Since the topic was protocols

switch(SerialNumberControl)
{
	case 1:					// Serial Number
	case 2:
	case 3:
	case 4:
	case 5:
	case 7:					// Control Number
		if(inChar >= '0' && inChar <= '9')
		{
			(*index)++;
		}
		else
		{
			nak = COMMAND_NACK;
			SerialNumberControl = 0;		// cancel config write
		}
		break;
	case 6:					// Part Number Suffix
	case 8:					// Control Number Revision
		if(inChar >= 'A' && inChar <= 'W')  //X, Y, and Z are packet delimiters
		{
			(*index)++;
		}
		else
		{
			nak = COMMAND_NACK;
			SerialNumberControl = 0;		// cancel config write
		}
		break;
	case 9:					// close S/N
		if(inChar == 'o')
		{
			(*index)++;
		}
		else
		{
			nak = COMMAND_NACK;
			SerialNumberControl = 0;		// cancel config write
		}
		break;
	default:					// Normal Diag commands
		if(strchr("VuNWwCcOG",inChar) != NULL)(*index)++;
		else nak = COMMAND_NACK;
}

It depends on the states and what you do with them