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4.5 Barbus
The Barbus is a proprietary bus, which combines RS485 serial data links and power lines.
As shown in Fig. 2, it is connected in a daisy chain layout between the main modules of the
MPU6 system.
Connection to each module is via an 11-way I.D.C connector, which includes a polarising key
at pin position 7 and has three elongated pins for the 0V connections.
This ensures that when a module is replaced the 0V line is always the first line to be
connected and the last to be disconnected.
In addition to the power supplies routed to modules around the Barbus, +24V dc and -12V dc
supplies are fed from the PSU directly to the MPU6 board only and do not form part of the
Barbus daisy chain.
To reduce the power supply currents flowing in the Barbus, Current is therefore supplied to
each module over two paths, halving the supply currents on the bus.
4.5.1 Serial link
The RS485 serial data link allows two-wire communications between the intelligent modules
at rates up to 100Kbytes/sec.
4.5.2 Addressing
The MPU6 board is the Master device on the Barbus, the MUX5 board(s) and REEL5 board
are slave devices.
As shown in Fig. 2 the MPU6 board is connected to the slave boards by six Barbus lines
only, the remaining four lines are used by slave boards to provide individual board addresses
by means of links wired into the Barbus connectors.
This allows up to eight slaves of one type (e.g. MUX5/MUX5E board) to be addressed, the
type address being transmitted by the MPU6 board via the serial data link.
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