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rtems/c/src/lib/libbsp/powerpc/mpc8260ads/network/README
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| 1 | Networking with HDLC |
| 2 | ==================== |
| 3 | |
| 4 | Author: Andy Dachs <a.dachs@sstl.co.uk> |
| 5 | Date: 31st August 2001 |
| 6 | Surrey Satellite Technology Limited |
| 7 | |
| 8 | |
| 9 | The network support in this BSP is not Ethernet support. The |
| 10 | "network" referred to here is a point to point HDLC communication link. |
| 11 | The ADS board does have a 10/100 ethernet port and it would be nice to |
| 12 | get support for that added. |
| 13 | |
| 14 | My requirement is for a WAN so I need the ability to send IP frames over |
| 15 | HDLC. Ultimately this will end up as frame relay support but in the meantime |
| 16 | I'm simply wrapping up the IP packet inside an HDLC frame. There is no |
| 17 | addressing mechanism or mac address attached to the start of the frame. |
| 18 | |
| 19 | This is what is physically transmitted: |
| 20 | <FLAG><IP Frame><CRC><FLAG> |
| 21 | |
| 22 | |
| 23 | The physical link consists of four lines, |
| 24 | TX DATA : Data transmitted |
| 25 | TX CLOCK: Clock for transmitted data. Data source provides clock. |
| 26 | RX DATA : Received data |
| 27 | RX CLOCK: Clock for received data. Data sink accepts clock. |
| 28 | |
| 29 | To connect two entities you require a NULL modem arrangement, i.e. TX data |
| 30 | and Tx clock from one end go into RX data and Rx clock on the other end. |
| 31 | |
| 32 | The MPC8260ADS side of the link is implemented using SCC3 in HDLC mode. The |
| 33 | TX clock is generated by BRG4. The RX clock in input to the board on the CLK5 |
| 34 | input. I built a LVTTL to RS422 converter. |
| 35 | |
| 36 | The other end of the link is a Windows NT PC with WANic400 synchronous |
| 37 | communication card. We bought the card from ImageStream |
| 38 | (http://www.imagestream.com/WANic400.html). You also need the NT |
| 39 | drivers and a cable. There are other distributors but I found these |
| 40 | guys helpful - and they also do Linux drivers. |
| 41 | |
| 42 | The NT WANic driver has a number of modes, one of which is ethernet emulation. |
| 43 | This mode is what is needed to transport IP packets in HDLC frames. |
| 44 | |
| 45 | In libnetworking/net you will find a file called if_hdlcsubr.c containing |
| 46 | the hdlc_input and hdlc_output routines required by network.c. This file was |
| 47 | created by taking out the addressing mechanisms from if_ethersubr.c in the |
| 48 | same directory. There are probably neater ways to do this <any |
| 49 | contributions welcome here>. |
| 50 | |
| 51 | |
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| 55 |
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