Motion Master on a Raspberry Pi 5: a card image, and 4 kHz with no skipped cycles

[markosankovic]

What this is

Motion Master now ships as a card image for the Raspberry Pi 5. Write it to a microSD card, put your Wi-Fi network in one file on the card, and the board comes up as an EtherCAT master with a real-time kernel and Motion Master already running as a service.

The board's Ethernet port carries EtherCAT, so it is not how you reach the board. That is why the Wi-Fi settings go on the card. Until the board has joined a network, there is no way to reach it and tell it which network to join.

Getting it

https://dezliul92qqoq.cloudfront.net/motion-master-rpi5.img.xz

About 1.3 GB compressed, 8 GB written to the card. Do not unpack it — Raspberry Pi Imager and balenaEtcher both read .xz directly. A checksum is published at the same address with .sha256 on the end.

The guide is docs/RASPBERRY_PI.md: download, write the card, set the Wi-Fi, find the board, reach it from the Console, and move it to another release. How the image is built is in rt/README.md.

4 kHz on a Raspberry Pi

The image carries a PREEMPT_RT kernel, one CPU core isolated from the scheduler, and the real-time thread pinned to that core. This is a Pi 5 with one SOMANET Circulo in OP, retimed to a 250 µs period from the Console's Game Loop page:

• 4000.0 Hz achieved against a 4000.0 Hz target.
• Zero skipped cycles over 661,580 of them.
• 44.87 µs of task time, 79.46 µs at the maximum. That is the EtherCAT frame round trip, which is what the loop spends its cycle on.
• SCHED_FIFO, mlockall and the pinned core all confirmed by the server itself.

Task time is the number that decides how far this goes. 79 µs against a 250 µs grid leaves room; against a 125 µs grid it would not, and one drive is not five.

Not synchronised yet

The loop holds its cycle and skips nothing. It is not synchronised with the drives: DC SYNC0 is not activated, so a drive acts on the frame as it arrives rather than on a clock it shares with the master. Synchronised actuation is work we will do.

Worth knowing

• The API has no authentication. Anything that can reach the board can enable a drive and command motion. Put it on a network you trust.
• The period is set at run time and is not saved, so a restart returns the board to the period in its configuration file.
• update-motion-master <version> moves the board to another release over SSH. Reflashing the card is never needed for a version change.
• The login is root, and the password is root. Every image also authorises the same SSH key, and its private half is published beside the image, at https://dezliul92qqoq.cloudfront.net/motion-master-rpi — chmod 600 it, then ssh -i. Both are public on purpose: the API has no authentication, so neither is what keeps anybody out. The password is for the console, with a monitor and a keyboard, which is the way in when the board has no network. The key is for SSH once it has one.

Reports from anyone who writes a card are welcome, particularly from a board that is not a Pi 5 Rev 1.1.

[markosankovic]

Why a Raspberry Pi 5, and why a microSD card

This came up, so here is the reasoning in one place. It is now also in the guide, under
Why a Raspberry Pi 5.

Motion Master runs on any 64-bit Linux machine. The image targets one board on purpose.

• A real-time kernel runs on it. Debian ships a PREEMPT_RT kernel for arm64. The image uses
  it, isolates one CPU core, and pins the real-time thread to that core. That is the 250 µs cycle
  with zero skipped cycles above.
• Ethernet and Wi-Fi are both on the board. EtherCAT needs the wired port to itself, so the
  wired port cannot also be how you reach the server. The Pi 5 has Gigabit Ethernet and dual-band
  802.11ac Wi-Fi as standard. No adapter, no second network card.
• Every board is the same board. We tune one machine and ship the result. The kernel, the
  isolated core, the pinned thread, the service and the configuration are all set the way we
  measured them. A laptop is a different machine every time, so nobody can hand you those settings.
• Setup is short. Write the image to a microSD card, put the card in, power it on.
• You can buy one anywhere. Amazon, Farnell, RS, Digi-Key and the Raspberry Pi resellers all
  stock it.
• It stays buyable. Raspberry Pi Ltd states that the Pi 5 "will remain in production until at
  least January 2036". A decade in which the hardware under this image does not change.
• It costs a fraction of a laptop.

Part	Price
Raspberry Pi 5, 8 GB	€185
27 W USB-C power supply	€11
Case with fan	€8
microSD card, 32 GB	€10
Micro-HDMI to HDMI cable	€5
Total	about €220

German retail prices on 28 August 2026, each including 19% VAT. Postage is on top, usually €5 to
€10 for the whole order. Treat the total as an order of magnitude. Prices moved hard through 2026:
the board rose three times in that year and more than doubled, because the price of LPDDR4 memory
rose. The same shortage raised the price of flash memory, so cards and SSDs both cost more
than they did a year ago.

Why 8 GB and not 4 GB

One thing on the board grows with your bus, and that is the process-data recorder. It keeps every
cycle the bus ran, and it is locked into RAM so the real-time loop never waits for a page of it.
Nothing else on the board scales the same way, so it is what decides the memory size.

bytes = recorder.capacity x (28 + IOmap bytes of the whole bus)

One SOMANET drive with the default mapping puts 82 bytes on the bus per cycle. recorder.capacity
counts cycles, not seconds. At the default 300000 cycles that is 5 minutes at a 1 ms period, and 75
seconds at 250 µs.

Bus	Recorder at 300000 cycles	Recorder for 5 minutes at 250 µs
1 drive	33 MB	132 MB
8 drives	205 MB	821 MB
32 drives	796 MB	3182 MB

A 4 GB board covers every row except the last one. 32 drives, a 250 µs period and a five-minute
window need 3.2 GB of locked memory, which 4 GB does not have to spare.

Three reasons to spend the €40 anyway:

1. The whole extra 4 GB reaches the recorder. Both boards run the same image, the same services
   and the same server, so the second 4 GB has nothing else to pay for. At 32 drives and 250 µs it
   is another 6.7 minutes of recording. At 8 drives and 1 ms it is another 105 minutes.
2. You cannot add memory later. The memory is soldered to the board. Every other part in the
   table is replaceable, so this is the only choice you make once.
3. You do not have to do the arithmetic before you record. On 4 GB a long window on a large bus
   is a number to check first. On 8 GB it is not.

The 4 GB board is what the measurements in this thread ran on, and it is not a bad buy at €144.

Why a card and not an SSD

• The board boots from a card as it comes. There is no storage on the board. NVMe needs the
  M.2 HAT+ at €13, the drive itself, and a case with room for the extra board.
• Any computer writes a card. Readers cost a few euros and many laptops have one. To write an
  NVMe drive you need the board, or a USB enclosure.
• The speed does not reach the loop. About 90 MB/s from a card against up to 500 MB/s from a
  drive; about 21 seconds to boot against about 15. Motion Master runs from memory after that, and
  the real-time loop reads and writes no file, so neither number touches the cycle.
• A worn-out card is a card. Write endurance is the one real argument for a drive. The board
  writes the system log and little else. If a card does fail, write another one. The image is the
  whole system, and your settings are one file on the card.