+-------+-------+ slave func |<----2 bytes-->|<----2 bytes-->|<----2 bytes-->|
The 2 Ref bytes are the datatable address starting at 0 (which corresponds to register 40001), in most significant byte first format (big endian). The 2 Num bytes are the count of registers to read, in big endian format, with a maximum of 125. The 2 CRC bytes are the CRC-16 calculated by preloading with 0xFFFF, not 0, and are output in little endian format.
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Format: 2 Two parameters: integer, integer |
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Sample output: see above where the 8 byte query is shown. |
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There is yet a simpler format string possible for this example, and that would be to hard code the Ref and Num words using 4 hex escapes, something that makes more sense if they do not need to be modifiable at runtime. In such a case no Params would be required, since the %#R conversion specification uses no Params itself.
Example 3. COMSCAN of Text with Integers | ||
In this example a string is expected on the Port in ASCII. The expected string looks like:
AK HM <variableWidthNonWhitespace> NNNCRLF
The AK HM are fixed. The <variableWidthNonWhitespace> is a field that will not have whitespace in it. This field's width will vary from 1 to 14 characters. The NNN is a decimal number like "123" and it is at most 3 characters wide. The CRLF are two characters: \r and \n respectively. They are whitespace according to our earlier definition.
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Format: 3 Two parameters: STRING element, integer |
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You almost always want to use precision in your COMSCAN conversion specifications, otherwise matching can erroneously succeed before all the bytes are received on the serial port.
Example 4. COMSCAN of Modbus RTU Response | ||
In example 2, COMPRINT was used to send a Modbus query. In this current example, the desired Modbus RTU response is for the Read Holding Registers query, Modbus function number 3 and coming back from slave 2:
+-------+-------+-------+-------+-------+-------+-------+ +-------+-------+
| 02 | 03 |# bytes|data0Hi|data0Lo|data1Hi|data1Lo|...| CRC Lo| CRC Hi|
+-------+-------+-------+-------+-------+-------+-------+ +-------+-------+
|<---- 125 words or 250 bytes ----->|
For discussion, we'll assume the query asked for 125 registers. Then the # bytes field will be 250 which is 0xFA in hex. COMSCAN is both a matcher and converter. If slave id, function, # bytes, or CRC do not match, then the ER bit is set along with the DN bit. However, the Params can still be modified even on COMSCAN mismatch and failure. Therefore it is critical to buffer the Params and copy them to a safe useable place when the DN bit is on without the ER bit.
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Format: 4 One parameter: integer |
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Example 5. Simple Modbus RTU Master | ||
This example is a complete working Modbus RTU master implemented with a COMPRINT and two COMSCANs. The COMPRINT is basically described by example 2. On the serial cable, in response to the COMPRINT one of three results are expected:
- A valid Modbus response packet
- A valid Modbus exception packet
- Garbage or no response at all
Matching a Modbus response was discussed in example 4, and it requires its own COMSCAN instruction with a taylor made format string. Matching a Modbus exception packet requires another COMSCAN instruction with a different taylor made format string. Because COMSCAN puts back all characters when any part of the matching process fails, this leaves them all available for the next COMSCAN to try and match against. If the second one also fails, any garbage characters can be cleared out of the receive buffer with the COMRCVCLEAR TLI.
The example is available both as a PDF printout and as a binary SOFTPLC.APP file. You can download either from our website. To use the SOFTPLC.APP file, put it into a new directory like \SoftPLC\app\MODBMAST\ or similar so that TOPDOC NexGen can find it. Acrobat Reader is needed to view the PDF file, and can also be used to print it out to a printer.
This example uses format strings 2, 4, and 5. Format strings 2 and 4 were discussed in examples 2 and 4 respectively. Format string 5 is designed to match an exception response, per the Modbus specification and capture the single byte exception code into the least significant byte of an integer Param.
Please read the PDF file and the rung comments for the full understanding of the program logic.