Recore A6

Recore is a 3D-printer control board running Linux. It is specifically tailored to be compatible with Klipper and OctoPrint. The main features are:

• Allwinner A64 SoC, quad core CPU running at 1 GHz.
• 1 GB of DDR3 RAM
• 8 GB of on board eMMC
• Gigabit Ethernet
• 6 TMC2209, 2 A stepper motor drivers
• 3 heater outputs + high power heated bed
• 4 USB High Speed ports
• 4 thermistor/thermocouple inputs (software selectable)
• Comes with Debian Linux with Klipper and OctoPrint installed

This document is for Recore Revision A6. For previous hardware revisions, please see:

• Recore_A5
• Recore_A4
• Recore_A3
• Recore_A2

Availability

Recore A6 can be purchased from the Intelligent Agent web shop https://www.iagent.no/product/recore/

Pinout

A64

The Allwinner A64 is a quad core Cortex A53 clocked at 1 GHz. It has an integrated RISC CPU (AR100) clocked at 300 MHz that handles the real time aspect for the 3D-printer. In the following table the pins used for End-stops and stepper control are described. The "bank and pin" can be used to specify the pin numbers in klipper. The "Number" column is useful for testing things on the command line in conjunction with gpiod.

Name	Bank and pin	Number	Name	Bank and pin	Number	Name	Bank and pin	Number
END STOP 0	PH4	228	DIR 6	PE14	142	USB-PWR-EN-0	PH0	224
END STOP 1	PH5	229	DIR 7	PE15	143	USB-PWR-EN-1	PH1	225
END STOP 2	PH6	230	STEP DIAG 0	PE0	128	USB-PWR-EN-2	PH2	226
END STOP 3	PH7	231	STEP DIAG 1	PE1	129	USB-PWR-EN-3	PH3	227
END STOP 4	PH8	232	STEP DIAG 2	PE2	130	GAIN-ENABLE-T0	PD4	100
END STOP 5	PH9	233	STEP DIAG 3	PE3	131	GAIN-ENABLE-T1	PH11	235
STEP 0	PL4	4	STEP DIAG 4	PE4	132	GAIN ENABLE T2	PE17	145
STEP 1	PL5	5	STEP DIAG 5	PE5	133	GAIN ENABLE T3	PB2	34
STEP 2	PL6	6	STEP DIAG 6	PE6	134	PU-ENABLE-T0	PG10	202
STEP 3	PL7	7	STEP DIAG 7	PE7	135	PU-ENABLE-T1	PG11	203
STEP 4	PL8	8	OC-ALERT	PF6	166	PU ENABLE T2	PG12	204
STEP 5	PL9	9	OC-RESET	PF4	164	PU ENABLE T3	PG13	205
STEP 6	PL10	10	EN-HP	PF5	165	OFFSET-T0	PG0	192
STEP 7	PL11	11	UC-INT-1	PG3	195	OFFSET-T1	PG1	193
DIR 0	PE8	136	UC-NRST	PG4	196	OFFSET-T2	PG3	194
DIR 1	PE9	137	UC-BOOT	PG5	197	OFFSET-T3	PG8	200
DIR 2	PE10	138	ES-EN-12V	PF0	160	STEPPER UART 2 TX	PB0	32
DIR 3	PE11	139	EN-HDMI-PWR	PG9	201	STEPPER UART 2 RX	PB1	33
DIR 4	PE12	140	EN-THERMISTORS	PF1	161	STEPPER UART 3 TX	PD0	96
DIR 5	PE13	141	EN-ENDSTOPS	PF2	162	STEPPER UART 3 RX	PD1	97

STM32F031

The STM32 is a Cortex M0 powered micro controller handling analog inputs and PWM outputs. It does not control any hard real time aspects of the print, that is handled by the AR100 core realized as a separate core in the A64 SoC.

Name	Bank and pin	Number	Name	Bank and pin	Number	Name	Bank and pin	Number
THERMISTOR 0	PA0	6	HEATER E0	PA8	18	FAN 0	PB0	14
THERMISTOR 1	PA1	7	HEATER E1	PA9	19	FAN 1	PB1	15
THERMISTOR 2	PA2	8	HEATER E2	PA10	20	ES1-PWM-OUT	PB3	26
THERMISTOR 3	PA3	9	HEATER BED	PA11	21	FAN 3	PB4	27
BOARD VOLTAGE	PA4	10	USER LED	PA12	22	FAN 2	PB5	28
BOARD CURRENT	PA5	11				PWM-ES2-OUT	PB6	29
BOARD TEMPERATURE	PA6	12				EXT 1	PB7	30
COLD JUNCTION	PA7	13				EXT 2	PB8	32

DBG header

This header is used for interacting with u-boot and getting early debug messages from the Linux kernel

Pin	Function
1	UART RX
2	UART TX
3	NC
4	GND

MCU header

This header is used for connecting additional MCU peripherals.

Pin	Function	Alt
1	PB7	I2C1_SDA
2	PB8	I2C1_SCL
3	3.3V
4	GND

LEDs

There are 15 white LEDs on the board. Here is what they mean

Name	Location	Meaning
D2	By PMIC	Board powered
D3	By heater BED	BED output on
D4	By fan 0	Fan 0 is on
D5	By fan 1	Fan 1 is on
D6	By heater H2	Heater H2 is on
D7	By fan 2	Fan 2 is on
D8	By heater H0	Heater H0 is on
D9	By fan 3	Fan 3 is on
D10	By heater H1	Heater H1 is on
D12	By input connector	High voltage domain is on
D15	By eMMC chip	eMMC activity
D17	By HDMI connector	USB activitiy
D18	By the USB C connector	Linux Heartbeat
D19	By the A64 SoC	CPU activity
D20	By the STM32	Klipper running on STM32

Buttons

There are 3 push buttons on the board.

Name	Meaning
FEL	Enter FEL-mode of the A64
BOOT	Boots or shuts down the board
RESET	Hard reset of the CPU

Current limits

Name	Limit
5V-ES	1 A
USB host each	1 A
Thermistors	1 A
MCU connector	0.5 A

Wiring

Below is a wiring diagram meant to aid in connecting motors, fans, extruders and hot ends to the control board. On the board, there is a small + sign next to output connectors where polarity is important. If passive heating elements are used, polarity is not important, but if those outputs are to be used with SSRs or relays, polarity must be observed.

Network connection

Recore is designed to be controlled through a browser on a computer, so the board must be connected to the local LAN or wifi network. Connect an ethernet cable between the board and a switch. The ping utility can be used to ensure that the board is discovered on the network.

LAN setup

Recore with Refactor is running avahi and should announce itself on the network once a connection has been established. On a Linux based host computer, it is possible to search for a booted device using the following command:

ping recore.local

If the device is present, an SSH connection can be established:

ssh root@recore.local

Furthermore, a web interface running OctoPrint can be found using a browser:

http://recore.local

Wifi setup

Wifi can be set up by connecting a wifi dongle to one of the USB host ports. Once the uSB dongle has been connected, and the board has been booted, access to the terminal command line can be given through a host computer. The board should appear as a TTY ACM device. On a Linux based host computer, the terminal program `screen` can be used to get access. From the host computer

screen /dev/ttyACM0 115200

If the host computer is running Windows, Putty can be used to get access.

Once a connection has been established, the wifi network can be set up using the command nmtui.

nmtui

Follow the instructions on screen to edit and activate the wifi connection.

Note

If the board is powered through the USB C connector, only the two USB host ports next to the HDMI port is powered.

Mating Connectors

The power connectors on the board are available either in the web shop as part of a connector pack or directly from Digi-key etc.

Four pin male: TBP01P1-508-04BE [| Digi-key]

Two pin male: TBP01P1-508-02BE [| Digi-key]

Configuration

There are no jumpers on Recore and no hard fuses. Everything is software configurable and resettable from software. This gives great power and flexibility without requiring physical access to the board once it has been installed. But with great power comes great responsibility. It is possible to break things if care is not taken during setup. There is one setting in particular that is worth mentioning: ES0 can have either +5V or +12V on the power pin. This is in order to allow a standard industrial inductive sensor to be used on that end stop. If a 5V only peripheral is connected, that could damage the connected peripheral.

The default config file for Recore A6 can be found in the repository: generic-recore.cfg

TMC2209

The 6 stepper drivers on Recore are of type TMC2209 from Trinamic. Each stepper has a configuration interface with a bidirectional UART port that is connected to a peripheral UART on the A64. The peripheral is not in use, but instead the GPIO bit-banging functionality of Klipper is used to communicate with the stepper drivers.

Here is a table with pins and addresses for the steppers

Stepper driver config
Name	Pins	Address
S0	PB0/PB1	0
S1	PB0/PB1	1
S2	PB0/PB1	2
S3	PB0/PB1	3
S4	PD0/PD1	0
S5	PD0/PD1	1
S6	PD0/PD1	2
S7	PD0/PD1	3

Temperature inputs

4 different temperature devices can be connected to the 4 analog inputs on Recore. Thermistor, Thermocouple, PT100 with INA826 and PT1000 without the use of a pre-amplifier.

Thermistor

Make sure the pull-up is enabled and the op-amp gain is set to 1.

Thermocouple

Make sure the pull-up is disabled and the op-amp gain is set to 100.

PT100 with INA826

In order to use a PT100 sensor, an external amplifier board must be used. There is an example configuration for Klipper that uses this. The ADC reference voltage is 5.0 by default, but for Recore it is 3.3 V. Here is an example:

[Extruder]
...
sensor_type: PT100 INA826
adc_voltage: 3.27

The pull-up must also be disabled.

PT1000

Make sure the pull-up is enabled and the gain is set to 1. In the latest version, the sensor must be prefixed with RECORE.

[Extruder]
...
sensor_type: RECORE PT1000
adc_voltage: 3.27
pullup_voltage: 3.3
offset_voltage: 3.2

Connecting other sensors

The board has been designed to work with a range of 3rd party sensors for automating the print.

BLtouch

When connected per wiring diagram above, setting up BLtouch can be done by adding the following section to printer.cfg.

[bltouch]
sensor_pin: ^ar100:PH6
control_pin: PB3
speed: 7
pin_move_time: 0.675
sample_retract_dist: 10
pin_up_reports_not_triggered: True
pin_up_touch_mode_reports_triggered: True
#Please change offsets below for your specific setup (these values are for EVA2.3.x BLtouch mount)
z_offset: 2.22
x_offset: -28 
y_offset: -13

Inductive Sensor

Connect the inductive sensor on ES0. You can choose either 5V or 12V output on the voltage pin.

ADXL345

Note

The ADXL345 seems to cause Klipper to shut down when trying to calibrate at a high rate. A workaround is to use a Pi Pico or similar MCU to communicate with the ADXL345, or to lower the rate. It's been tested working at 400, but not 800 which is the recommended minimum.

A popular accelerometer sensor for doing input shaping with Klipper is the ADXL345. This can be connected to the connectors S6/S7. There is no hardware SPI interface available, but a software implementation in Klipper exists. Note: Only experimental support has been tested for this. https://learn.adafruit.com/adxl345-digital-accelerometer

Config:

[adxl345]
cs_pin: ar100:PL11
spi_software_sclk_pin: ar100:PL10
spi_software_mosi_pin: ar100:PE15
spi_software_miso_pin: ar100:PE14
rate: 400

[resonance_tester]
accel_chip: adxl345
probe_points:
    100, 100, 20  # an example

Using a Raspberry Pi Pico:

[adxl345]
cs_pin: pico:gpio17
spi_bus: spi0c

RC Servo

Endstop 1 and 2 can be used as PWM outputs with 5V levels. Rev A6 is the first board with support for this. More testing and configuration documentation is needed.

Software for Recore

Recore comes with the Linux distro Refactor (Armbian) pre-installed on the eMMC. For more instructions about software for Recore, see the Refactor wiki page: Refactor#Recore

AR100

Please see: AR100

Manual control and testing

All functions on Recore are meant to be controlled from a graphical user interface such as OctoPrint, Fluid, Mainsail etc. This in turn in controlled by Klipper. Sometimes it can be beneficial to get low level access to certain functions and test things directly on the command line. This section is meant to give an overview of what capabilites are available from the command line (SSH/terminal).

Power domain overview

Power domains

There are 9 power domains around the board controlling voltage on different pins:

• Input power controls power to 4 high power outputs, the 4 fans and the 6 stepper drivers. This input has current monitoring, fast acting over current protection, voltage monitoring, temperature monitoring and reverse polarity protection.
• Thermo couple power Controls +5V/1A output to the ADCs. This can be used for turning on power to the analog pins.
• End stop power Controls +5V/1A output on the six endstops. Ganged. ES5 can be switched to have 12V output. The 12V output has a

100mA internal current limit.

• 4 USB host power domains Controls 5V/1A current output to the usb host connectors. These are turned on by u-boot.
• HDMI 5V output Controls the 5V/1A HDMI output. Turned on by u-boot.

Input stage

Reset over current protection and set it in "one-shot" mode.

gpioset 1 164=0
gpioset 1 164=1

The over current protection can also be set in "transparent" mode, where the current alarm will be reset automatically. Note that this is a bad idea for general operation, only use for testing.

gpioset 1 164=0

Enable 24V input

gpioset 1 165=0

End stop 5V /12V

End stop 0 to 4 has a programmable +5V output voltage. End stop 5 has +5V or +12V selectable.

To enable +5V on ES 0...4

gpioset 1 162=1

To disable again

gpioset 1 162=0

To switch to 12V output on ES5

gpioset 1 160=1

To disable again

gpioset 1 160=0

End stops values

To see what value the end stops have

gpioget 1 228 # ES 5
gpioget 1 229
gpioget 1 230
gpioget 1 231
gpioget 1 232
gpioget 1 233 # ES 0

TMC2209

In window/shell 1:

stty -F /dev/ttyS2 raw -echoe -echo
xxd -c 4 /dev/ttyS2

In window/shell 2:

echo -n -e '\x5\x0\x6\x6F' > /dev/ttyS2

The return should be along the lines of

00000000: 0500 066f  ...o
00000004: 05ff 0620  ... 
00000008: 0001 4058  ..@X

To check that no errors are reported by the steppers write this on the command line. If a "1" shows up, that is an indication that the motor is reporting an error.

gpioget 1 128
gpioget 1 129
gpioget 1 130
gpioget 1 131
gpioget 1 132
gpioget 1 133

STM32F031

There is a small firmware to test all functionality that is controlled by the STM32F031 chip on the board. Once uploaded to the board using the flash script, the firmware will turn on all LEDs it controls and send back ADC readings.

To flash this firmware to the STM32:

gpioset 1 197=1
gpioset 1 196=0
stm32flash -i -196,196 -w Sumato-f031.bin -v -g 0x00 /dev/ttyS1
gpioset 1 197=0
stm32flash -i -196,196 /dev/ttyS1

To see the ADC readings:

stty -F /dev/ttyS3 38400 raw
cat /dev/ttyS3

You should see something like

ADC T0: 0
ADC T1: 0
ADC T2: 0
ADC T3: 0
ADC U: 2338
ADC I: 0
ADC TB: 3417

The following one-liners assumes gawk is installed.

Reading Current

There is board current reading implemented in OctoPrint. The current shunt resistor is 1 mOhm, the amplifier is 20 times, the vref is 3.3V, so we get: (adc_val/4096*3.3)*1000/20 = (adc_val/4096)*165

If the testing firmware is installed on the STM32, you can convert the current (as in amps) reading to something useful like this:

cat /dev/ttyS4 | awk '/I:/ { printf "scale=4; (%i/4096)*165\n", $3; fflush(); }' | bc -l

Current limit programming

The current limit must be set in the device tree by programming the voltage on dldo2. dldo2 can have values from 0.7 V to 3.4 V. Setting the voltage level to 3.4 V effectively disables the fast acting current limit. Setting it to the lowest value gives an over current limit of 35 A.

Reading Board Temperature

To see the temperature readings, try this :

export beta=4327
cat /dev/ttyS1 | awk '/TB:/ { printf "scale=4; 4327/l( (4700/((3.3/(3.3*%i/4096))-1.0))/0.05306820342563400000) -273.15\n", $3; fflush(); }' | bc -l | awk '{print strftime("%k:%M:%S"), $0; fflush();}'

Reading Input Voltage

To see voltage on the input, it can be converted like this:

exec 4</dev/ttyS4 5>/dev/ttyS4
echo -n ";A4;" >&5
read -t1 reply <&4
echo $reply | awk '{ printf "scale=4; (%i/4096)*3.3*((100+10)/10)\n", $4; fflush(); }' | bc -l

Thermistor inputs

To enable pull-up on input for using thermistors for temperature measurements:

gpioset 1 202=1
gpioset 1 203=1
gpioset 1 204=1
gpioset 1 205=1

Setting Op-amp gain to 1:

gpioset 1 100=0
gpioset 1 235=0
gpioset 1 145=0
gpioset 1 34=0

Setting Op-amp gain to 100:

gpioset 1 100=1
gpioset 1 235=1
gpioset 1 145=1
gpioset 1 34=1

Adding a 0.33 V offset to the input:

gpioset 1 192=1
gpioset 1 193=1
gpioset 1 194=1
gpioset 1 200=1

Do not add 0.33 V offset to the input:

gpioget 1 192
gpioget 1 193
gpioget 1 194
gpioget 1 200

In the device tree file, the gpio/ldos must be set to gpio input:

&gpio0_ldo {
  function = "gpio_in";
};

Thermocouple input

Either a thermocouple or a thermistor can be used as input on the T0-T3 inputs.

Bias

The thermocouple is set up to have a Bias of 0.7V/100 on the input which enables negative values (lower than the board temperature) as input to the ADCs. With the input short circuited, the bias can be recorded. Typical values are 750 This needs to be subtracted from the result.

Cold junction compensation

Another important aspect is cold junction compensation. There is a thermistor connected on the board close to the thermocouple inputs which can be used to measure the temperature close to the input junction. This temperature must be added in software.

The connected thermistor is a TDK NTCG104EF104FT1X The connected Thermocouple has a (25/100) beta value of 4327 K and a (25/85) beta value of 4308 K. The 25/85 beta value is what is used in Klipper.

In order to use the inputs for thermocouple, the pull-up-down resistors must be set to pull-down:

gpioset 1 102=0
gpioset 1 120=0
gpioset 1 160=0
gpioset 1 161=0

Also, the op-amp needs a gain of 100:

gpioset 1 100=0
gpioset 1 235=0
gpioset 1 145=0
gpioset 1 34=0

Finally, the input bias needs to be enabled. The following LDOs needs to be enabled:

reg_ldo_io0
reg_ldo_io1
reg_dldo3
reg_dldo4

Performance

Start-up time

With a maximum clock for MMC2 of 200 MHz, here is the start-up time:

root@recore-3:~# systemd-analyze 
Startup finished in 5.128s (kernel) + 11.567s (userspace) = 16.695s 
graphical.target reached after 11.421s in userspace

Measured voltages

The final stages of Recore manufacturing consists of testing, calibration and flashing of firmware. The calibration stage measures a number of voltages on the board and classifies the input stages gain and offset. Instruments used:

• HP 3458A, 8.5 digit multimeter
• HP 3245A, Universal Source

Microcontroller voltages

VDDA uC: 3.312 V
VDD uC: 3.312 V

Voltage from the PMIC supplied as bias:
0.706 V

5V Buck converter ripple and noise

The 5V step down/Buck converter is a AOZ2151PQI-10. It can provide 4A and has an input voltage range of 12-28V. It is hard wired to be in PWM mode. PFM mode results in a loud whine.

Below is a screenshot of the ripple and noise across the the 5V rail with a normal kernel running.

Input stage

An experiment with a constant current load drawing 20 A from the bed connector with and without a 92 mm fan on top of the board for forced convection. The temperature measured was with the on board thermistor. Using a thermal camera, the temperature displayed matches well with the temperature reported by the camera. However, in Rev A2, this is not the hottest point on the board. The MOSFET controlling the bed seems to consistently get the highest temperature. The reason for this is probably due to a 3.3 V VGS.

Inrush current

During turn on of the high power stage, there will be an inrush current to charge up the bank of capacitors present on the board. If no loads are turned on, the inrush current reaches 31 A at peak. This is important to consider when setting the current limit with device tree.

Todo: These measurements are from rev A3. Redo measurements with Rev A6.

Temperature low pass filter

A Low pass filter has been added on each of the temperature input channels. The cut-off frequency is below what my oscilloscope can handle, but a plot in the time domain verifies a 7 Hz -3dB point.

ADC offset and gain measurements

To measure the ADC offset error, measure the voltage on the uC input pin and record when the transition from 0 to 1 occurs. It should occur at 0.5 LSB = 3.3/4096/2 = 0.4028 mV. Offset: 55.35 mV - 0.4028 mV = 54.95 mV

Temperature calibration

A set of experiments have been carried out to calibrate the different sensors using a dry-well calibrator. The reference probe was a Sensing Devices SDL385 PRT. The levels of calibration was: room temperature, 50, 100, 150, and 200 degrees C.

Thermistor

The thermistor in use was a standard EPCOS 100K B57560G104F. This thermistor has a resistance tolerance of 2 %. The value of the pull-up resistor as measured during calibration was 4740.17501. This value seemed to work well at all levels of calibration, although a little on the high side compared to the reference.

PT1000

The PT1000 in use was a Heraeus 31500989 [1] works well. The tolerance class given in the datasheet is "F 0.10 / Class 1/3 B". There are two different tolerances here: F0.10 and one third of Class B. The accuracy looks good at all temperatures, perhaps a bit on the high side if using the calibrated value for the pull-up. During the testing, 10 ohm was subtracted from the calibrated value.

PT100

The PT100 used in the calibration was the E3D PT100 with the INA826 amplifier board. There is no datasheet available for the PT100 sensor and no mention of what tolerance there is on the reference resistors used. Furthermore Klipper expects the reference voltage to be 5V exactly, and there is no way to supply a measured value. Measurements on Recore has shown that the value is often a bit lower due to the PMIC for the 5V output on the "input" ports. 4.979 V has been measured and is likely an expected value.

Thermocouple

Using the calibration values directly will give a slight offset to the thermocouple measurements. The gain and ADC voltage can be used as measured during calibration. It should be in the region of 101 for gain and 3.307 for ADC voltage. As for the offset, that can be increased slightly to compensate for the cold junction temperature difference. For instance, the measured offset for the test board was 0.0032960, but a value of 0.00340 fit the data points well. The measurements were done on T2, so it it possible that the distance from the cold junction caused the offset. The cold junction thermistor is located between T0 and T1.

FAQ

Where can I ask for help?

You can join the Discord: https://discord.gg/bCnp9H5SB5

What comes in the box with the Recore? Specifically, terminal connectors for power, etc?

It's only the board, no cables or connectors. There is a bag of connectors available to purchase in the web shop.

Do I need access to those 3 buttons on the board?

It is not necessary to have access to the three buttons for normal use.

What's the maximum current draw for heat bed?

This has been tested up to 20 A

Why are there unused and unbroken out pins on the STM32 MCU?

The pins that are needed from the MCU have been routed out plus two pins available on an external header.

I have a gadget that I want to connect with SPI. Is that possible?

There are 6 pins available from the main SoC that can be used for various things.

There is no hardware SPI, but you can make a program that uses soft SPI. There is no dedicated I2C, but perhaps that would be a nice addition.

What exactly is the role of the AR100 device?

The AR100 handles the fast realtime aspects, mainly the stepper motors

What voltage fans are supported?

The same voltage as the input voltage, either 12 V or 24 V. You can experiment with PWM for running lower voltage fans on 24 V input.

Where is the board layout file?

The Layout file will not be available. It is an "open schematic" board, not open source hardware

Where are you shipping from?

Boards are shipping from Norway. Eventually shipping will be from a warehouse in the US

Known hardware issues for rev A6

T1 offset is not consistent

On Rev A6, during calibration it has been discovered that the offset on T1 is often varying around the predetermined set point of 0.33 V. It seems the cause of this is the batch of op-amps used, so it is a hardware issue. The good news is that it seems to be OK to use the values from the calibration document. The gain should be about the same (101).

Board can not handle 24 V input

Connector with capacitor

Board with capacitor

The design without alterations can not handle 24 V input. A bare board can only handle about 20 V. A workaround is to add an electrolytic capacitor to the input. This can either be soldered on to the step down converter input MLCC capacitors, or it can be screwed directly on to the input connector. Be aware of the polarity of the capacitor when mounting it. The "minus" should be towards the black wire. The recommended value is a 47 uF capacitor with a voltage rating of 35 V or higher. 33 uF has also been tested and working. 100 uF and higher should also work, but has not been tested specifically.

The capacitor that will be added to remaining boards from the Rev A6 batch is ESH476M035AE3AA from Kemet: https://www.digikey.com/en/products/detail/kemet/ESH476M035AE3AA/2712504

It seems the reason for the issue is a combination of long wires with high inductance, too low capacitance on the input of the board and too little headroom on the step down converter. This can cause voltage spikes that makes the input voltage go above the rated input voltage during load changes. Adding a capacitor to the input "buffers" or "decouples" the long inductance wires and limits the voltage spikes.