To make manual building of Modbus frames easier...

modhelp.c

#include <stdio.h>
#include <signal.h>
#include <inttypes.h>
#include <string.h>
#include <lightmodbus/master.h>
#include <pthread.h>

ModbusMaster master;
pthread_t pt;

void sigexit( int signum )
{
	printf( "\nBye!\n" );
	pthread_cancel( pt );
	exit( 0 );
}

int promptval( const char *name )
{
	int val = 0;
	char buf[1024];

	printf( "%s>", name );
	scanf( "%1023s", buf );
	sscanf( buf, "%d", &val );
	return val;
}

void *prompt( void *dummy )
{
	unsigned int i;
	uint16_t func;
	uint16_t addr;
	uint16_t index;
	uint16_t count;
	uint16_t andmask;
	uint16_t ormask;
	uint16_t val;
	uint8_t ec = 0;
	uint8_t *data = NULL;
	char strbuf[256];

	addr = promptval( "addr" );
	func = promptval( "func" );

	switch ( func )
	{
		case 1:
		case 2:	
			index = promptval( "index" );
			count = promptval( "count" );
			ec = modbusBuildRequest0102( &master, func, addr, index, count );
			break;

		case 3:
		case 4:
			index = promptval( "index" );
			count = promptval( "count" );
			ec = modbusBuildRequest0304( &master, func, addr, index, count );
			break;

		case 5:
			index = promptval( "index" );
			val = promptval( "val" );
			ec = modbusBuildRequest05( &master, addr, index, !!val );
			break;

		case 6:
			index = promptval( "index" );
			val = promptval( "val" );
			ec = modbusBuildRequest06( &master, addr, index, val );
			break;

		case 15:
			index = promptval( "index" );
			count = promptval( "count" );
			data = calloc( BITSTOBYTES( count ), sizeof( uint8_t ) );
			if ( data == NULL ) raise( SIGINT );

			for ( i = 0; i < count; i++ )
			{
				memset( strbuf, 0, 256 );
				snprintf( strbuf, 256, "%.2d:%.2d:%.2d", index, index + i, index + count );
				val = promptval( strbuf );
				modbusMaskWrite( data, BITSTOBYTES( count ), i, val );
			}

			ec = modbusBuildRequest15( &master, addr, index, count, data );
			free( data );
			break;

		case 16:
			index = promptval( "index" );
			count = promptval( "count" );
			data = calloc( count, sizeof( uint16_t ) );
			if ( data == NULL ) raise( SIGINT );

			for ( i = 0; i < count; i++ )
			{
				memset( strbuf, 0, 256 );
				snprintf( strbuf, 256, "%.2d:%.2d:%.2d", index, index + i, index + count );
				val = promptval( strbuf );
				data[i] = val;
			}

			ec = modbusBuildRequest16( &master, addr, index, count, (uint16_t*) data );
			free( data );
			break;

		case 22:
			index = promptval( "index" );
			andmask = promptval( "and" );
			ormask = promptval( "or" );
			ec = modbusBuildRequest22( &master, addr, index, andmask, ormask );
			break;


		default:
			printf( ":unsupported function code!\n" );
			break;
	}

	if ( ec != MODBUS_OK ) printf( ":error %d\n", ec );
	else
	{
		for ( i = 0; i < master.request.length; i++ )
			printf( "%.2X ", master.request.frame[i] );
		printf( "\n" );
	}
	return NULL;
}

void sigreprompt( int signum )
{
	pthread_cancel( pt );
	printf( "\n" );
	signal( SIGQUIT, sigreprompt );
}

int main( int argc, char **argv )
{
	signal( SIGINT, sigexit );
	signal( SIGQUIT, sigreprompt );

	modbusMasterInit( &master );
	printf( "SIGINT to exit\n" );
	
	while ( 1 )
	{
		pthread_create( &pt, NULL, prompt, NULL );
		pthread_join( pt, NULL );
	}

	modbusMasterEnd( &master );
	return 0;
}