MAX_MESSAGE_LENGTH and MAX_REGISTERS deviate from Modbus standard

[n1ywb]

"#define MAX_MESSAGE_LENGTH 256" is incorrect for Modbus TCP.
Modbus_Application_Protocol_V1_1b.pdf Chapter 4 Section 1 Page 5:
RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256 bytes.
TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.

"#define MAX_REGISTERS 100" is incorrect for Modbus.
Modbus_Application_Protocol_V1_1b.pdf Chapter 6 Section 3 Page 15:
Quantity of Registers: 2 Bytes: 1 to 125 (0x7D)

[n1ywb (jeff-laughlin)]

This patch against 2.0.2 changes MAX_REGISTERS to 125. It also adds a MAX_MESSAGE_LENGTH_RTU and _TCP #defs and a "max_message_length" field to modbus_params_t which is initialized to the appropriate limit in the init functions. I replaced all references to MAX_MESSAGE_LENGTH with a reference to mb_params->max_message_length. It passes unit testing.

[Stéphane Raimbault (sra)]

I'm not really happy to add yet another variable to modbus_param_t but I'm intend to apply your patch, just some comments:
- the MAX_REGISTERS change is missing (minor)
- could you use MAX_MESSAGE_LENGTH_TCP in the tests because the concerned code only works on TCP devices and I don't like to use internal value (modbus_param_t) in external code (not modbus.c)?
- could you provide two unit tests with the two length (or TCP at least)?
Thank you.

[n1ywb (jeff-laughlin)]

One alternative might be to declare a global constant array of message
lengths and index it on the connection type, something like

const int message_length[] = { 256, 260 };

assert(message_length[RTU] == 256);
assert(message_length[TCP] == 260);

since type_com is already in mb_params.

On Fri, Mar 13, 2009 at 6:05 PM, Stéphane Raimbault
<email address hidden> wrote:
> I'm not really happy to add yet another variable to modbus_param_t but I'm intend to apply your patch, just some comments:
> - the MAX_REGISTERS change is missing (minor)
> - could you use MAX_MESSAGE_LENGTH_TCP in the tests because the concerned code only works on TCP devices and I don't like to use internal value (modbus_param_t) in external code (not modbus.c)?
> - could you provide two unit tests with the two length (or TCP at least)?
> Thank you.
>
> --
> MAX_MESSAGE_LENGTH and MAX_REGISTERS deviate from Modbus standard
> https://bugs.launchpad.net/bugs/341702
> You received this bug notification because you are a direct subscriber
> of the bug.
>
> Status in A groovy libmodbus!: New
>
> Bug description:
> "#define MAX_MESSAGE_LENGTH 256" is incorrect for Modbus TCP.
> Modbus_Application_Protocol_V1_1b.pdf Chapter 4 Section 1 Page 5:
> RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256 bytes.
> TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
>
> "#define MAX_REGISTERS 100" is incorrect for Modbus.
> Modbus_Application_Protocol_V1_1b.pdf Chapter 6 Section 3 Page 15:
> Quantity of Registers: 2 Bytes: 1 to 125 (0x7D)
>

--
73 de n1ywb