Using servos and controlling unused IO pins

Author: Tony (and 2 other contributors)

The Gcodes M42 Switch I/O pin and M280 set servo position allow direct control of some pins, or re-purposing of those pins to control PWM hobby servos

When a servo stops moving, it can pump a large amount of energy into the +5v rail due to regenerative braking. If unchecked, this may increase the voltage of the +5V rail above the safe limit. The 1.04 revision of the Duet 2 WiFi/Ethernet has an extra 220uF capacitor on the 5v rail to help absorb this, so does the Duet 2 Maestro. Older Duets don't. So when using servos with them it is advisable to add your own 220uF capacitor between the +5V and ground servo wires. This applies whether the servo is connected directly to the Duet or to an attached DueX expansion board or expansion breakout board.

Starting with firmware version 1.16, pin numbers in the M42 command are logical pin numbers as listed below, not Arduino-like pin numbers. The M280 servo control command uses a subset of the same pin numbers.

Note that these logical pin numbers are not the physical pin numbers on the expansion header, or the internal pin numbering within the firmware.

To see where these pins are on the 2nd Generation Duets, see the Duet wiring diagrams

Pin numbers	Usual function	PWM capable?	Usable for servos?	Notes
0-7	Heaters 0-7	yes	yes	Disable the heater using M307 H# A-1 C-1 D-1 to make the pin available, where # is the heater number, and make sure that no tool is configured to use that heater. The outputs on the expansion connector are active low with 3.3V signal level. On the Duex 5 expansion board, heater channels 3-7 are available as 5V active-high signals on the PWM1 - PWM5 connectors respectively. On the DueX2 from version 0.8a onwards, heater channels 3-7 are available on PWM1 - PWM5, prior to that heater channels 3-5 only are available on PWM1 - PWM3 . Caution: the heater output terminals will also be driven by the PWM signal or servo pulses.
20-27	Fans 0-7	yes	20-22 only	Disable the fan using M106 P# I-1 to make the pin available, where # is the fan number. Fans 3-7 are on the Duex 5 and Duex 2 expansion boards.
40-49	Endstop connectors 0-9	no	no	40-44 are input-only. On the Duet 2 WiFi and Duet 2 Ethernet, if a Duex 2 or Duex 5 expansion board is connected then 45-49 are the endstop connectors on the expansion board and are input-only; else they are additional endstop connections in the expansion connector. On the Duet 0.6 and 0.8.5, 45-48 are the endstop inputs on the expansion connector.
60-	Miscellaneous pins	varies	varies	The pins available depend on which Duet you have - see below
100-	Pins on expansion devices	varies	varies	The pins available depend on which Duet you have - see below

The miscellaneous pin numbers 60-63 correspond to the CS5-CS8 pins on the expansion connector. These were originally allocated for use as the E2-E6 endstop pins and are labelled as such on the expansion connector, but they are now used to address thermocouple and PT100 daughter boards on a Duex 2 or Duex 5 instead. Because most users don't need more than the four thermocouple and PT100 channels supported on the Duet 2, we make CS5-8 available as general purpose I/O if you do not have any daughter boards fitted to the Duex 2 or 5.

It will be helpful to refer to the wiring diagram for the Duet 2 Wifi/Ethernet and the Duex to identify the pins.

Here is the mapping between pin numbers, signal names, and the labels on the expansion connector:

Logical pin	Signal name	Expansion connector label	Expansion connector pin number	PWM capable?	Servo capable?	Notes
60	CS5	CS5	50	no	no
61	CS6	E3_STOP	9	no	no
62	CS7	E4_STOP	14	no	no
63	CS8	E5_STOP	19	no	no
64	ENC_SW	ENC_SW on CONN_SD connector	7 on CONN_SD connector	no	no
65	Z_PROBE_MOD	MOD on Z-probe connector	3 on Z-probe connector	no	no
100-103	GPIO pins 1-4			yes	no	These are on the Duex 2 and Duex 5 expansion boards
120-135	SX1509B expansion I/O pins			yes	no	These are on an SX1509B-based expander board at I2C address 0x71 connected to a Duet 2 Wifi/Ethernet

Logical pins 64 and 65 are supported in firmware 2.01 and later only. Note that the Z_PROB_MOD pin is driven automatically when some Z probe modes are selected.

It would be helpful to refer to the wiring diagram for the Duet 2 Maestro to identify the pins.

Logical pin	Signal name	Expansion connector label	Expansion connector pin number	PWM capable?	Servo capable?
60	PA21/RXD1/AD8	EXP0	4	no	no
61	PA22/TXD1/AD9	EXP1	5	no	no
62	PA3/TWD	TWD0	9	no	no
63	PA4/TWC	TWCK0	8	no	no
64	Z_PROBE_MOD	MOD on Z_PROBE connector, also SERVO on expansion connector	3 on Z_PROBE connector, 11 on expansion connector	yes	yes

The mapping between logical pin numbers for miscellaneous pins and the corresponding signal names on the expansion connector is as follows.

Logical pin number	Expansion connector signal name
60	PA10/RXD0
61	PA11/TXD0
62	PA12/RXD1
63	PA13/TXD1
64	PA14/RTS1
65	PB12/TWD1
66	PB13/TWCK1
67	PB16/DAC1 (also used as CS signal on external SD card socket)
68	PB21/AD14
69	PC4

Image from wikipedia

5 Comments

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Dear David,

is there any information available for RADDS pin assignments.

There are three Servo pins available on a RADDS board, labeled as PWM1-3. But I can not really make out on how to call them in RRF notation.

Thanks for your help,

Andreas

Andreas Melzer - 09/10/2018 Reply

Hello,

I am looking to directly control the unused pins with C/C++ code (like an arduino). Is there a way to do this in the firmware or is there separate firmware I can load onto the Duet Wifi?

Thank You,

Caleb

caleb young - 09/17/2018 Reply

Could we add a column with frequency-ranges for those ports with PWM and adjustable freq?

Duet3D

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Using servos and controlling unused IO pins

Introduction

Important note on connecting servos

Logical pin numbers

Board dependencies

Duet 2 WiFi and Duet 2 Ethernet

Duet 2 Maestro

Duet 0.6 and 0.8.5

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