FixmeBus

Overview

FixmeBus is a field bus based on the half duplex RS485 famous hardware protocol. It is a master / slave architecture, with every slave chained together. The communication protocol has following parameters:

• 38,400 bauds
• 8 data bits
• 1 start and 1 stop bit
• no parity
• no flow control

Each end of the bus is terminated with a 1% 120R resistor.

Every data frame on the bus is like this (byte on the left = first byte):

• 2 bytes of slave adress (ID_HIGH and ID_LOW) => 65536 slave adress
  • ID = 0 => Master
  • ID = 65535 => order to all slaves (no ack)
  • ID = 65000 to 65499 => order to a group of slave (no ack)
  • ID = 1000 to 64999 => order to an unique slave (ack)
• 2 bytes for the function (FCT_HIGH and FCT_LOW)
• 1 byte for the number of data bytes to transmit (N_BYTE)
• n byte(s) of data (DATA) bytes (max. 200 bytes per frame)
• 2 bytes for the CRC (CRC_HIGH and CRC_LOW)

b0	b1	b2	b3	b4	b5	...	bn+4	bn+5	bn+6
ID_HIGH	ID_LOW	FCT_HIGH	FCT_LOW	N_BYTE	DATA[0]	...	DATA[n-1]	CRC_HIGH	CRC_LOW

Here is the first brainstorming on the Fixme's dashboard:

Technical information

1. Hardware
2. Software