I/O Controller 1204: 12DI, 4DO & Serial RS-232 port over IP

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All input and output operations are performed through ports. A port is a pointer into a possibly infinite stream of data often a filean opening through which programs may draw bytes or characters from the stream or place bytes or characters into the stream. A port may be an input port, an output port, or both simultaneously. Ports are first-class objects, like any other object in Scheme.

Like procedures, ports do not have a printed representation the way strings and numbers do. There are initially three ports: Several ways to open new ports are provided. An input port often points to a finite stream, e. If one of the input operations, e. Ports are either binary or textual. A binary port allows a program to read or write 8-bit unsigned bytes, or "octets," from or to the underlying stream.

A textual port allows a program to read or write characters. In many cases, the underlying stream is organized as a sequence of bytes, continuing on binary input and output ports these bytes should be treated as encodings for characters.

In this case, a textual port may be created with a transcoder to decode bytes to characters for input or encode characters to bytes for output. A transcoder encapsulates a codec that determines how characters are represented as bytes. Three standard codecs are provided: For the latin-1 encoding, each character is represented by exactly one byte. For utf-8each character is represented by from one to four bytes, and for utfeach character is represented by two or four bytes.

A transcoder also encapsulates an eol style that determines whether and how line endings are recognized. If the eol style is noneno line endings are recognized. The six other standard eol styles are the following: Unicode next-line character ls: Unicode line-separator character crlf: For input, any eol style except none causes each of the line-ending characters or two-character sequences to be converted into a single line-feed character.

For output, any eol style except none causes line-feed characters to be converted into the specific one- or two-character sequence associated with the eol style. In the input direction, all eol styles except none are equivalent, while in the output direction, the eol styles none and lf are equivalent.

In addition to the codec and eol style, a transcoder encapsulates just one other piece of information: The error-handling mode is ignoreraiseor replace. If the error-handling mode is ignorethe offending sequence of bytes or the character is ignored. If the error-handling mode is replacea replacement character or character encoding is produced: A port may be buffered for efficiency, to eliminate the overhead of a call into the operating system for each byte or character.

Three standard buffer modes are supported: With block buffering, input is drawn from a stream and output is sent to the stream in chunks of some implementation-dependent size. With line buffering, buffering is performed on a line-by-line basis or on some other implementation-dependent basis. Line buffering is typically distinguished from block buffering only for textual output ports; there are no line divisions in binary ports, and input is likely to be drawn from a stream as it becomes available.

With buffer-mode none, no buffering is performed, so output is sent immediately to the stream and input is drawn only as needed. Transcoders As described above, transcoders encapsulate three values: This section describes the procedures that create or operate on transcoders and the values that transcoders encapsulate. The eol-style syntax provides useful documentation as well.

The continuing on binary input and output ports syntax provides useful documentation as well. Opening Files The procedures in this section are used for opening file ports. Procedures for opening other kinds of ports, e. Each of the file-open operations accepts a path argument that names the file to be opened.

It must be a string or some other implementation-dependent value that names a file. Some of the file-open procedures accept optional optionsb-modeand? Binary ports created by the procedures in this section support continuing on binary input and output ports port-position and set-port-position! Whether textual ports created by the procedures in this section support these operations is implementation-dependent.

There are three standard file options: With the default file options, i. If the no-fail option is included, no exception is continuing on binary input and output ports if the file already exists; instead, the file is opened and truncated to zero length. The no-create option implies the no-fail option.

The no-truncate option is relevant only if the no-fail option is included or implied, in which case if an existing file is opened, it is not truncated, but the port's position is still set to the beginning of the file. Continuing on binary input and output ports is perhaps easier to imagine that the default file options are the imaginary option symbols createfail-if-existsand truncate ; no-create removes createno-fail removes fail-if-existsand no-truncate removes truncate.

Implementations may support additional file option symbols. The buffer-mode syntax provides useful documentation as well. Otherwise, continuing on binary input and output ports procedure returns a binary input port. See the lead-in to this section for a description of the constraints on and effects of the other arguments.

Otherwise, this procedure returns a binary output port. Standard Ports The procedures described in this section return ports that are attached to a process's standard input, standard output, and standard error streams. The first set returns "ready-made" textual ports with implementation-dependent transcoders if any and buffer modes.

Thus, these procedures are typically appropriate only when a program no longer needs to use any existing ports attached to the standard streams. String and Bytevector Ports The procedures in this section allow bytevectors and strings to be used as input or output streams. The effect of modifying bytevector after this procedure is called is unspecified. The new port may or may not have a transcoder, and if it does, the transcoder is implementation-dependent.

While not required, implementations are encouraged to support port-position and set-port-position! Otherwise, the port value is a binary output port. The extraction procedure is a procedure that, when called without arguments, creates a bytevector containing the accumulated bytes in the port, clears the port of its accumulated bytes, resets its position to zero, and returns the bytevector.

The accumulated bytes include any bytes written beyond the end of the current position, if the position has been set back from its maximum extent. The accumulated characters include any characters written beyond the end of the current position, if the position has been set back from its maximum extent.

Otherwise, procedure is continuing on binary input and output ports with a binary bytevector output port. If procedure returns, a bytevector containing the bytes accumulated in the port is created, the accumulated bytes are cleared from the port, the port's position is reset to zero, and the bytevector is returned from call-with-bytevector-output-port. These actions occur each time procedure returns, if it returns multiple times due to the invocation of a continuation created while procedure is active.

If procedure returns, a string containing the characters accumulated in the port is created, the accumulated characters are cleared from the port, the port's position is reset to zero, and the string is returned from call-with-string-output-port.

Opening Custom Ports procedure: These arguments are described below. It is called with three arguments: Continuing on binary input and output ports the byte stream is at end of file, r! Otherwise, it should read at least one and at most n bytes from the stream, store these bytes in consecutive locations of bytevector starting at startand return as an exact positive integer the number of bytes actually read.

If it is fthe port will not support port-position. If it is not fit will be passed zero arguments and should return the current position as a displacement in bytes from the start of the byte stream as an exact nonnegative integer. If it is fthe port will not support set-port-position! If it is not fit will be passed one argument, an exact nonnegative integer representing the new position as a displacement in bytes from the start of the byte stream, and it should set continuing on binary input and output ports position to this value.

If it is f continuing on binary input and output ports, no action will be taken to close the byte stream when the new port is closed. If it is not fit will be passed zero arguments and should take whatever actions are necessary to close the byte stream.

For the same reason, a call to port-position after an input operation may not return an accurate position if the sp! If the character stream is at end of file, r! Otherwise, it should read at least one and at most n characters from the stream, store these characters in consecutive locations of string starting at startand return as an exact positive integer the number of characters actually read. If it is not fcontinuing on binary input and output ports will be passed zero arguments and should return the current position, which may be an arbitrary value.

If it is not fit will be passed one argument, posa value representing the new position. If pos is the result of a previous call to gpsp! If it is fno action will be taken to close the character stream when the new port is closed. If it is not fit will be passed zero arguments and should take whatever actions are necessary to close the character stream. Since the representations of port positions are not specified, it is not possible for the implementation to adjust the gp return value to account for the number of buffered characters.

For the same reason, a call to port-position after an input operation may not return an accurate position, even if the sp! It should, however, be possible to perform output reliably after reading if the position is reset to the starting position. Port Operations This section describes a variety of operations on ports that do not directly involve either reading from or writing to a port.

The input and output operations are described in subsequent sections. Once a port has been closed, no more input or output operations may be performed on the port.

Because the operating system may place limits on the number of file ports open at one time or restrict access to an open file, it is good practice to continuing on binary input and output ports any file port that will continuing on binary input and output ports longer be used for input or output.

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In SISC all procedures that create ports for accessing files, e. Here are some examples of valid URLs:. For further details on the format of URLs please consult this specification. What types of URLs are supported by a particular installation of Java depends on the configured protocol handlers.

Relative file names or URLs are resolved in relation to the following parameter:. It is initialized on start up with the path to the current directory.

All parameters and the returned value are strings. The algorithm for resolving relative URLs is defined in this specification. For compatibility with other Schemes, SISC also supports the current-directory procedure, which is a simple wrapper around current-url.

A convenience procedure exists for executing a procedure while the current-url is temporarily set to a different value:. When called with one string argument, normalize-url returns the normalized version of the given URL. Normalization involves, amongst other things, the replacement of relative path references such as. When called with two string arguments, the procedure returns the normalized version of the second URL when interpreted as a being relative to the first URL.

Buffered ports are provided in SISC to layer over any existing port, and read or write in larger, more efficient chunks. Buffered ports are created with the following constructors, which accept the underlying port and an optional size, indicating the number of bytes or characters to buffer before making an underlying read or write.

Creates a binary-input-port which is buffered, and reads its bytes from the provided port. Creates a binary-output-port which is buffered, and writes its bytes to the provided port. Creates a character-input-port which is buffered, and reads its characters from the provided port. Creates a character-output-port which is buffered, and writes its characters to the provided port.

With buffered output ports, individual writes may not actually reach the eventual output source, so the programmer must explicitly flush the port when it the output data must reach its destination.

Causes the specified output-port 's buffered data to be written immediately. This operation is allowed on any output port, but may have no affect on some.

Character ports read characters from input sources and treat the data as characters in a given character set. Correspondingly, character ports output bytes from characters according to a given character set's encoding rules.

By default character ports use the value of the string parameter character-set as the character encoding name. A list of many possible encoding names can be found in the Java Platform Documentation. One may temporarily change the default character set using the with-character-set function.

Changes the value of the character-set parameter, and the default character set to the named encoding while executing the body of thunk. Creates an input port from the specified url. If the optional encoding parameter, a string, is supplied input will be decoded from the specified encoding rather than the default.

Creates an output port to the specified url. If the optional encoding parameter, a string, is supplied output will be encoded in the specified encoding rather than the default. If the optional auto-flush argument is provided and is non-false, the port will automatically flush after each write call. If the specified file exists, it will be overwritten silently when the port is opened.

Creates an input port which reads from the provided binary input port. One may specify the desired character set as a string, otherwise the character set is retrieved from the character-set dynamic parameter. Creates an output port which writes to the provided binary output port.

Reads up to count characters from the implicit or specified character port into the string buffer starting from the position specified by the integer offset.

The number of characters successfully read which may be fewer than count is returned, or the end-of-file value if the end-of-file was reached before any characters were encountered. Writes exactly count characters from the given string buffer starting from the integer position offset to the implicit or specified character output port.

The next set of procedures assists in creating a port, followed by calling a given procedure with that port. When the procedure returns, the port is closed. Invoking escaping continuations from inside the procedure does not close the port, and invoking a continuation captured inside the procedure does not open the port.

Calls procedure with a new input port attached to url. The result of the thunk is returned. If the optional encoding parameter is provided, the character port created will use the specified encoding rather than the default. Calls procedure with a new character output port attached to url. The following procedures wrap a thunk, redirecting the input and output of the thunk while it is evaluating to an input or output port other than the current-input-port and current-output-port.

Invoking escaping continuations from inside the procedure restores the original port, and invoking a continuation captured inside the procedure restores the redirection. Evaluates thunk with input-port as the current-input-port for the duration of the evaluation. Evaluates thunk with output-port as the current-output-port for the duration of the evaluation. Evaluates thunk with an input port attached to a file opened for reading from url as the current-input-port for the duration of the evaluation.

The port is closed when thunk returns normally. Evaluates thunk with an input port attached to a file opened for writing to url as the current-output-port for the duration of the evaluation. Returns t if value is an input port, f otherwise. Returns t if value is an output port, f otherwise.

String ports are input or output ports that read or write to a string rather than a file or other stream. String ports can be used to parse or emit formatted strings using the standard Scheme port operations. A String Input port will read from a given string until the end of string is reached, at which point! String ports deal with characters as the atomic unit, and as such preserve full unicode width characters at all times. Creates a string input port whose characters are read from the provided string.

Characters will be returned from any read operation on the port until the end of the string is reached. Read calls after reaching the end of the string will return! Creates a string output port, which behaves as an ordinary output port, except that writes are used to create a string as output. The results of all the write operations are retrieved using get-output-string.

Returns the string that was created by zero or more writes to a string output port. If no writes were performed on the string output port, an empty string "" is returned. After this call, the provided string output port is reset to its initial, empty state.

Calls procedure with a new string input port created from string. Calls procedure with a new string output port. The contents of the string-output-port are returned when the procedure returns. Evaluates thunk with a string-input-port created from string as the current-input-port for the duration of the evaluation. Evaluates thunk with a string-output-port created as the current-output-port for the duration of the evaluation. When the thunk returns, the contents of the string-output-port are returned.

Returns t if value is a string input port, f otherwise. Returns t if value is a string output port, f otherwise. Using character-oriented operations such as the traditional R 5 RS functions read , read-char , display , etc. Creates an input port in the same manner as R 5 RS open-input-file , producing an input port that does no character-set decoding on the bytes read as input.

Creates an output port in the same manner as open-output-file , producing an output port that does no character-set encoding. Calls procedure with a new binary input port attached to url. Calls procedure with a new binary output port attached to url. Evaluates thunk with a binary input port attached to a file opened for reading from url as the current-input-port for the duration of the evaluation.

Evaluates thunk with a binary output port attached to a file opened for writing to url as the current-output-port for the duration of the evaluation. There are several operations specifically available for use on binary ports.

Similar to peek-char , reads ahead one byte in the stream, returning the next byte available but not advancing the stream. The byte is returned as an integer. The current input port is used unless specified.

Reads a single byte from the stream, advancing the stream and returning the byte as an integer. Reads up to count bytes of data from the current input port or the binary-input-port parameter if provided, into the binary buffer buffer starting at position offset. Note that less than count bytes may be read. The number of bytes actually read is returned. If the end-of-file is encountered before any bytes could be read,!

Writes a single byte specified as an integer to the given binary-output-port if provided, current output port otherwise. Writes count bytes of data from the provided buffer at starting point offset to the given binary-output-port or to the current output port if unspecified.

Exactly count bytes will be written. Returns t if value is a binary input port, f otherwise. Returns t if value is a binary output port, f otherwise. Creates a buffer input port whose bytes are read from the provided buffer.

Bytes will be returned from any read operation on the port until the end of the buffer is reached.