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 is a standard UART...

... which has these 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. The A/B lines are biased once at the gateway with 2x 1kOhms resistors.

The maximum length @38,400 bauds is 4 kFt = 1.2 km (see the graphic on the right).

Every data frame contain chronologically:

• 2 bytes of slave adress (ID_HIGH and ID_LOW) => 65536 slave adress at all
  • 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 a 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 Modbus (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

The CRC Modbus can be calculated here

Technical Documents

1. Hardware
2. ...