ASCII support

[jbysewski]

This patch extracts common serial driver functionality into modbus-serial*.c/h and adds modbus-ascii upon this common serial driver. I was able to test the implementation against a chain of RS485/Modbus ASCII pressure sensors.

The setting of serial mode and rts is done via a forward to the modbus-serial module which keeps the current API but is not very clean IMHO.

Heads-up:
Some of the ascii implementation is based upon patches/pull-requests surrounding this project.

Google groups thread: https://groups.google.com/forum/#!topic/libmodbus/2A-ocabMhrE

[stephane]

First review: good work!
I think a few 'switch/case' could be improved and we need to add tests and documentation.
I intend to work on it this month...

[AlexLonardi]

Hi,
we had stressed this patch and some minor bugs are emerged when the modbus interface is set as slave:

1. Every reply misses of ':' at start, so we edited _modbus_ascii_build_response_basis as follows:

static int _modbus_ascii_build_response_basis(sft_t *sft, uint8_t *rsp)
{
  rsp[0] = ':';
  rsp[1] = sft->slave;
  rsp[2] = sft->function;

  return _MODBUS_ASCII_PRESET_RSP_LENGTH;
}

Redefining _MODBUS_ASCII_PRESET_RSP_LENGTH macro as follows

_MODBUS_ASCII_PRESET_RSP_LENGTH 3

2. We'had got stored a float value of 8.86 in register, which translates to unsigned values (49807,16653), in hex (0xC28F,0x410D). When replying to a read register request, _modbus_ascii_send detects 0X0D as a '\r' character, skipping its translation to ascii. So, we've modified the code as follows, specializing the if statement:

static ssize_t _modbus_ascii_send(modbus_t *ctx, const uint8_t *req, int req_length)
{    
    uint8_t ascii_req[3 + (MODBUS_ASCII_MAX_ADU_LENGTH * 2)];
    ssize_t i, j = 0;

    for (i = 0; i < req_length; i++) {
        if ((i == 0 && req[i] == ':') ||
                ((i == req_length - 2) && (req[i] == '\r')) ||
                ((i == req_length - 1) && (req[i] == '\n')))
        {
            ascii_req[j++] = req[i];
        } else {
            ascii_req[j++] = nibble_to_hex_ascii(req[i] >> 4);
            ascii_req[j++] = nibble_to_hex_ascii(req[i] & 0x0f);
        }
    }
    ascii_req[j] = '\0';

    ssize_t size = _modbus_serial_send(ctx, ascii_req, j);
    return ((size - 3) / 2) +3;
}

[stephane]

Thank you @jbysewski and @AlexLonardi .

I created a new branch called https://github.com/stephane/libmodbus/tree/ascii-support to work on that, I need help to work on:

1. https://github.com/stephane/libmodbus/blob/ascii-support/src/modbus-ascii.c#L211, I think MODBUS_MAX_ADU_LENGTH must depend on backend...
2. many possible overflows
3. run unit tests with success
4. missing documentation for modbus_ascii_new (yes I intend to rename modbus_rtu_new too !)

[ulimitedCoder]

I am looking to use the ascii-support in libmodbus. What is the current working status for the branch? Is it stable enough to work with?

[lyesba]

hi
thank you for i job
i want ,can i use the ASCII support with the libmodbus, if yes I would like to know how to add this ?
great regard

[yuanfanggo]

good job

[ghost]

ascii-support branch has a double free bug in rtu mode, this line needs to be deleted,

libmodbus/src/modbus-rtu.c

Line 185 in 787e6ba

free(ctx->backend_data);