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>The Linux 2.4 Parallel Port Subsystem</TH
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><H1
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><A
NAME="AEN123"
></A
>The IEEE 1284.3 API</H1
><P
> The ability to daisy-chain devices is very useful, but if every
device does it in a different way it could lead to lots of
complications for device driver writers. Fortunately, the IEEE
are standardising it in IEEE 1284.3, which covers daisy-chain
devices and port multiplexors.
</P
><P
> At the time of writing, IEEE 1284.3 has not been published, but
the draft specifies the on-the-wire protocol for daisy-chaining
and multiplexing, and also suggests a programming interface for
using it. That interface (or most of it) has been implemented in
the <TT
CLASS="LITERAL"
>parport</TT
> code in Linux.
</P
><P
> At initialisation of the parallel port <SPAN
CLASS="QUOTE"
>"bus"</SPAN
>,
daisy-chained devices are assigned addresses starting from zero.
There can only be four devices with daisy-chain addresses, plus
one device on the end that doesn't know about daisy-chaining and
thinks it's connected directly to a computer.
</P
><P
> Another way of connecting more parallel port devices is to use a
multiplexor. The idea is to have a device that is connected
directly to a parallel port on a computer, but has a number of
parallel ports on the other side for other peripherals to connect
to (two or four ports are allowed). The multiplexor switches
control to different ports under software control---it is, in
effect, a programmable printer switch.
</P
><P
> Combining the ability of daisy-chaining five devices together with
the ability to multiplex one parallel port between four gives the
potential to have twenty peripherals connected to the same
parallel port!
</P
><P
> In addition, of course, a single computer can have multiple
parallel ports. So, each parallel port peripheral in the system
can be identified with three numbers, or co-ordinates: the
parallel port, the multiplexed port, and the daisy-chain
address.
</P
><DIV
CLASS="MEDIAOBJECT"
><P
><IMG
SRC="parport-multi.png"></P
></DIV
><P
> Each device in the system is numbered at initialisation (by
<TT
CLASS="FUNCTION"
>parport_daisy_init</TT
>). You can convert between
this device number and its co-ordinates with
<TT
CLASS="FUNCTION"
>parport_device_num</TT
> and
<TT
CLASS="FUNCTION"
>parport_device_coords</TT
>.
</P
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN142"
></A
><P
></P
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <parport.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>parport_device_num</TT
></CODE
>(int <TT
CLASS="PARAMETER"
><I
>parport</I
></TT
>, int <TT
CLASS="PARAMETER"
><I
>mux</I
></TT
>, int <TT
CLASS="PARAMETER"
><I
>daisy</I
></TT
>);</CODE
></P
><P
></P
></DIV
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN153"
></A
><P
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>parport_device_coords</TT
></CODE
>(int <TT
CLASS="PARAMETER"
><I
>devnum</I
></TT
>, int *<TT
CLASS="PARAMETER"
><I
>parport</I
></TT
>, int *<TT
CLASS="PARAMETER"
><I
>mux</I
></TT
>, int *<TT
CLASS="PARAMETER"
><I
>daisy</I
></TT
>);</CODE
></P
><P
></P
></DIV
><P
> Any parallel port peripheral will be connected directly or
indirectly to a parallel port on the system, but it won't have a
daisy-chain address if it does not know about daisy-chaining, and
it won't be connected through a multiplexor port if there is no
multiplexor. The special co-ordinate value
<TT
CLASS="CONSTANT"
>-1</TT
> is used to indicate these cases.
</P
><P
> Two functions are provided for finding devices based on their IEEE
1284 Device ID: <TT
CLASS="FUNCTION"
>parport_find_device</TT
> and
<TT
CLASS="FUNCTION"
>parport_find_class</TT
>.
</P
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN170"
></A
><P
></P
><TABLE
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><TR
><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <parport.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>parport_find_device</TT
></CODE
>(const char *<TT
CLASS="PARAMETER"
><I
>mfg</I
></TT
>, const char *<TT
CLASS="PARAMETER"
><I
>mdl</I
></TT
>, int <TT
CLASS="PARAMETER"
><I
>from</I
></TT
>);</CODE
></P
><P
></P
></DIV
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN181"
></A
><P
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>parport_find_class</TT
></CODE
>(parport_device_class <TT
CLASS="PARAMETER"
><I
>cls</I
></TT
>, int <TT
CLASS="PARAMETER"
><I
>from</I
></TT
>);</CODE
></P
><P
></P
></DIV
><P
> These functions take a device number (in addition to some other
things), and return another device number. They walk through the
list of detected devices until they find one that matches the
requirements, and then return that device number (or
<TT
CLASS="CONSTANT"
>-1</TT
> if there are no more such devices). They
start their search at the device after the one in the list with
the number given (at <TT
CLASS="PARAMETER"
><I
>from</I
></TT
>+1, in other
words).
</P
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