Control Chain Protocol
Contents
About
Control Chain is an open hardware assignment protocol developed to reflect the LV2 controls properties. In other words the Control Chain enables you to control LV2 plugins control ports using a hardware device like Arduino.
Although the Control Chain has been created focused on MOD, it isn't a must to have it working with LV2 plugins. This can be done using, for example, an Arduino, connected via USB to your PC.
This page concentrates technical information of Control Chain. If you are looking for a startup guide to create new controllers, please refer to MOD Arduino Shield article, there you will find a tutorial and Arduino cases that will help you.
Physical interface
In theory, Control Chain does not requires a physical interface to deal with a host. But in practice is what happen and some of decisions about the protocol were taken looking at the physical interface limitations.
The MOD hardware uses the RS-485(TIA/EIA-485) electrical standard, over serial communication, accessible via the RJ45 connector, which allows the connection of external peripherals using ethernet cables. The communication is full-duplex and the speed used is 1Mbps.
Probably the better way to prototype Control Chain controllers is using an Arduino. In this case the physical interface will be serial over USB.
Protocol
Control Chain uses a binary protocol which has your data encapsulation using SLIP (RFC 1055).
Message Structure
The below table shows the elementary structure of all messages.
| field | size |
|---|---|
| header | 6 |
| data | N |
The header contain the following informations: destination, origin, command,data size and check. The data field holds the data bytes related to command.
Expanding the fields:
| field | subfield | size |
|---|---|---|
| header | destination | 1 |
| origin | 1 | |
| command | 1 | |
| data size | 2 | |
| check | 1 | |
| data | data 0 | 1 |
| data 1 | 1 | |
| ... | N |
The destination and origin fields are addresses that says from where the message is going to and coming from, respectively. The value 0x00 stands for the host address and the devices addresses values must be between 0x80 and 0xFF, inclusively. The check is the byte message verification. It's calculated XOR'ing all message bytes.
Size fields marked with 1B_str stands for a string started with one byte, that indicates your size, and followed by your data characters. For example, the "hello" string will be represented by:
| 0x05 | h | e | l | l | o |
Commands
The valid commands are:
| value | description |
|---|---|
| 0x01 | handshaking |
| 0x02 | device descriptor |
| 0x03 | control assignment |
| 0x04 | data request |
| 0x05 | control unassignment |
| 0xFF | error report |
Handshaking
Devices can only send the handshaking to host when the host isn't sending data. New devices doesn't have any origin address yet thus the origin, in the header message field, must contain the 0x00 value. Once that host received the handshaking it will generate an address to new device, this address value must be between 0x80 and 0xFF, inclusively, and must be informed to device using the destination field. The data field of original message must be given back to device untouched, allowing the device discover its address.
The channel field is used to differentiate devices of the same type, i.e. same device URI. Each device can choose its own way to select the channel.
The protocol version always must be verified by the host. In the case of device has newer protocol implementation, the host have to raise an error report message to device, and the device, by its side, can optionally, if possible, pop up the received error to user. And finally that device has to stop sending handshaking. The host have to support backwards compatibility.
device to host or host to device data field
| field | size | description |
|---|---|---|
| device URI | 1B_str | a string that identifies the device |
| channel | 1 | channel configured on device |
| protocol version (major) | 1 | major number version control |
| protocol version (minor) | 1 | minor number version control |
device descriptor
The device descriptor should contain all information about the actuators of the device. The host can optionally request the device descriptor of the device. Actually, only devices unknown by host need to be requested.
host to device data field
| field | size |
|---|---|
| [none] | 0 |
device to host data field
| field | size | subfield 1 | subfield 2 | subfield 3 | subfield 4 | description |
|---|---|---|---|---|---|---|
| device label | 1B_str | friendly device name | ||||
| actuators count | 1 | amount of actuators of the device | ||||
| actuators list | 1 | actuator 1 | id | actuator id | ||
| 1B_str | name | actuator name | ||||
| 1 | modes count | amount of supported assignment modes | ||||
| 1 | modes list | mode 1 | relevant | relevant (mask 1) | ||
| 1 | mandatory | mandatory (mask 2) | ||||
| 1B_str | label | mode label | ||||
| ... | ||||||
| 1 | max assignments | amount of controls that can be assigned to this actuator | ||||
| 1 | steps count | amount of values in the steps list | ||||
| 2 | steps list | value 1 | step value 1 | |||
| 2 | value 2 | step value 2 | ||||
| ... | ||||||
| ... |
The actuator id is number defined by device developer used as short way to indentify an actuator of the device.
Some devices developers can like to have a stacking assignment feature, which would be possible to have more than one assignment to same actuator. This have to be defined on the field max assignments. Using max assignments greater than one is possible, for example, use a button to navigate through of the assignments.
The steps list contains the values of steps that the device developer recommends to be used with that actuator.
The modes describe to the host the LV2 properties supported by the actuator. Each mode is composed by two masks and one label. Each bit of each mask represents one LV2 control property. The first mask, the relevant, it's used to define if the bits of mandatory mask has to be evaluated.
The label of mode is a friendly name to help the user understand what the mode does.
The masks structure is:
| bit 7 | bit 6 | bit 5 | bit 4 | bit 3 | bit 2 | bit 1 | bit 0 |
|---|---|---|---|---|---|---|---|
| integer | logarithmic | toggled | trigger | scale points | enumeration | tap tempo | bypass |
Example:
The footswitch actuator could implement two operation modes: ON/OFF and PULSE. In this case the following masks can be used:
| mode | mask | integer | logarithmic | toggled | trigger | scale points | enumeration | tap tempo | bypass |
|---|---|---|---|---|---|---|---|---|---|
| ON/OFF | relevant | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
| mandatory | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | |
| PULSE | relevant | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
| mandatory | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
In the above example, in the ON/OFF case, the user can only assign a control to this actuator if this control port has "toggled" implemented but not "trigger". In the another case, the PULSE one, the assignment can be done only if both properties are implemented, "toggled" and "trigger".
The below table shows a realistic footswitch implementation:
| mode | mask | integer | logarithmic | toggled | trigger | scale points | enumeration | tap tempo | bypass | mask value |
|---|---|---|---|---|---|---|---|---|---|---|
| ON/OFF | relevant | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0x7F |
| mandatory | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0x20 | |
| PULSE | relevant | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0x7F |
| mandatory | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0x30 | |
| TAP TEMPO | relevant | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0xFF |
| mandatory | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0x02 | |
| ENUMERATION | relevant | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0x7F |
| mandatory | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 0x0C |
- ON/OFF if toggled is implemented but not logarithm, trigger, scale points, enumeration and tap tempo. integer is irrelevant.
- PULSE if toggled AND trigger are implemented but not logarithm, scale points, enumeration and tap tempo. integer is irrelevant.
- TAP TEMPO if tap tempo is implemented and no other.
- ENUMERATION if enumeration AND scale points are implemented but not logarithm, toggled, trigger e tap tempo. integer is irrelevant.
control assignment
The device will receive this command when the user assing a control to one of your actuators. The device must reply with a error code indicating whether was possible assigned the requested control to the actuator.
host to device
| field | size | subfield 1 | subfield 2 | description |
|---|---|---|---|---|
| actuator id | 1 | actuator identifier | ||
| assignment id | 1 | assignment identifier | ||
| port mask | 1 | LV2 control port properties | ||
| chosen mode | 1 | relevant | relevant (mask 1) of chosen mode | |
| 1 | mandatory | mandatory (mask 2) of chosen mode | ||
| label | 1B_str | control label | ||
| value | 4 | current control value | ||
| minimum | 4 | minimum control value | ||
| maximum | 4 | maximum control value | ||
| default | 4 | default control value | ||
| step | 2 | control step | ||
| unit render | 1B_str | control unit reder | ||
| scale points count | 1 | amount of control scale points | ||
| scale points list | 1B_str | scale point 1 | label | scale point label |
| 4 | value | scale point value | ||
| ... |
device to host
| field | size | description |
|---|---|---|
| error code | 1 | indicates whether assignment was well done |
The assignment id is a number that is short way to reference the assignments. It's used in the data request and unassignment commands. The port mask field defines the LV2 control port properties, the mask use the same structure as presented in the device descriptor. The chosen mode is used to device indentify which of your modes has to be applied to this assignment.
The unit render is printf format string for rendering a value (eg. "%f dB"). The fields value, minimum, maximum, default and the value of scale points use the float standard IEEE 754.
data request
This command is used to ask data of the devices. The host send data request to device and the devices must always send a response back to the host. Preferably, the response should have only the values of the modified actuators.
host to device data field
| field | size |
|---|---|
| [none] | 0 |
device to host data field
| field | size | subfield 1 | subfield 2 | description |
|---|---|---|---|---|
| assignments count | 1 | amount of assignments in the list | ||
| assignments list | 1 | assignment 1 | id | assignment identifier |
| 4 | value | current assignment value | ||
| ... |
The value uses the float standard IEEE 754.
control unassignment
This command is sent to device as soon as the user remove the assignment (unassignment) of the actuator. The device response have no data field. In the case of the device not reply, the host must raise a timeout and understand the control as unassigned. Once that all actuators of one specific device are unassigned, the host must stop the data requisting in this device.
host to device
| field | size | description |
|---|---|---|
| assignment id | 1 | assignment identifier |
device to host
| field | size |
|---|---|
| [none] | 0 |
error report
host to device or device to host
| field | size | description |
|---|---|---|
| error on command | 1 | indicates which command raise the error |
| error code | 1 | a number that represents the error |
| error message | 1B_str | string that describes the error |
Operation
Initially host is waiting for devices handshaking, once the first device sends the handshaking, the host must response back informing its device id in the destination field. After that host can request the device descriptor and wait for device response or timeout. Since the device is identified on host, it should wait until the host send to it an assignment request.
The host start to polling the identified devices for data request as soon as an assignement is done. The polling period is determinated by the host considering the count of identified devices. In each polling devices cycle the host is responsible to reserve a short period of time to allow that new devices connect to it or send other requests, this because devices can only send messages to host when it isn't sending any message. The host must to watch the device time response, i.e. implement a timeout verification.
The host can control when the devices will message it keeping your transmission line sending data.
