## Summary of changes for rlink v3 to v4

### Background
The protocol was initially designed as debug interface used over serial 
port connections. From the beginning the protocol had crc error checking
and a simple error recovery mechanism.
When the protocol was used in the IO emulation of the w11, features like
block transfers, attentions, and command groups were added. Over time the
original simple concept for error recovery became practically unusable.
When the protocol was used on boards with a Cypress FX2 USB interface
the number of round trips became the sole performance limiting factor.

### Goals for rlink v4
- 16 bit addresses (instead of 8 bit)
- more robust encoding, support for error recovery at transport level
- add features to reduce round trips
  - improved attention handling
  - new 'list abort' command

### Changes in detail

#### Encoding
- framing (comma) char representation changed
  - now 2 byte sequence for comma char, with internal redundancy
  - optimized for robustness. Also more compact for larger `rblk`/`wblk`.

#### Framing
- unexpected `EOP` in `sl_idle` now silently ignored

          --> before: send NAK+EOP
          --> now allows to send EOP+NAK to start a retransmit

- command aborts send now an error code, the abort sequence
 
          NAK <nakbyte> EOP

- the nakbyte has the redundant format

          10<!nakcode><nakcode>

- the abort sequence sequence is not protected by a crc, but has enough
  redundancy that transmission errors can be detected.
- all unexpected commas after `SOP` will cause an abort. This adds robustness
  in case transmission error converts a data byte into a comma.

#### Commands
- 16 bit addresses
- 16 bit `rblk`/`wblk` transfer size counts. Now cnt rather cnt-1 used.
- `rblk`/`wblk` now return 'done count', number of successfully transfered
  words Note: rblk always transfers cnt words, rest is padded.
- `babo` state flag added
  - `babo` is cleared when `rblk`/`wblk` is started, and set when they 
    are aborted
  - `babo` is not changed by commands other then rblk and wblk
- `stat` command removed (functionality not needed anymore)
- `labo` command added
  - returns the babo flag
  - if babo set, all remaining commands in the list will be ignored
- `stat` byte layout changed
  - `cerr` and `derr` flags removed (not needed anymore)
  - now 4 (instead of 3) external `RB_STAT` bit
- 16 bit crc used (instead of 8 bit)

#### attn handling
- a message with the current attn pattern is send, not only an attn comma.
  This give the attn handler a priori knowledge of `LAM` sources.
  An `attn` command must still be used to harvest the attn pattern.
- attn poll always returns attn notify, usage of idle comma removed

#### General
- reserve 0xff00-0xffff range for rlink system usage
- implement 4 default registers (in rlink_core)

          ffff    cntl
          fffe    stat   (holds rtbuf size)
          fffc/d  sysid  (32 bit system identifier)

- rlink initialization now via wreg, not with `init` anymore
- has now retransmit buffer, size configurable (2,4,8,.. kB)
- used for `wblk` dcrc validation in addition
- a `NAK` outside a `SOP`/`EOP` frame will trigger a retransmit of last 
  response
- retransmit buffer cleared when first cmd processed

        --> an empty SOP-EOP does not reset the retransmit buffer

- no internal/external `init` distinction, 'we' always 0 when init=1