# Over CAN CAN is the gripper's interface: the one cable carries it next to the power, and a robot's control board, a PC with the CANvas adapter or any microcontroller with a CAN transceiver drives the gripper with it. The gripper is a Spectral Micro driver on the bus, so the frames are the driver's; this page is the part a gripper needs. ## The frame CAN 2.0 data frames with 11-bit identifiers at 1 Mbit/s. The identifier carries the node, the command and the error state: ![Bits 10 to 7 of the identifier are the node ID, bits 6 to 1 the command ID, bit 0 the error bit](https://docs.source-robotics.com/SSG48-gripper-docs/2/assets/CANID.PNG){ width="600" } - **Node ID**, bits 10 to 7: which device. Every gripper ships as node 0; set another with Send_CAN_ID or `#CANID` and save it. Use 0 to 13: the driver's bootloader reserves 14 and 15, and a device on one of those cannot be updated over CAN. - **Command ID**, bits 6 to 1: which message. The gripper command is 61, its answer 60, the calibration 62. - **Error bit**, bit 0: in a frame from the gripper, 1 while any error is active. In a frame from the host it is ignored. So the gripper command to node 0 travels on identifier `(0 << 7) | (61 << 1)` = 0x7A, and the answer comes back on `(60 << 1) | error` = 0x78 or 0x79; to node 3 the command is 0x1FA and the answer 0x1F8. Multi-byte values are big-endian; flags inside a byte are numbered from the most significant bit. ## The messages a gripper needs | Message | ID | What for | |---|---|---| | [Send_gripper_data_pack](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/gripper/#send_gripper_data_pack) | 61 | the move command: position, speed, current, the activate, action and E-stop bits; 0 bytes to poll | | [Respond_Gripper_data_pack](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/gripper/#respond_gripper_data_pack) | 60 | the answer: position, current, the state and error bits | | [Send_gripper_calibrate](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/gripper/#send_gripper_calibrate) | 62 | the gripper calibration, after every power-up | | [Send_Clear_Error](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_clear_error) | 1 | clears the errors | | [Send_ESTOP](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_estop), [Send_Idle](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_idle) | 0, 12 | stop | | [Send_Watchdog](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_watchdog) | 15 | the gripper stops when the host stops talking | | [Send_CAN_ID](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_can_id), [Send_Save_config](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/system/#send_save_config) | 11, 13 | another node ID, saved | | [Send_Respond_State_of_Errors](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/status/#send_respond_state_of_errors), [Send_Respond_Temperature](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/status/#send_respond_temperature) | 26, 23 | the driver's error flags, the motor temperature | The [CAN reference](https://docs.source-robotics.com/SSG48-gripper-docs/2/reference/can/) has the bytes of each; the driver's other messages, which a gripper does not need, are on the [Spectral Micro site](https://docs.source-robotics.com/Spectral-BLDC-docs/latest/reference/can/). ## A control loop 1. At power-up: Send_Clear_Error, then Send_gripper_calibrate; poll with empty Send_gripper_data_pack frames until the calibrated bit is 1 (a few seconds; the routine gives up after 30 s). Send nothing else meanwhile. 2. For each grip: one Send_gripper_data_pack with the position, speed and current and the activate and action bits set. Its answer still carries the state from before it; ignore it. Then poll with empty frames until the object-detection field reads 1, 2 or 3, or your own timeout passes. 3. Keep polling at a steady rate (50 Hz is plenty) if you set the watchdog: every frame resets it, and the gripper stops by itself when your program stops. The answer to every frame tells the whole state, so a host never has to ask twice. ## Host checklist - **Clear, then calibrate.** With an error active, the driver drops the calibration request; a clear sent during the calibration ends it. - **During the calibration, only empty frames.** A full Send_gripper_data_pack ends the calibration as well, and the gripper stays uncalibrated. Wait for the calibrated bit; the routine gives up after 30 s. - **The first answer is old.** The answer to a full command is sent before the gripper acts on it. Ignore it, and give the move about 100 ms before you trust the object detection: at the start of a move the jaws are slow, which can briefly read as an object found. - **The action status repeats your bit.** It is the action bit you last sent and stays 1 while the gripper holds. Follow a move by its object detection. - **An identical command does nothing.** To repeat a move to the position the gripper holds, change the speed or the current by one. - **E-stop recovery:** first a command with the E-stop bit at 0 (and the action bit at 0), then Send_Clear_Error. While the last command carries the E-stop bit, a clear does not hold. - **Watchdog:** with Send_Watchdog set, keep the polls coming. The gripper commands and the status requests reset it; Send_Clear_Error and Send_gripper_calibrate do not. After a timeout, fix the host, clear, and resume. - **Current:** keep the commanded current at 1500 mA or less for a grip, and low for a long hold; the firmware does not clamp it ([Force and speed](https://docs.source-robotics.com/SSG48-gripper-docs/2/03-technical-data/force-and-speed/)). ## From a PC The [CANvas adapter](https://source-robotics.com/products/canvas-usb-to-can-adapter) shows up as a serial port and speaks slcan; `python-can` opens it with `bustype='slcan'`, and the [Python library](https://docs.source-robotics.com/SSG48-gripper-docs/2/06-operation/python/) wraps the messages. Any CAN adapter that `python-can` supports works the same way. ## On a bus with other devices A gripper on a robot shares the bus with the robot's joints or other grippers. Give each device its own node ID (connect one at a time, Send_CAN_ID, Send_Save_config), terminate both ends of the bus ([CAN termination](https://docs.source-robotics.com/SSG48-gripper-docs/2/04-installation/can-termination/)), and keep in mind that a lower node ID wins the bus arbitration.