Class Bus

Initialize Dynamixel Bus

INPUT: 
  port -- serial port string or None to autoconfig

ATTRIBUTES:
  ser -- serial handle
  buf -- buffer string '
  expect -- number of expected bytes. default: 6
  eSync -- number of SYNC errors
  eChksum -- number of checksum errors  
  eLen -- number of length errors   
  eID -- number of ID errors 
  count -- a count of bytes received  
  rxPkts -- a count of valid packets received
  txPkts -- a count of packets sent

Overrides: object.__init__

returns the current Bus statistics 

OUTPUTS:
  -- list -- strings indicating current counts, error, etc ...

(private) Parse and output error returned from dynamixel servos as per 3-4-1 pp. 25

Parses a message into human readable form
INPUT:
  msg -- one or more message in a string
OUTPUT:
  the messages in human-readable form

reads a single packet off of the RS485 or serial bus 

INPUTS:
  None
OUTPUTS:
  pkt -- string -- valid packet from Dynamixel hardware bus
PRECONDITIONS:
  existence of serial object
  reset() has been called at least once
POSTCONDITIONS:
  pkt is returned given that all values of packet check OUTPUTS

THEORY OF OPERATION:
  Parse for valid packets as per EX-106 section 3-3 pp. 17
  - While there are bytes to be read from serial buffer or from buf
    - Read in expected number of bytes if available
    - Check for start frame, otherwise drop byte
    - Check for valid ID, otherwise drop byte
    - Check for valid Length, otherwise drop byte
    - Once Length bytes arrive, check if CRC is valid, otherwise drop byte
  - If there are no more bytes to read or parse then return None

Build a message and  transmit, update rxPkts, and return message string

INPUTS:
  nid -- int -- node ID of module
  cmd -- int -- command value. ie: CMD_PING, CMD_WRITE_DATA, etc ...
  pars -- string -- parameter string
OUTPUTS: 
  msg -- string -- transmitted packet minus sync          
PRECONDITIONS:
  existance of serial object        
POSTCONDITIONS:
  None

THEORY OF OPERATION:
  Assemble packet as per EX-106 section 3-2 pp. 16,
  Compute checksum, transmit and then return string

Build a SYNC_WRITE message writing a value

INPUTS:
  nid -- int -- node ID of module
  addr -- string -- address
  pars -- string -- value (after marshalling)
OUTPUTS: 
  msg -- string -- transmitted packet minus sync          
PRECONDITIONS:
  existance of serial object        
POSTCONDITIONS:
  None

THEORY OF OPERATION:
  Set a value remotely, without expecting a reply

Send a low level ping command to a given nid 

INPUTS:
  nid -- int -- node ID of module
OUTPUTS: 
  None 
PRECONDITIONS:
  instantiation of dynamixel.Bus
POSTCONDITIONS:
  No response occurs 

THEORY OF OPERATION:
  Send CMD_PING for given nid as per section 3-5-5 pp. 37

Send a low level reset command to a given nid 

INPUTS:
  nid -- int -- node ID of servo to reset 

THEORY OF OPERATION:
  Send CMD_RESET for given nid as per section 3-5-6 pp. 38
  Resets all values in the servo's control table to factory
  defaults, INCLUDING the ID, which is reset to 0x01. 

  By convention, we reserve ID 0x01 for configuration purposes

Send a command in synchronous form, waiting for reply

INPUTS:
  nid, cmd, pars -- same as for .send()
  timeout -- float-- maximal wait per retry
  retries -- number of allowed retries until giving up
  rate -- wait time between retries
OUTPUTS: 
  pkt -- string -- response packet's payload          

THEORY OF OPERATION:
  Plan out sending times for all retries. When a retry goal time is passed
  sends out the message. In between, poll for a reply for that message at a
  hard coded rate (currently 100Hz)

Unpack reply header and payload
OUTPUT: nid, len, cmd, tail
  nid -- node ID
  len -- message length 
  cmd -- command code in message
  tail -- remainder of the message, as a string