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Fixed spelling mistakes and defined timeouts

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Sebastian 2015-05-20 22:07:04 +02:00
parent 676ae7c52d
commit 3382ec240b
1 changed files with 12 additions and 10 deletions
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README.md
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@ -11,33 +11,35 @@ Physical Layer
--------------
The physical layer is based on a RS248 three wire bus (a differential pair for data and ground),
using the *MCP2551* is used as transceiver chip.
Since it is actually a CAN-Transceiver,
Since this chip is actually a CAN-transceiver,
it has a well defined behaviour in case of collisions (no short circuits possible)
and it allows to read back the bits which are actually on the bus while sending.
and it allows to read back the bits which were actually on the bus while sending.
This enables any device with a decent UART (microcontroller, raspi ...) to communicate over long distances (2000m+).
The data is send at 9600 Baud (aka. 9,6kbit/s) with a maximum baudrate error of +-5%,
to ensure that even the slowest micro controller can participate in the communication.
to ensure that even the slowest microcontroller can participate in the communication.
A slow bitrate also necessary to keep communication and collision detection reliable over longer
distances.
Transport Layer
---------------
A device is allowed to send data after the bus has been idle for at least 24 bit times (aka. 2.5ms).
A device is allowed to send data after the bus has been idle for at least *48 bit times* (aka. 5ms).
After this period the sender is allowed to send a single frame.
The frame format is simply the first byte is the length of the entire frame in bytes (= N + 3),
The frame format is simply the first byte is the length of the entire frame in bytes (`= N + 3`),
followed by N bytes of payload.
The reaming 2 bytes of the are a 16bit CRC checksum over the frames payload.
If a received frame is shorter than it length field or if its CRC sum does not match its payload,
The remaining 2 bytes of the are a 16bit CRC checksum over the frames payload.
If a received frame has timed out or if its CRC sum does not match its payload,
any receiving devices should assume a collision and drop the frame.
Additionally each frame is associated to a priority between 0 (highest) and 5 (lowest),
which is not send over the bus.
A frame can be assumed as timed out if it was not fully received yet and no
additional data has been received for at least *24 bit times* (aka. 2.5ms).
Furthermore each frame is associated to a priority between 0 (highest) and 5 (lowest),
which is not part of the frame send over the bus.
Instead the priority is used to calculate the necessary back off in case of a collision.
The sender has to detect collisions by reading back each byte written to the bus immediately after writing it.
If the read byte does not match the written byte, a collision occurred.
In this case the sender has to wait for random back off interval until attempting a retransmit.
The back off interval is calculated as `(8 * bit_time) * (priority + rand(1,10))`,
The back off interval is calculated as `(8 * bit_time) * (priority + rand(1,5))`,
where `bit_time = 1 / 9600 s` and priority is the frames priority.
Device receiving on the bus are not necessarily able to detect collisions,
unless they show up as timing error at their UART.