SEN500 Engineering Preview

February 5, 2025by Phil Strong

SEN500 Secure Linux Computer

SEN500 is a secure Linux computer engineered for edge applications that require a high standard of security, flexibility and resilience.

The product is available for sampling and pre-orders in 25Q1, and generally available for volume shipments in April 2025.

Upgrades & Differences to SEN400

SEN500 product family builds upon the successful SEN400 products, and is available in standard ‘SEN500’ and redundant ‘SEN500R’ configurations.

Models, CM5 vs CM4

SEN 400 is powered by Raspberry Pi Compute Module 4 (CM4) with 8GB SDRAM and 32GB eMMC, see CM4 spec sheet: https://datasheets.raspberrypi.com/cm4/cm4-product-brief.pdf. This is the maximum configuration of CM4 available. OEM versions with cost optimized CM4 options are available subject to availability and minimum order quantities.
SEN 500 is powered by Raspberry Pi Compute Module 5 (CM5) with 4GB SDRAM and 32GB eMMC*, see CM5 spec sheet: https://datasheets.raspberrypi.com/cm5/cm5-product-brief.pdf. CM5 is also available in higher spec configurations, such as the 8GB / 64GB configuration.
SEN 500R is a higher resilience version of SEN 500, available for custom order. SEN 500R has all the SEN 500 features plus the following options:
- Fully encapsulated HSM interposer (see below)
- Redundant power supply input option (2x power inputs with enclosure sense loops)
- Heatsink option for the M.2 slot (required for AI accelerators)
- Dual ethernet option (via the M.2 slot)
Both SEN 400 and SEN 500 use the same motherboard and use very similar enclosures. The core compute and security differences are as follows:
- CM4 vs CM5
- Zymbit supervisor vs. Zymbit interposer (see below)
- SEN 500 is available with higher capacity eMMC storage — up to 64GB initially, and up to 256GB possible in OEM configuration via storage on the interposer.
- SEN 500 has notably higher performance, higher power consumption and higher maximum heat output. This is entirely due to the differences between CM4 and CM5.
- SEN 500 has significantly higher encrypted I/O performance due to the AES accelerator in the CM5 CPU, which CM4 lacks. This is a major difference that results in notably higher effective performance of SEN 500 vs SEN 400 because the CPU cycles are not spent on encrypted disk I/O or network I/O. This factor also lessens the effective increase in heat output, though SEN 500 still, generally, runs hotter.

Security Supervisor vs Interposer

The SEN 400 uses Zymbit Secure Compute Module 4 (SCM4), which consists of a Zymbit security supervisor and Pi CM4, which are physically encapsulated and permanently potted with a security rated compound.
In contrast, SEN 500 uses a Zymbit Interposer HSM and Pi CM5, which are optionally physically encapsulated on SEN 500R models only. On the non-encapsulated models, CM5 and the interposer may be unplugged from each other. However, if a CM5 is cryptographically bound to a given interposer, the interposer will not work with any other CM, as is the case for most Zymbit HSMs and modules.
The security supervisor in SCM4 and SEN 400 has dual secure CPUs in a partially redundant configuration, connects to the CM4 CPU via USB 2.0, and has the ability to access, verify, and repair the boot filesystem on CM4 prior to allowing CM4 to boot. SCM4 exclusively uses Zymbit Supervised Boot and does not support Pi Secure Boot (partially because it was available significantly before Pi secure boot was introduced).
The Interposer HSM in SEN 500 does not have the dual CPUs and therefore runs slower, consumes less power, and is connected to the CM5 CPU via the I2C bus. The interposer uses Pi secure boot to maintain boot code integrity, and the interposer is not able to access, verify, and repair the boot partition directly prior to booting the CM.
Both SCM4 and the interposers share all the same core security and cryptographic features and protocols, same physical protection features (2x encoded perimeter loops), same supercapacitor “last gasp” features, same power and temperature monitoring abilities, and the same hardware root of trust capabilities.
Both SCM4 and the interposers secure the boot chain. The Zymbit Secure Compute module (SEN 400) uses Zymbit supervised boot whereas the Zymbit Secure Interposer uses Raspberry Pi secure boot. The main difference is that supervised boot has the ability to repair boot files, while Pi secure boot does not. Both methods enforce verification before running boot code.

Form Factor

Production SEN 400 and SEN 500 have the same form factor and dimensions. Early SEN 400 models and pre-production SEN 500 units may have different heights due to differences in heatsinks.

Power

The SEN 400 and SEN 500 have the same power input and voltage/connector requirements (12-24VDC)
The CM5 powering the SEN 500 draws more current and therefore should have a power supply capable of providing at least 3A at 12V, or equivalent at other voltages.

POE Power

Powering the SEN 400 with 802.11at POE+ is generally sufficient unless additional user hardware connected to it draws significant power. For the SEN 500, 802.11bt POE++ is often required if additional devices (e.g. NVMe or USB storage) are present.

IO

Because SEN 400 and SEN 500 share the same motherboard, they have similar I/O interfaces, with the following key differences:

USB 3.0 Fully Supported on SEN 500

SEN 500 has 2x USB 3.0 external connectors + 1x USB 2.0 internal, while on SEN 400 all USB connections are USB 2.0. This is because only CM5 has native USB 3.0 support.

M.2 Slot Potentially Faster on SEN 500

Both SEN 400 and SEN 500 have a type M M.2 slot for 2242 and 2280 devices. The slot supports NVMe drives, network adapters, and AI accelerator cards, with up to 10W power consumption. By default, the slot runs at PCIe 2.0 x1, however, SEN 500 supports PCIe 3.0 x1, subject to device and driver limitations.

MIPI I/O for Cameras and Displays More Flexible

There are differences in internal MIPI camera and display connector functionality mirroring the same changes between CM4 and CM5. CM4 has dedicated camera and display ports, where CM5 has bidirectional ports. The motherboard supports both.

SEN 500 Engineering Samples vs Production Units

SEN 500 Engineering Samples do not pass through the external button press events to the CM5, the production unit will have this functionality enabled.
SEN 500 Engineering Samples have the Raspberry Pi Red power status LED inverted from how they are displayed on the Pi5. Pi5 has the red LED default OFF and the green LED default ON and flash off to indicate activity, on the SEN 500 Engineering samples the red LED is default ON and the green LED is default on and will flash OFF to indicate activity. This results in the Pi LED indicator defaulting to an ORANGE state then flashing to red in normal operation. The final units will have this addressed so that the red LED is only on if there is indeed a power issue which is consistent with the SEN 400
Pre-production units have pre-production heatsinks and have not been validated for the full temperature range that SEN 400 units support. Final design and thermal performance of the SEN 500 are being validated. Expect changes in appearance and minor changes in exterior dimensions. Production SEN 500 units will be passively cooled.
Pre-production units do not yet have the FCC / CE EMC certifications.