ArtNet, RDM and Packet Interleaving - Revision history

66.92.11.32 at 22:57, 6 September 2010

2010-09-06T22:57:02Z

66.92.11.32: /* Send a single RDM command at once */

2010-09-06T20:44:44Z

‎Send a single RDM command at once

66.92.11.32: /* Possible Solutions */

2010-09-06T18:55:49Z

‎Possible Solutions

Nomis52 at 17:43, 6 September 2010

2010-09-06T17:43:24Z

Nomis52: Created page with '== RDM over Artnet == ArtNet nodes behave as pass-through devices for non-discovery RDM data. The normal sequence of RDM commands looks like this: [[Image:ArtNetRDMSeq…'

2010-09-06T17:12:25Z

Created page with '== RDM over Artnet == ArtNet nodes behave as pass-through devices for non-discovery RDM data. The normal sequence of RDM commands looks like this: [[Image:ArtNetRDMSeq…'

New page

== RDM over Artnet ==

[[ArtNet]] nodes behave as pass-through devices for non-discovery [[RDM]] data. The normal sequence of RDM commands looks like this:

[[Image:ArtNetRDMSequence.png]]

== The Potential Problem ==

The problem occurs when the RDM device responds with an ACK_OVERFLOW message.

From the standard:

''To receive the remaining data, the controller should continue to send GET_COMMANDS for the same PID. The responder shall send subsequent blocks of data with a response type of RESPONSE_TYPE_ACK_OVERFLOW until the remaining data can fit in a single message. The responder shall set the response type to RESPONSE_TYPE_ACK on the final response message in the sequence, to indicate completion of the data transfer. ''

Assuming that PID A causes an ACK_OVERFLOW, the sequence now looks like this:

[[Image:ArtNetRDMOverflowSequence.png]]

But the standard also states:

''The responder shall abort a partial transfer of overflow data for a PID when receiving a command for a different PID before the overflow data transfer is complete. A subsequent command for the overflow PID will result in a new data transfer starting at the beginning of the data set.''

So any RDM packet interleaved between the two ''GET PID A' commands in the above example will cause the transfer to abort.

* Note that the [http://www.goddarddesign.com/rdm_lab_pack.html Labpack] has a bug where it will not abort the transfer so this continues to work. This is present in version 0x20009

== References ==

* [http://www.artisticlicence.com/WebSiteMaster/User%20Guides/art-net.pdf ArtNet Protocol Specification]
* [http://www.esta.org/tsp/documents/published_docs.php E1.20 RDM Specification]

@@ Line 38: / Line 38: @@
 === Send multiple packets at once without waiting for a response ===
-This appears to be what [[DMX Workshop]] does and has the potential to break ack_overflow sequences.
+This appears to be what [[DMX Workshop]] does and has the potential to break ACK_OVERFLOW sequences.
 === Send multiple packets at once and back off when an ACK_OVERFLOW is received ===
@@ Line 49: / Line 49: @@
 * PROXIED_DEVICES
-* STATUS_MESSAGES
+* STATUS_MESSAGES - but this uses it's own queuing mechanism
 * SUPPORTED_PARAMETERS
 * LANGUAGE_CAPABILITIES

@@ Line 34: / Line 34: @@
 === Send a single RDM command at once ===
-This is obviously the most reliable but has the drawback of requiring a full round trip between the controller and responder for each message. Testing with a [[Net-Lynx]] showed an average response time of 7ms as seen on the controller when the RTT was 1.3ms. This solution also has the desirable property of not over-running the ArtNet node's buffers.
+This is obviously the most reliable but has the drawback of requiring a full round trip between the controller and responder for each message. Testing with a [[Net-Lynx]] showed an average response time of 7ms as seen on the controller when the RTT was 1.3ms. This solution also has the desirable property of not over-running the ArtNet node's buffers. Obviously the controller needs a timeout to detect dropped packets and either re-transmit or move on to the next packet.
 === Send multiple packets at once without waiting for a response ===

@@ Line 36: / Line 36: @@
 This is obviously the most reliable but has the drawback of requiring a full round trip between the controller and responder for each message. Testing with a [[Net-Lynx]] showed an average response time of 7ms as seen on the controller when the RTT was 1.3ms. This solution also has the desirable property of not over-running the ArtNet node's buffers.
-=== Send multiple packets at once ===
+=== Send multiple packets at once without waiting for a response ===
 This appears to be what [[DMX Workshop]] does and has the potential to break ack_overflow sequences.

@@ Line 4: / Line 4: @@
-[[Image:ArtNetRDMSequence.png]]
+[[Image:ArtNetRDMSequence.png|center]]
 == The Potential Problem ==
-The problem occurs when the RDM device responds with an ACK_OVERFLOW message.
+The problem occurs when the RDM device responds with an ACK_OVERFLOW message. From the standard:
 ''To receive the remaining data, the controller should continue to send GET_COMMANDS for the same PID. The responder shall send subsequent blocks of data with a response type of RESPONSE_TYPE_ACK_OVERFLOW until the remaining data can fit in a single message. The responder shall set the response type to RESPONSE_TYPE_ACK on the final response message in the sequence, to indicate completion of the data transfer. ''
@@ Line 17: / Line 15: @@
 Assuming that PID A causes an ACK_OVERFLOW, the sequence now looks like this:
-[[Image:ArtNetRDMOverflowSequence.png]]
+[[Image:ArtNetRDMOverflowSequence.png|center]]
 But the standard also states:
@@ Line 25: / Line 23: @@
 So any RDM packet interleaved between the two ''GET PID A' commands in the above example will cause the transfer to abort.
-* Note that the [http://www.goddarddesign.com/rdm_lab_pack.html Labpack] has a bug where it will not abort the transfer so this continues to work. This is present in version 0x20009
+When the Controller sends any of these PIDs it can wait to see if an ACK_OVERFLOW occurs before sending any other RDM commands. Of course this doesn't solve the problem for manufacturer specific PIDs, instead it relies on the back off mechanism above.
 == References ==