CNC-Slot aims to simplify wiring of 3D printers, laser cutters or CNC routers by putting commonly used signals and wires on a card-edge connector. This will allow to easily swap controllers.
This was originally discussed on a gist.
Note, Version 1.0 is not released yet, there might still be small changes till
Version 1 is concluded after comments have been integrated. In particular
some currently N/C pins might be connected and the last pins in the
wide 36-pin configuration might change.
Status quo is, that there are many boards out there that each provide their own connectors for motors, endswitches and PWM. Typically there is a mix of Molex connectors, screw terminals and solder connections, often on all sides of the controller boards. Wiring up any machine to a motor controller board is a mess and requires careful labeling of wires. Any 3D printer cabling at the controller board I have seen has always been a mess.
Replacing a board with some other board of another manufacturer requires wiring in different places, possibly re-crimping cables etc.
The goal of this effort is to define a standardized connection between typical small CNC machines or 3D printers that allows to minimize the effort to cable everything and make it simple to exchange controller boards.
CNC and 3D printer companies can provide the standardized card-slot connector and a bay to plug in the control board (and will probably provide their own controller board with it).
The control board can be provided from independent vendors that provide the same compatible electrical interface.
With the hurdle of the machine specific wiring out of the way, this will allow competition and innovation in the field of motion control hardware.
This effort defines a card edge connector that should cover the typical use-cases in these machines. The electrical connection is
- a 0.1" pitch card edge connector on the controller board side (Let's call this the "controller cartridge")
- a PCB with card edge 'fingers' on the machine side. That board is the endpoint of all the wire harness for that particular machine.
- There are three defined sizes with 25pos, 31pos and 36pos card edge connectors with increasing features.
Further definition for physical size options of the controller cartridge cases, fan options etc. have to be defined.
The goals are to cover typical use cases in these small machines:
- Controls bipolar stepper motors up to 3-4A.
- Provides 2 high power and 2 low power PWM outputs.
- Strongly suggests and requires a couple of safety features.
- Has a fixed layout, so that a particular pin always is at the same place no matter the manufacturer of the controller cartridge.
- Suggests a couple of serial busses that are needed in modern devices.
- Multiple options available to cover different amount of I/O lines from 25pos to 36pos card-slot. The smallest 25 Pos configuration good for a typical 3D printer.
The fingers on the card edge come in two lengths
- All signals including GND: connect first, all the way to the edge.
- Emergency stop switches connect with recessed fingers so that they are triggered first when the connectors are separated.
All output pins can only switch on iff the two e-stop wires on pin 1 and 50 are connected with each other (a current loop of 10mA-20mA). The fact that the two pins are recessed and at opposite ends of the connector will make sure that the outputs are switched off before the connector is being disconnected completely or is 'wiggled' in/out at an angle. This prevents fried motor drivers and accidental shorts.
Even a brief interruption of the e-stop shall require a reset from software.
N/C = Not Connected. Typically place holders for Version #1 of this spec. Must not be connected to anything to allow future use.
| Pos | Connection | BTM | TOP | Connection | Remarks |
|---|---|---|---|---|---|
| 1 | EStop_1 | 1 | 2 | GND | Emergency Stop Normally Closed, 10-20mA current loop. |
| 2 | 5V-out | 3 | 4 | N/C | -> 5V, max 1A out (for sensors) |
| 3 | PWM_1 | 5 | 6 | PWM_1 | 4 parallel pins: 12A high side switch |
| 4 | PWM_1 | 7 | 8 | PWM_1 | (optional: push pull offering H-Bridge with PWM-2) |
| 5 | PWM_2 | 9 | 10 | PWM_2 | 4 parallel pins: 12A high side switch |
| 6 | PWM_2 | 11 | 12 | PWM_2 | (optional: push pull offering H-Bridge with PWM-1) |
| 7 | PWM_3 | 13 | 14 | PWM_4 | two 3A low side switching PWM (maybe need more?) |
| 8 | M1_A+ | 15 | 16 | M1_A- | Stepper 1 (Bipolar stepper motor) |
| 9 | M1_B+ | 17 | 18 | M1_B- | |
| 10 | M2_A+ | 19 | 20 | M2_A- | Stepper 2 |
| 11 | M2_B+ | 21 | 22 | M2_B- | |
| 12 | M3_A+ | 23 | 24 | M3_A- | Stepper 3 |
| 13 | M3_B+ | 25 | 26 | M3_B- | |
| 14 | M4_A+ | 27 | 28 | M4_A- | Stepper 4 |
| 15 | M4_B+ | 29 | 30 | M4_B- | |
| 16 | GND | 31 | 32 | GND | |
| 17 | SW_1 | 33 | 34 | SW_2 | end-stop/prober switches to GND,… |
| 18 | SW_3 | 35 | 36 | SW_4 | …Current Loop 10-20mA |
| 19 | SW_5 | 37 | 38 | SW_6 | (prefer Normaly Closed for end-stops) |
| 20 | RS422_TX+ | 39 | 40 | RS422_TX- | RS422-TX. N/C if not supported. |
| 21 | RS485_RXTX+ | 41 | 42 | RS485_RXTX- | RS485 Bus and/or RS422-RX; N/C if not supported. |
| 22 | GND | 43 | 44 | GND | |
| 23 | Ain_1 | 45 | 46 | Ain_2 | Analog inputs. 4-20mA to GND. |
| 24 | N/C | 47 | 48 | N/C | Not connected in Version #1. |
| 25 | GND | 49 | 50 | EStop_2 | [---- END 25 Pos configuration ----] |
| 26 | SW_7 | 51 | 52 | SW_8 | similar to SW_1 to SW_6 |
| 27 | N/C | 53 | 54 | N/C | N/C in Version #1 (future: CAN+/CAN- or I²C SDA/SCL ?) |
| 28 | M5_A+ | 55 | 56 | M5_A- | Stepper 5 |
| 29 | M5_B+ | 57 | 58 | M5_B- | |
| 30 | M6_A+ | 59 | 60 | M6_A- | Stepper 6 |
| 31 | M6_B+ | 61 | 62 | M6_B- | [---- END 31 Pos configuration ----] |
| 32 | M7_A+ | 63 | 64 | M7_A- | Stepper 7 (Needed ? Maybe instead use 63..68 for 3 quadrature encoders to allow for servo control ?) |
| 33 | M7_B+ | 65 | 66 | M7_B- | |
| 34 | N/C | 67 | 68 | N/C | TODO: what else we want ? 8th motor? Ain? Quadrature Encoder ? |
| 35 | N/C | 69 | 70 | N/C | |
| 36 | GND | 71 | 72 | GND | [---- END 36 Pos configuration ----] |
-
Q: why are the motors numbered Motor_1, Motor_2, ... and not e.g. X, Y, Z ?
A: Depending on the geometry or kinematics of the machine, this can have different meanings. The mapping of motor number to axis happens in your motion control system configuration. -
Q: Why didn't you include feature foo or bar ? It would only use two pins.
A: There are a myriad of potential features, but the pins are limited. The specification intentionally leaves it open to add separate additional connectors to the control board if needed, while the bulk of commonly needed features are covered by this CNC-Slot.
It is encouraged to implement purely digital features using the high-speed serial busses provided and are already commonly used in the industry.
Finally, there are couple of pins left unconnected in version 1 for which we might find good use in the future. Please propose one in the issue tracker to discuss if you think it might be vitally important. -
Q: Why is the analog input reading this weird current range ? I can't simply connect a NTC Thermistor to it.
A: Using thermistors directly in particular over longer cables results in noisy measurements as they have a high impedance which results in less robust readings. Luckily, there is a standard way out: in industry settings, analog transducers of all sorts are very commonly translating their measurement range into a current of 4..20mA. The low impedance makes this very robust. The CNC-Slot connector encourages this best practice and also makes it compatible with a large number of existing industry standard sensors.
Do adapt the thermistor, you need a little op-amp circuit on the machine side (best: physically close to the thermistor to minimize noise). -
Q: I see the end-switches are also labelled as current loop. Is that complicated as with the thermistor ?
A: No. You just connect a microswitch between the connector and GND.
Current loop in that case means that the controller board will send a current of 10-20mA through the switch to avoid noise.
There is a schematic symbol library in the kicad-library/
directory featuring three symbols to be used for implementing the control
board or machine side in the cnc-slot.kicad_sym file.
The symbols are all the same size with fixed pin locations so that it is easy to upgrade without having to re-wire in the schematic.
For the card edge, there is a footprint including necessary Edge.Cuts
layer in the cnc-slot.pretty directory.
This is the footprint to be used on the machine side and typically you'll use it to create a breakout board to have the machine cabeling harness terminate in this connector.
The library provides the three variants of card-edge footprints including the recessed pins for the Emergency stop.
(some grab-bag of loosely formulated goals used in the design process)
- Interoperability: Provides a standardized output to allow easy exchange of controller boards and allow independent competition in the controller world, all providing this standard connector. Competition can focus on software features, microstepping, motor current capability etc.; the standardized connector encourages the users experimenting with different solutions.
- Goal is to cover the electrical connection to the machine (motors, switches, ...), that will allow to have the complicated cable harness inside the machine terminate in one place with a specific interface.
- Non Goals: Explicitly does not define the interfacing on the data side - USB, Ethernet, Wireless ? GCode or simple Sub-D25 stepper input ? LCD display and user interface or not ? SD-Card reading ? This is part of the feature-set provided by the cartridge, but not part of the electrical connection to the actuators and switches of the machine.
- Should cover typical 3D printers, smaller CNC routers or laser cutters.
- Of course, not every possible configuration can be supported with a limited connector with fixed pins but that is explicitly a non-goal. This is to define the basic functions that is needed for 98% of all devices. It is encouraged to provide additional features via screw terminals or the specified robust RS485 bus.
- Within the same standard connector, there then can be distinguishing features of various controllers: provided current for motors, or if the two power PWMs can be used together as an H-Bridge.
- Simplify wire harnesses. They are now fixed part of the printer all connecting to the card-edge connector. Easy to pre-fabricate with the right cable lengths. Less issues with confusing 'cable salads'.
- Multiple configurations from minimal 3D printer with up to four steppers (with a 25 Pos card edge connector that is shorter than a 70mm PCB) to larger machines with specific requirements: Next level with 31 Pos and 36 pos level adding more motors.
- Enforce some safety features and electric recommendations directly in the
specification that are not yet commonly seen in many controllers today:
- The high power PWM outputs are only high-side switches to minimize dangerous heater-stuck-on failure scenarios.
- Emergency Stop Switch is part of the specification. Contacts strategically placed so that card insertion issues or hot-unplugging trigger it.
- Have all kinds of external switch inputs be current loops and normally closed for more resilience.
- For periphery: suggest busses that are proper in the noisy environment (RS422/485) instead of flaky I²C not well suited for an electrically noisy machine.
- There are two analog inputs, and they define a particular input range in voltage. That means that adapter amplifiers need to be done on the machine side. This allows to adapt various inputs (e.g. Thermistors, PT100, ...) without assuming a particular analog interface.
- (After initial inclusion of power connection, this is now removed as this might be handled differently per device)