EEZ Bench Box 3

The EEZ BB3 (short for Bench Box 3) represents the next step in making EEZ H24005 project even more flexible and modular. It is built to simplify adding of new T&M functionalities in the future. During the course of H24005 project we've learned a lot and identify various obstacles and issues caused by different reasons, from inexperience, lack of information, wrong presumption, etc. The process of learning is continued and the EEZ BB3 project is addressing the following issue that we found in the H24005 project:

• We found that the H24005's front panel assembling and disassembling is tedious and difficult task. In the same time it limits modularity since it includes components that really belongs to the module not the chassis (i.e. output terminals, LED indicators, I/O ports). A new level of modularity is achieved by splitting the Digital control board (i.e. Arduino shield) that is mounted on the enclosure front panel back into two parts: the MCU board and the backplane. A module will includes its own front panel that provides access to all parts that have to be exposed (e.g. terminals, switches, LEDs, etc.)
• Move from Arduino Due to more capable solution. The H24005 firmware with its extensive list of functionalities pushed Due to the limits, both in case of computing power and memory capacity. Also, lack of "enhanced" features such as multitasking (like FreeRTOS, etc.) make Due unsuitable in many ways. Therefore STM32F7 is selected as the new platform, but thanks to new modularity it should be possible to choose other MCU, SoC or SBC as main computing power in the future releases.
• Increase total number of modules that was fixed on H24005 to only two. The BB3 offers enough room to host up to three modules. It is slightly higher, but what is important occupy exactly the same benchtop area.
• Bigger screen, and better presentation. Screen size, resolution and speed depends of deployed display controller. Selected new STM32F7 is capable of directly driving displays with RGB interface, reducing the cost of display and simplify HMI programming. The screen size is now increased from 3.2" to 4.3" making more room to comfortably present information and parameters for all three modules. Fonts can be also finally rendered with anti-aliasing make is more legible and improve aesthetics.

With the EEZ BB3 we continue the practice of design in a manner that it can be easily build (or at least doable :) by DIYers/makers with certain level of patience and intermediate skills in soldering, assembling, testing and have basic understanding of software and firmware installations and uploading. Although it includes custom made enclosure, builder has a freedom to make his/her own enclosure or adopt some general purpose enclosure of similar dimensions.

Additionally, when custom enclosure is selected, one can choose to increase the number of peripheral modules by extending its backplane to some higher number (up to 7).

Thanks to modular design, the EEZ BB3 based solution can be build gradually in accordance with ones needs and budget. Peripheral modules can have their own processing resources or can be controlled from master MCU that keeps costs low. The EEZ BB3 can be controlled locally (TFT touchscreen, encoder) or remotely (USB, Ethernet).

Quick facts

• Fully open source
• Modular design
• Full range autoswitch AC input (115 / 230 Vac)
• Minimalist wire harness for simplified assembling and servicing
• Up to 3 peripheral modules
• STM32F7 ARM 32-bit MCU
• 4.3” TFT touchscreen display
• Front panel AC power switch
• Incremental encoder and user-defined switch
• USB 2.0 OTG and 10/100 Mbit/s Ethernet connectivity
• Software controlled 80 mm cooling fan
• Remotely controlled by SCPI commands using the EEZ Studio of similar SCPI controllers
• Compact size: 290 (W) x 123 (H) x 240 (D) mm

Building blocks

EEZ DIB v1.0

The EEZ BB3 modularity is based on DIB (short for DIY Instrument Bus). It's a work in progress and the current specification version is 1.0. It defines electrical, mechanical and software part of the modular system. Mechanical part of specification is more or less done, electrical defines connector's pin mappings and is also presented below. Software part is under development and has to define how MCU master on the MCU board should communicated with various peripheral modules that could come with or without on-board processing resources (MCU, FPGA, etc.). A sort of "plug-and-play" functionality should be achieved that new modules can be easily added and that already installed modules can be freely moved around without compromising module related configuration parameters and data (e.g. calibration data, working hours counters, etc.).

This specification defines four physical connectors: two mandatory (MCU 40-pin and Peripheral module 28-pin) that are related to DIB backplane and two optional, one (16-pin) that defines power connection when the EEZ DIB AUX Power supply is used for powering MCU and backplane, and another (20-pin) that allows coupling of power sourcing module output terminals such as DCP405 power module.

Connector's pin mappings are shown on Fig. 1. All connectors contain two rows hence pin mappings description is separated in two columns (A and B). Direction (Dir column) when applicable shows unidirectional signal flow referenced from the inserted module, not backplane.

For example, NRESET on the MCU module connector is assigned as an output (O). Therefore the same signal represents an input (I) on the peripheral module connector. Bidirectional signals have the same direction mark (I/O) on both sides (MCU and peripheral module).

MCU module connectivity

A 40-pin (2 x 20-pin) right angled pin header and receptacle with 0.1” pitch are used for making connection between MCU board and DIB backplane. Receptacle is used on the MCU board side while header is on the backplane side. The MCU module provides the following power and signaling lines:

• +5 V pass-thru power output (from the AUX Power Supply)
• +12 V pass-thru power output (from the AUX Power Supply)
• +3.3 V low power output (provided from the MCU board LDO, max. 20 mA per module)
• +3.3 V backup, low power battery backup output
• Reset output (active low) and Fault open-collector signal
• I2C bus (SSCL and SSDA, 3.3 V level) shared between all peripheral modules and AUX Power Supply
• Async UART (RX and TX, 3.3 V level) shared between all peripheral modules that can be used for firmware uploading of the peripheral modules' on-board MCU if it does not support SPI for such operation
• Three dedicated SPI buses (SCLK, MISO and MOSI for CH1, CH2, CH3) and two device select lines (CSA and CSB for CH1, CH2, CH3) that allows addressing of up to four SPI devices on the peripheral module (using 2-to-4 line decoder like SN74LVC1G139)
• Peripheral module interrupt (IRQ for CH1, CH2, CH3)
• SYNC output for synchronizing activities between two or more peripherals, e.g. OE (Output Enable) of power sourcing peripheral modules.

Although only three SPI buses are defined with DIB v1.0, that doesn't limit max. number of peripheral modules to three. The DIB backplane can be designed in a way that two or more peripheral module connectors share the same SPI bus. That slots could be used for low-speed devices. If more device select (CS) lines are required, a SIPO register (e.g. 74HC595) can be used to address up to eight SPI devices. Addressing more SPI devices can be accomplished by deploying two or more daisy-chained SIPO registers.

Peripheral modules connectivity

Power and signaling lines for the peripheral modules are provided with 28-pin (2 x 14-pin) 0.1” pitch right angled pin header on the side of the peripheral module and receptacle on the backplane side. The peripheral module connection includes the following power and signal lines:

• +5 V input (from the AUX Power Supply)
• +12 V input (from the AUX Power Supply)
• +3.3 V low power input (provided from the MCU board LDO, max. 20 mA per module)
• +3.3 V backup , low power battery backup input for e.g. standby
• Reset input (active low) and Fault open-collector signals
• I2C bus (SSCL and SSDA) shared between all peripheral modules and AUX Power Supply
• SPI bus (SCLK, MISO and MOSI) and two device select lines (CSA and CSB)
• Interrupt output (IRQ)
• SYNC input for synchronizing activities between two or more peripherals initiated by MCU
• Module identification inputs (A0, A1, A2)
• BOOT input
• Shared async UART (optional)

The BOOT input can be used with peripheral modules that comes with on-board MCU and which firmware could be uploaded via SPI or UART. If BOOT input has to be dedicated (i.e. one per module) that only one on-board MCU can be put into bootloader mode after the reset (power up) a DIB backplane can be equipped with I2C I/O expander that could provide one BOOT control signal per module.

The module identification inputs can be used as device selection for I2C peripherals such as on-board EEPROM, temperature sensors, etc. When used for EEPROM addressing the 000 address cannot be used since it's assigned to the I2C EEPROM on the MCU board. Inputs A0-A2 require pull-up resistors connected to +3.3 V (e.g. 10 KΩ) on the peripheral module PCB. Module position on the backplane is defined by “hardcoded” wiring of A0-A2 to GND.

Please note that #0 position should not be used to avoid possible conflict with already mentioned EEPROM on the MCU board and also other devices in the future.

When selecting I2C EEPROM pay close attention on its addressing capabilities even if it provides three address inputs. Many low capacity devices can effectively use only a single input for addressing purposes.

If two or more modules have to be galvanically isolated (e.g. like in case of power modules with floating outputs) use appropriate isolators (e.g. Silabs Si86xx, Maxim MAX14850) for control and data lines.

AUX PS module connectivity

The main purpose of this connection is power delivery for MCU board and peripheral modules. Two voltages are specified with DIB v1.0: +5 V and +12 V. Additionally, two lines are assigned to +3.3 V auxiliary powers: a) +VAUX as (battery) backup and b) +3.3 V that is sourced from the MCU board LDO and can be used to power e.g. an I2C low-power (max. 20 mA) device like fan controller. This connection contains also the following functions:

• Shared I2C bus (SSCL, SSDA) for fan controller or similar devices
• AC Soft-start/Standby control inputs (PWR_SSTART, PWR_DIRECT)
• Reset input (active low) and Fault open-collector signal
• PE (Protective Earth) output
• MBOOT output for define MCU bootloader mode

Power sourcing module connectivity

This connection is optional, but its physical location should be taken into account when peripheral module PCB is designed to leave that area unpopulated to avoid possible issues with inserting peripheral module into the DIB backplane that includes this connectivity (e.g. EEZ DIB BP3C backplane). Therefore it is highly recommended to follow the peripheral module PCB template available on the project's GitHub repository.

A 20-pin (2 x 10-pin) 0.1” pitch right angled receptacle is used on the side of peripheral module and straight pin header on the DIB backplane side. Pinout scheme is rather simple: it provides power terminal inputs and outputs that are assigned on multiple pins for increased current capacity. Allocation of five pins per power lines should be sufficient to carry 5 A continuously.

The idea behind output terminal coupling is to avoid usage of external wiring as it is accomplished on the EEZ H24005's Arduino Shield board by using power relays for reliable connection under MCU control.

If peripheral module such as DCP405 power module is deployed, this connectivity becomes mandatory on the DIB backplane. If no coupling capabilities are provided, a simple pass-thru connection (i.e. IN+ to OUT+ and IN- to OUT-) has to be made to ensure normal operation of such module.

Peripheral module PCB dimensions

The DIB v1.0 specifies only the physical dimensions of the peripheral module and related connectors (i.e. 28-pin peripheral module connector, and optional 20-pin power sourcing module connector) positions. Dimensions of other parts of the system (e.g. MCU board, backplane, power supply module) can differ from project to project depending of e.g. chosen enclosure dictated by builder budget and appearance preferences. Allowed dimension of the peripheral module is shown on Fig. 2.

Thanks to the fact that DIB connectors are located at the bottom end, the PCB width can vary and it could be anything from 90 to 185 mm. In fact it could be even wider, but this is recommended if the EEZ Bench Box 3 design is used. Recommended PCB height is 95 mm but it can also vary in accordance with selected enclosure, and that is a max. height for the EEZ Bench Box 3 design.

Distance (horizontal pitch) between two peripheral module connectors is 35.5 mm (7 HP) and could appear that is somewhat too large for today's standards and components. Such distance is selected to allow mounting of larger heatsink elements on the peripheral module that is beneficial for power sourcing and sinking modules where increased power dissipation is expected. Still, the specified modules distance does not limit builder to design a backplane that combine few DIB v1.0 modules (i.e. 7 HP wide to “comply” with specification) with thinner modules (e.g. 5 HP).

Note that even if peripheral module does not require 20-pin power sourcing module receptacle, related PCB area has to remain unpopulated to avoid issue with inserting peripheral module into DIB backplane which has that connector.

Peripheral module front panel is shown on Fig. 3. It contains two pairs of holes for fixing it on the peripheral module PCB and to the DIB enclosure front panel.

DIB 3-slot backplane example

The DIB backplane shown on Fig. 4. is an example of possible DIB v1.0 backplane design. Its design is used for making the BP3C backplane for the EEZ Bench Box 3. It provides connectivity to up to three peripheral modules which can also be a power sourcing modules. Therefore both 28-pin peripheral module connector and 20-pin power sourcing module connector are included for each slot.

Peripheral modules are inserted in this backplane vertically, and this is a mandatory, while MCU board is connected horizontally which is not a mandatory orientation. If a backplane is designed to accept MCU board vertically, and positioned on the far right of the backplane, the same horizontal pitch (7 HP) has to be used for MCU 40-pin connector as for the peripheral module connectors.

It is recommended that PCB with min. two layers is used and filled with huge GND planes.