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$Id: README_Rlink_V4.txt 614 2014-12-20 15:00:45Z mueller $
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Summary of changes for rlink v3 to v4
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Background
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The protocol was initially designed as debug interface used over serial
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port connections. From the beginning the protocol had crc error checking
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and a simple error recovery mechanism.
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When the protocol was used in the IO emulation of the w11, features like
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block transfers, attentions, and command groups were added. Over time the
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original simple concept for error recovery became practically unusable.
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When the protocol was used on boards with a Cypress FX2 USB interface
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the number of round trips became the sole performance limiting factor.
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Goals for rlink v4
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- 16 bit addresses (instead of 8 bit)
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- more robust encoding, support for error recovery at transport level
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- add features to reduce round trips
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- improved attention handling
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- new 'list abort' command
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Changes in detail
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- encoding
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- framing (comma) char representation changed
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- now 2 byte sequence for comma char, with internal redundancy
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- optimized for robustness. Also more compact for larger rblk/wblk.
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- framing
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- unexpected EOP in sl_idle now silently ignored
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--> before: send NAK+EOP
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--> now allows to send EOP+NAK to start a retransmit
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- command aborts send now an error code, the abort sequence
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NAK EOP
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- the nakbyte has the redundant format
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10
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- the abort sequence sequence is not protected by a crc, but has enough
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redundancy that transmission errors can be detected.
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- all unexpected commas after SOP will cause an abort. This adds robustness
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in case transmission error converts a data byte into a comma.
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- commands
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- 16 bit addresses
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- 16 bit rblk/wblk transfer size counts. Now cnt rather cnt-1 used.
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- rblk/wblk now return 'done count', number of successfully transfered words
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Note: rblk always transfers cnt words, rest is padded.
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- babo state flag added
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- babo is cleared when rblk/wblk is started, and set when they are aborted
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- babo is not changed by commands other then rblk and wblk
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- stat command removed (functionality not needed anymore)
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- labo command added
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- returns the babo flag
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- if babo set, all remaining commands in the list will be ignored
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- stat byte layout changed
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- cerr and derr flags removed (not needed anymore)
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- now 4 (instead of 3) external RB_STAT bit
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- 16 bit crc used (instead of 8 bit)
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- attn handling
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- a message with the current attn pattern is send, not only an attn comma.
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This give the attn handler a priori knowledge of LAM sources.
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An attn command must still be used to harvest the attn pattern.
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- attn poll always returns attn notify, usage of idle comma removed
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- general
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- reserve 0xff00-0xffff range for rlink system usage
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- implement 4 default registers (in rlink_core)
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ffff cntl
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fffe stat (holds rtbuf size)
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fffc/d sysid (32 bit system identifier)
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- rlink initialization now via wreg, not with init anymore
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- has now retransmit buffer, size configurable (2,4,8,.. kB)
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- used for wblk dcrc validation in addition
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- a NAK outside a SOP/EOP frame will trigger a retransmit of last response
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- retransmit buffer cleared when first cmd processed
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-> an empty SOP-EOP does not reset the retransmit buffer
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- no internal/external init distinction, 'we' always 0 when init=1
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