Fixed typos in RS485 and made stuff clearer
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@ -9,12 +9,17 @@ The actual application layer will be developed independent of this specification
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Physical Layer
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Physical Layer
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The physical layer is based on a RS248 three wire bus (a differential pair for data and ground),
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The physical layer is based on a RS485 three wire bus (a differential pair for data and ground),
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using the *MCP2551* is used as transceiver chip.
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using the *MCP2551* is used as transceiver chip.
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Since this chip is actually a CAN-transceiver,
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Since this chip is actually a CAN-transceiver,
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it has a well defined behaviour in case of collisions (no short circuits possible)
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it has a well defined behaviour in case of collisions (no short circuits possible)
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and it allows to read back the bits which were actually on the bus while sending.
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and it allows to read back the bits which were actually on the bus while sending.
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This enables any device with a decent UART (microcontroller, raspi ...) to communicate over long distances (2000m+).
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Additionally for most regular RS485 transceivers a collision on the bus results
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in one transceiver pulling the bus while the other one pulls it high,
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effectively creating a temporary short circuit.
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Therefore using a CAN-transceiver is preferable for our bus configuration.
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Using a sss7modem any device with a decent UART (microcontroller, raspi ...) can communicate over long distances (2000m+).
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The data is send at 9600 Baud (aka. 9,6kbit/s) with a maximum baudrate error of +-5%,
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The data is send at 9600 Baud (aka. 9,6kbit/s) with a maximum baudrate error of +-5%,
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to ensure that even the slowest microcontroller can participate in the communication.
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to ensure that even the slowest microcontroller can participate in the communication.
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A slow bitrate also necessary to keep communication and collision detection reliable over longer
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A slow bitrate also necessary to keep communication and collision detection reliable over longer
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