The printer is accessible through /dev/lp0; in the same way, the parallel port itself is accessible through /dev/parport0. The difference is in the level of control that you have over the wires in the parallel port cable.
With the printer driver, a user-space program (such as the printer spooler) can send bytes in "printer protocol". Briefly, this means that for each byte, the eight data lines are set up, then a "strobe" line tells the printer to look at the data lines, and the printer sets an "acknowledgement" line to say that it got the byte. The printer driver also allows the user-space program to read bytes in "nibble mode", which is a way of transferring data from the peripheral to the computer half a byte at a time (and so it's quite slow).
In contrast, the ppdev driver (accessed via /dev/parport0) allows you to:
examine status lines,
set control lines,
set/examine data lines (and control the direction of the data lines),
wait for an interrupt (triggered by one of the status lines),
find out how many new interrupts have occurred,
set up a response to an interrupt,
use IEEE 1284 negotiation (for telling peripheral which transfer mode, to use)
transfer data using a specified IEEE 1284 mode.
Programming interface
The ppdev interface is largely the same as that of other character special devices, in that it supports open, close, read, write, and ioctl. The constants for the ioctl commands are in include/linux/ppdev.h.
Starting and stopping: open and close
The device node /dev/parport0 represents any device that is connected to parport0, the first parallel port in the system. Each time the device node is opened, it represents (to the process doing the opening) a different device. It can be opened more than once, but only one instance can actually be in control of the parallel port at any time. A process that has opened /dev/parport0 shares the parallel port in the same way as any other device driver. A user-land driver may be sharing the parallel port with in-kernel device drivers as well as other user-land drivers.
Control: ioctl
Most of the control is done, naturally enough, via the ioctl call. Using ioctl, the user-land driver can control both the ppdev driver in the kernel and the physical parallel port itself. The ioctl call takes as parameters a file descriptor (the one returned from opening the device node), a command, and optionally (a pointer to) some data.
PPCLAIM
Claims access to the port. As a user-land device driver writer, you will need to do this before you are able to actually change the state of the parallel port in any way. Note that some operations only affect the ppdev driver and not the port, such as PPSETMODE; they can be performed while access to the port is not claimed.
PPEXCL
Instructs the kernel driver to forbid any sharing of the port with other drivers, i.e. it requests exclusivity. The PPEXCL command is only valid when the port is not already claimed for use, and it may mean that the next PPCLAIM ioctl will fail: some other driver may already have registered itself on that port.
Most device drivers don't need exclusive access to the port. It's only provided in case it is really needed, for example for devices where access to the port is required for extensive periods of time (many seconds).
Note that the PPEXCL ioctl doesn't actually claim the port there and then---action is deferred until the PPCLAIM ioctl is performed.
PPRELEASE
Releases the port. Releasing the port undoes the effect of claiming the port. It allows other device drivers to talk to their devices (assuming that there are any).
PPYIELD
Yields the port to another driver. This ioctl is a kind of short-hand for releasing the port and immediately reclaiming it. It gives other drivers a chance to talk to their devices, but afterwards claims the port back. An example of using this would be in a user-land printer driver: once a few characters have been written we could give the port to another device driver for a while, but if we still have characters to send to the printer we would want the port back as soon as possible.
It is important not to claim the parallel port for too long, as other device drivers will have no time to service their devices. If your device does not allow for parallel port sharing at all, it is better to claim the parallel port exclusively (see PPEXCL).
PPNEGOT
Performs IEEE 1284 negotiation into a particular mode. Briefly, negotiation is the method by which the host and the peripheral decide on a protocol to use when transferring data.
An IEEE 1284 compliant device will start out in compatibility mode, and then the host can negotiate to another mode (such as ECP).
The ioctl parameter should be a pointer to an int; values for this are in incluce/linux/parport.h and include:
IEEE1284_MODE_COMPAT
IEEE1284_MODE_NIBBLE
IEEE1284_MODE_BYTE
IEEE1284_MODE_EPP
IEEE1284_MODE_ECP
The PPNEGOT ioctl actually does two things: it performs the on-the-wire negotiation, and it sets the behaviour of subsequent read/write calls so that they use that mode (but see PPSETMODE).
PPSETMODE
Sets which IEEE 1284 protocol to use for the read and write calls.
The ioctl parameter should be a pointer to an int.
PPGETMODE
Retrieves the current IEEE 1284 mode to use for read and write.
PPGETTIME
Retrieves the time-out value. The read and write calls will time out if the peripheral doesn't respond quickly enough. The PPGETTIME ioctl retrieves the length of time that the peripheral is allowed to have before giving up.
The ioctl parameter should be a pointer to a struct timeval.
PPSETTIME
Sets the time-out. The ioctl parameter should be a pointer to a struct timeval.
PPGETMODES
Retrieves the capabilities of the hardware (i.e. the modes field of the parport structure).
PPSETFLAGS
Sets flags on the ppdev device which can affect future I/O operations. Available flags are:
PP_FASTWRITE
PP_FASTREAD
PP_W91284PIC
PPWCONTROL
Sets the control lines. The ioctl parameter is a pointer to an unsigned char, the bitwise OR of the control line values in include/linux/parport.h.
PPRCONTROL
Returns the last value written to the control register, in the form of an unsigned char: each bit corresponds to a control line (although some are unused). The ioctl parameter should be a pointer to an unsigned char.
This doesn't actually touch the hardware; the last value written is remembered in software. This is because some parallel port hardware does not offer read access to the control register.
The control lines bits are defined in include/linux/parport.h:
PARPORT_CONTROL_STROBE
PARPORT_CONTROL_AUTOFD
PARPORT_CONTROL_SELECT
PARPORT_CONTROL_INIT
PPFCONTROL
Frobs the control lines. Since a common operation is to change one of the control signals while leaving the others alone, it would be quite inefficient for the user-land driver to have to use PPRCONTROL, make the change, and then use PPWCONTROL. Of course, ea
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