SCPI parser library

SCPI Parser library aims to provide parsing ability of SCPI commands on instrument side. All commands are defined by their patterns eg: “STATus:QUEStionable:EVENt?”.

Source code is published with open source Simplified BSD license.

Command pattern definition

A command pattern is defined using the well known command representation from SCPI instruments. A pattern is case insensitive but uses lower and upper case letters to show the short and long form of the command.

Pattern "SYSTem" matches strings "SYST", "syst", "SyStEm", "system", ...

A command pattern is divided by colon “:” to show command hierarchy

Pattern "SYSTem:VERsion?" mathes strings "SYST:version?", "system:ver?", "SYST:VER?", ...

The SCPI standard also uses brackets “[]” to define optional parts of a command.

Pattern "SYSTem:ERRor[:NEXT]?" mathes "SYST:ERR?", "system:err?" and also "system:error:next?", ...

Command callback

Command callback is defined as a function with a context parameter, e.g.:

    int DMM_MeasureVoltageDcQ(scpi_context_t * context)

The “Q” at the end of the function name indicates that this function is a Query function (command with “?”).

The command callback can use predefined functions to parse input parameters and to write output.

Reading input parameters is done by using the functions SCPI_ParamInt, SCPI_ParamDouble, SCPI_ParamString and SCPI_ParamNumber.

Writing output is done by using the functions SCPI_ResultInt, SCPI_ResultDouble, SCPI_ResultString, SCPI_ResultText. You can write multiple output variables. They are automaticcaly separated by comma “,”.

Source code organisation

Source codes are divided into a few files to provide better portability to other systems.

libscpi/src/parser.c - provides the core parser library
libscpi/src/error.c - provides basic error handling (error queue of the instrument)
libscpi/src/ieee488.c - provides basic implementation of IEEE488.2 mandatory commands
libscpi/src/minimal.c - provides basic implementation of SCPI mandatory commands
libscpi/src/utils.c - provides string handling routines and conversion routines
libscpi/src/units.c - provides handling of special numners (DEF, MIN, MAX, …) and units
libscpi/src/fifo.c - provides basic implementation of error queue FIFO
libscpi/src/debug.c - provides debug functions
examples/test-parser - is the basic non-interactive demo of the parser
examples/test-interactive - is the basic interactive demo of the parser
examples/test-tcp - is the basic interactive tcp server (port 5025)
examples/common - common examples commands

Implementation to your instrument

First of all you need to fill the structure of SCPI command definitions

scpi_command_t scpi_commands[] = {
	{ .pattern = "*IDN?", .callback = SCPI_CoreIdnQ,},
	{ .pattern = "*RST", .callback = SCPI_CoreRst,},
	{ .pattern = "MEASure:VOLTage:DC?", .callback = DMM_MeasureVoltageDcQ,},
	SCPI_CMD_LIST_END
};

Then you need to initialize the interface callbacks structure. If you don’t want to provide some callbacks, just initialize it as NULL. The write callback is mandatory and is used to output data from the library.

scpi_interface_t scpi_interface = {
	.write = myWrite,
	.error = NULL,
	.reset = NULL, /* Called from SCPI_CoreRst */
	.test = NULL, /* Called from SCPI_CoreTstQ */
	.control = NULL,
};

An important library component is the command buffer. The maximum size is up to you and should be larger than the largest possible command.

#define SCPI_INPUT_BUFFER_LENGTH 256
static char scpi_input_buffer[SCPI_INPUT_BUFFER_LENGTH];

The last structure is the scpi context used in the parser library.

scpi_t scpi_context = {
	.cmdlist = scpi_commands,
	.buffer = {
		.length = SCPI_INPUT_BUFFER_LENGTH,
		.data = scpi_input_buffer,
	},
	.interface = &scpi_interface,
	.registers = scpi_regs,
	.units = scpi_units_def,
	.special_numbers = scpi_special_numbers_def,
};

All these structures should be global variables of the C file or allocated by function like malloc. It is a common mistake to create these structures inside a function as local variables of this function. This will not work. If you don’t know why, you should read something about function stack..

Now we are ready to initialize SCPI context. It is possible to use more SCPI contexts and share some configurations (command list, registers, units list, error callback…)

SCPI_Init(&scpi_context);

A test implementation of function myWrite, which outputs everything to stdout, could be

size_t myWrite(scpi_t * context, const char * data, size_t len) {
	(void) context;
	return fwrite(data, 1, len, stdout);
}

An interactive demo can beimplemented using this loop

#define SMALL_BUFFER_LEN
char smbuffer[SMALL_BUFFER_LEN];
while(1) {
	fgets(smbuffer, SMALL_BUFFER_LEN, stdin);
	SCPI_Input(&scpi_context, smbuffer, strlen(smbuffer));
}

Implementation of command callback

Command callback is defined as a function with return value of type scpi_result_t and one parameter - scpi context

	scpi_result_t DMM_MeasureVoltageDcQ(scpi_t * context)

Command callback should return SCPI_RES_OK if everything goes well.

You can read command parameters and write command results. There are several functions to do this.

Every time you call a function to read a command parameter, it shifts pointers to the next parameter. You can’t read specified parameter directly by its index - e.g.

	// pseudocode
	param3 = read_param(3); // this is not possible

	read_param();           // discard first parameter
	read_param();           // discard second parameter
	param3 = read_param();  // read third parameter

If you discard some parameters, there is no way to recover them.

These are the functions, you can use to read command parameters

SCPI_ParamInt - read signed 32bit integer value (dec or hex with 0x prefix)
SCPI_ParamDouble - read double value
SCPI_ParamNumber - read double value with or without units or represented by special number (DEF, MIN, MAX, …). This function is more universal then SCPI_ParamDouble.
SCPI_ParamText - read text value - may be encapsuled in ""
SCPI_ParamString - read unspecified parameter not encapsulated in ""
SCPI_ParamBool - read boolean value (ON, OFF, 0, 1)
SCPI_ParamChoice - read enumeration value eg. (BUS, IMMediate, EXTernal) defined by parameter

These are the functions, you can use to write command results

SCPI_ResultInt - write integer value
SCPI_ResultDouble - write double value
SCPI_ResultText - write text value encapsulated in ""
SCPI_ResultString - directly write string value
SCPI_ResultBool - write boolean value

You can use the function SCPI_NumberToStr to convert number with units to textual representation and then use SCPI_ResultString to write this to the user.