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+This is the 6pack-mini-HOWTO, written by
+Andreas Könsgen DG3KQ
+Last update: April 7, 1998
+1. What is 6pack, and what are the advantages to KISS?
+6pack is a transmission protocol for data exchange between the PC and
+the TNC over a serial line. It can be used as an alternative to KISS.
+6pack has two major advantages:
+- The PC is given full control over the radio
+ channel. Special control data is exchanged between the PC and the TNC so
+ that the PC knows at any time if the TNC is receiving data, if a TNC
+ buffer underrun or overrun has occurred, if the PTT is
+ set and so on. This control data is processed at a higher priority than
+ normal data, so a data stream can be interrupted at any time to issue an
+ important event. This helps to improve the channel access and timing
+ algorithms as everything is computed in the PC. It would even be possible
+ to experiment with something completely different from the known CSMA and
+ DAMA channel access methods.
+ This kind of real-time control is especially important to supply several
+ TNCs that are connected between each other and the PC by a daisy chain
+ (however, this feature is not supported yet by the Linux 6pack driver).
+- Each packet transferred over the serial line is supplied with a checksum,
+ so it is easy to detect errors due to problems on the serial line.
+ Received packets that are corrupt are not passed on to the AX.25 layer.
+ Damaged packets that the TNC has received from the PC are not transmitted.
+More details about 6pack are described in the file 6pack.ps that is located
+in the doc directory of the AX.25 utilities package.
+2. Who has developed the 6pack protocol?
+The 6pack protocol has been developed by Ekki Plicht DF4OR, Henning Rech
+DF9IC and Gunter Jost DK7WJ. A driver for 6pack, written by Gunter Jost and
+Matthias Welwarsky DG2FEF, comes along with the PC version of FlexNet.
+They have also written a firmware for TNCs to perform the 6pack
+protocol (see section 4 below).
+3. Where can I get the latest version of 6pack for LinuX?
+At the moment, the 6pack stuff can obtained via anonymous ftp from
+db0bm.automation.fh-aachen.de. In the directory /incoming/dg3kq,
+there is a file named 6pack.tgz.
+4. Preparing the TNC for 6pack operation
+To be able to use 6pack, a special firmware for the TNC is needed. The EPROM
+of a newly bought TNC does not contain 6pack, so you will have to
+program an EPROM yourself. The image file for 6pack EPROMs should be
+available on any packet radio box where PC/FlexNet can be found. The name of
+the file is 6pack.bin. This file is copyrighted and maintained by the FlexNet
+team. It can be used under the terms of the license that comes along
+with PC/FlexNet. Please do not ask me about the internals of this file as I
+don't know anything about it. I used a textual description of the 6pack
+protocol to program the Linux driver.
+TNCs contain a 64kByte EPROM, the lower half of which is used for
+the firmware/KISS. The upper half is either empty or is sometimes
+programmed with software called TAPR. In the latter case, the TNC
+is supplied with a DIP switch so you can easily change between the
+two systems. When programming a new EPROM, one of the systems is replaced
+by 6pack. It is useful to replace TAPR, as this software is rarely used
+nowadays. If your TNC is not equipped with the switch mentioned above, you
+can build in one yourself that switches over the highest address pin
+of the EPROM between HIGH and LOW level. After having inserted the new EPROM
+and switched to 6pack, apply power to the TNC for a first test. The connect
+and the status LED are lit for about a second if the firmware initialises
+the TNC correctly.
+5. Building and installing the 6pack driver
+The driver has been tested with kernel version 2.1.90. Use with older
+kernels may lead to a compilation error because the interface to a kernel
+function has been changed in the 2.1.8x kernels.
+How to turn on 6pack support:
+- In the linux kernel configuration program, select the code maturity level
+ options menu and turn on the prompting for development drivers.
+- Select the amateur radio support menu and turn on the serial port 6pack
+- Compile and install the kernel and the modules.
+To use the driver, the kissattach program delivered with the AX.25 utilities
+has to be modified.
+- Do a cd to the directory that holds the kissattach sources. Edit the
+ kissattach.c file. At the top, insert the following lines:
+ #ifndef N_6PACK
+ #define N_6PACK (N_AX25+1)
+ Then find the line
+ int disc = N_AX25;
+ and replace N_AX25 by N_6PACK.
+- Recompile kissattach. Rename it to spattach to avoid confusions.
+Installing the driver:
+- Do an insmod 6pack. Look at your /var/log/messages file to check if the
+ module has printed its initialization message.
+- Do a spattach as you would launch kissattach when starting a KISS port.
+ Check if the kernel prints the message '6pack: TNC found'.
+- From here, everything should work as if you were setting up a KISS port.
+ The only difference is that the network device that represents
+ the 6pack port is called sp instead of sl or ax. So, sp0 would be the
+ first 6pack port.
+Although the driver has been tested on various platforms, I still declare it
+ALPHA. BE CAREFUL! Sync your disks before insmoding the 6pack module
+and spattaching. Watch out if your computer behaves strangely. Read section
+6 of this file about known problems.
+Note that the connect and status LEDs of the TNC are controlled in a
+different way than they are when the TNC is used with PC/FlexNet. When using
+FlexNet, the connect LED is on if there is a connection; the status LED is
+on if there is data in the buffer of the PC's AX.25 engine that has to be
+transmitted. Under Linux, the 6pack layer is beyond the AX.25 layer,
+so the 6pack driver doesn't know anything about connects or data that
+has not yet been transmitted. Therefore the LEDs are controlled
+as they are in KISS mode: The connect LED is turned on if data is transferred
+from the PC to the TNC over the serial line, the status LED if data is
+sent to the PC.
+6. Known problems
+When testing the driver with 2.0.3x kernels and
+operating with data rates on the radio channel of 9600 Baud or higher,
+the driver may, on certain systems, sometimes print the message '6pack:
+bad checksum', which is due to data loss if the other station sends two
+or more subsequent packets. I have been told that this is due to a problem
+with the serial driver of 2.0.3x kernels. I don't know yet if the problem
+still exists with 2.1.x kernels, as I have heard that the serial driver
+code has been changed with 2.1.x.
+When shutting down the sp interface with ifconfig, the kernel crashes if
+there is still an AX.25 connection left over which an IP connection was
+running, even if that IP connection is already closed. The problem does not
+occur when there is a bare AX.25 connection still running. I don't know if
+this is a problem of the 6pack driver or something else in the kernel.
+The driver has been tested as a module, not yet as a kernel-builtin driver.
+The 6pack protocol supports daisy-chaining of TNCs in a token ring, which is
+connected to one serial port of the PC. This feature is not implemented
+and at least at the moment I won't be able to do it because I do not have
+the opportunity to build a TNC daisy-chain and test it.
+Some of the comments in the source code are inaccurate. They are left from
+the SLIP/KISS driver, from which the 6pack driver has been derived.
+I haven't modified or removed them yet -- sorry! The code itself needs
+some cleaning and optimizing. This will be done in a later release.
+If you encounter a bug or if you have a question or suggestion concerning the
+driver, feel free to mail me, using the addresses given at the beginning of