From 0c333567f599206cb0be6b74f02e59820536e0b2 Mon Sep 17 00:00:00 2001
From: cbdev <cb@cbcdn.com>
Date: Sun, 31 Mar 2019 09:00:12 +0200
Subject: Move backend documentation out of main README

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+### The `sacn` backend
+
+The sACN backend provides read-write access to the Multicast-UDP based streaming ACN protocol (ANSI E1.31-2016),
+used for lighting fixture control. The backend sends universe discovery frames approximately every 10 seconds,
+containing all write-enabled universes.
+
+#### Global configuration
+
+| Option	| Example value		| Default value 	| Description		|
+|---------------|-----------------------|-----------------------|-----------------------|
+| `name`	| `sACN source`		| `MIDIMonster`		| sACN source name	|
+| `cid`		| `0xAA 0xBB 0xCC` ...	| `MIDIMonster`		| Source CID (16 bytes)	|
+| `bind`	| `0.0.0.0 5568`	| none			| Binds a network address to listen for data. This option may be set multiple times, with each descriptor being assigned an index starting from 0 to be used with the `interface` instance configuration option. At least one descriptor is required for transmission. |
+
+#### Instance configuration
+
+| Option	| Example value		| Default value 	| Description		|
+|---------------|-----------------------|-----------------------|-----------------------|
+| `universe`	| `0`			| none			| Universe identifier	|
+| `interface`	| `1`			| `0`			| The bound address to use for data input/output |
+| `priority`	| `100`			| none			| The data priority to transmit for this instance. Setting this option enables the instance for output and includes it in the universe discovery report. |
+| `destination`	| `10.2.2.2`		| Universe multicast	| Destination address for unicast output. If unset, the multicast destination for the specified universe is used. |
+| `from`	| `0xAA 0xBB` ...	| none			| 16-byte input source CID filter. Setting this option filters the input stream for this universe. |
+| `unicast`	| `1`			| `0`			| Prevent this instance from joining its universe multicast group |
+
+Note that instances accepting multicast input also process unicast frames directed at them, while
+instances in `unicast` mode will not receive multicast frames.
+
+#### Channel specification
+
+A channel is specified by it's universe index. Channel indices start at 1 and end at 512.
+
+Example mapping:
+```
+sacn1.231 < sacn2.123
+```
+
+A 16-bit channel (spanning any two normal 8-bit channels in the same universe, also called a wide channel) may be mapped with the syntax
+```
+sacn.1+2 > sacn2.5+123
+```
+
+A normal channel that is part of a wide channel can not be mapped individually.
+
+#### Known bugs / problems
+
+The DMX start code of transmitted and received universes is fixed as `0`.
+
+The (upper) limit on packet transmission rate mandated by section 6.6.1 of the sACN specification is disregarded.
+The rate of packet transmission is influenced by the rate of incoming mapped events on the instance.
+
+Universe synchronization is currently not supported, though this feature may be implemented in the future.
+
+To use multicast input, all networking hardware in the path must support the IGMPv2 protocol.
+
+The Linux kernel limits the number of multicast groups an interface may join to 20. An instance configured
+for input automatically joins the multicast group for its universe, unless configured in `unicast` mode.
+This limit can be raised by changing the kernel option in `/proc/sys/net/ipv4/igmp_max_memberships`.
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