nwDaq-R3 subrack platform

The nwDaq-R3 is the third revision of a subrack platform designed to house remote data acquisition and logging devices in the form of plug-in units. It is compliant with the IEC 60297-3 standard and utilizes common components that are widely available from multiple manufacturers and distributors.

1. Introduction

The nwDaq-R3 platform is based on a standard IEC 60297-3 subrack. For detailed information on the subrack assembly and its components, including dimensions, please refer to the Mechanical design section. This section covers topics such as side panels, horizontal rails, rail positions, subrack sizes and depths, backplane mounting, front panels and blinds for plug-in units, plug-in units themselves, and backplane connector layouts.

The Electrical design section provides details on the signals on the backplane, including power rail types, power classes, and redundant communication buses for both low-power, slow and high-speed data transfer.

To ensure software compatibility, the Communication protocols section outlines the protocols used on the nwdaq-nbus2 bus, as well as the 10Base-T1S and 100Base-T1/1GBase-T1 buses. Additionally, this section describes the protocol used for the nwdaq-pbus bus, which is employed for communicating power resource information.

2. Mechanical design

2.1. Horizontal rail position and side panel dimensions

The platform uses only 3U high subracks. Height of the subrack assembly is 132.35 mm.

Width of the subrack is variable. The following rules must be respected:

• one horizontal pitch unit HP is 5.08 mm wide

• unit width is exactly 4 HP, yielding 20.32 mm. We call it a module.

• the first HP starts 2.54 mm (0.5 HP) from the left

• there is an additional 0.5 HP at the end of the rail

Fig. 2.1 Front horizontal rails with rack-mountable side panels (drawing not to scale)

The resulting horizontal rail length is (n * 20.32 + 5.08) mm, n being the count of modules in the subrack assembly. Table below lists some common combinations and rail lengths.

Table 2.1 Common horizontal rail lengths

Modules	Width in HP	Rail length in mm	Comment
10	41	208.28	Size for 10” racks
21	85	431.80	Size for 19” racks

Side view of the subrack with the corresponding dimensions is pictured below.

Fig. 2.2 Side panel with horizontal rails attached, side view

2.2. Plug-in unit front panels

The cceptable thickness of the front panel is 2, 2.5 and 3 mm. Width of the front panel is calculated as n * 20.32 - 0.30 mm, where n represents the width of the unit in modules with each module being 20.32 mm wide. For a 1-module wide plug-in unit, the nominal width is 20.02 mm. The height of a 3U front panel is 128.4 mm.

Front panels contain two holes for attaching to a PCB, they are horizontally centered to the second HP from the left. Vertical distance of the holes is 99.00 mm, diameter is 2.7 mm and they are vertically centered.

Near the bottom PCB holder hole, there are multiple helper holes to aid in the alignment of the front bracket/holder. These holes are spaced 5.08 mm apart, typically one hole to the left and two holes to the right of the PCB holder hole. Please refer to the figure below for further clarification.

Front panels are secured to the subrack front rails using two M2.5 screws for 1 module wide units and four M2.5 for units that are 2 modules or wider. The attachment holes are horizontally centered in the second HP from the left (and second HP from the right for 2-module and wider units). The holes are vertically centered and their vertical distance is 122.5 mm.

Fig. 2.3 Front panel for 1 module wide plug-in unit

2.3. Plug-in unit PCB dimensions

A single plug-in unit comprises of at least these components:

• front panel with the specified design

• front panel bracket/holder to assist with pullting the unit out of the subrack

• two PCB holders

• one or more backplane connectors on the back

• PCB

For more elaborate designs there may be more components in a single unit, eg. multiple PCB holders when more than one PCB is used, helper connectors, PCB cover, etc.

Fig. 2.4 Dimensions of a plug-in unit main PCB

3. Electrical design

3.1. Overall concept

Plug-in units are interconnected with a backplane mounted on the rear horizontal rails within the subrack. Power to the backplane is provided using bidirectional/source units from their respective front panels. Data communication and any field connections are only done on plug-in unit front panels.

3.2. Backplane connections

The backplane provides redundant low power rails, a high power rail, redundant nqdaq-nbus communication buses, nwdaq-pbus power resource communication bus, 10Base-T1S multidrop ethernet bus and 100Base-T1/1GBase-T1 point-to-point links. WIth the exception of the last one, all interconnections are using the bus topology. Point-to-point ethernet links use star topology, more on that in the corresponding section. There are no external connections going to the backplane except connections allowing backplane stacking.

As a result, nwdaq-r3 platform backplane can be fully passive. There are no mandatory components on the backplane except:

• nwdaq-nbus2 bus termination (both ends)

• 10Base-T1S bus termination (both ends)

• nwdaq-pbus pull-up resistor

All connections are organized into 8 backplane connectors with designations JM0 to JM7. The following table lists all connections within the particular backplane connector.

Table 3.1 List of backplane connectors with their respective usage

Connector	Name	Description
JM0	Basic power and data	Contains the required minimum for a unit. It carries VBUS_LP low power bus, nwdaq-nbus2 communication bus, 10Base-T1S ethernet bus, card detect input/output, chassis ground connection.
JM1	Redundant power and data	This is a redundancy and data speed extension. It carries an independent VBUS_LP2 low power bus, an independent redundant nwdaq-nbus2 communication bus and a 100Base-T1/1GBase-T1 point-to-point ethernet link (the actual connected target depends on the backplane design).
JM2	App specific, star	Application-specific fanout of 10 differential lanes, connections to other units are dictated by the backplane design. See JM2 addendums for commonly used connections.
JM3	Reserved
JM3	Reserved
JM3	Reserved
JM3	Reserved
JM3	High power bus	Carries signals and buses for high power units. VBUS_HP high power bus capable of delivering 10 A at 28.8 V maximum, nwdaq-pbus power resource bus, card detect input/output and a chassis ground connection.

The following subsections briefly describe all available power and data buses.

3.2.1. nwdaq-nbus2 data bus

nwdaq-nbus is designed with the following constraints and requirements in mind:

• very low power during the idle state with commonly available CAN bus transceivers

• moderate speeds achievable (1-4 mbit/s on the backplane, 250 kbit/s nominally over long runs)

• initiator driven MAC (token passing) or CSMA/CA/CD MAC

• wired-OR operation to avoid power peaks during collisions

• source-based communication using topics to mimic MQTT (publish/subscribe) or request/response communication to follow the RPC concept

• packet-switched

• native encryption using pre-shared keys providing basic security on the MAC level

• packet size up to 1024 B (this limit is TBD)

On the physical layer (L1) level, the bus uses CAN signalling as specified in ISO 11898-2. Common CAN transceivers can be used to access the nwdaq-nbus2 bus. CAN MAC is not used.

Detailed description of the nwdaq-nbus2 protocol on the MAC and higher levels can be found in the corresponding section nwdaq-nbus2.

Note

There is also a legacy possibility of tunneling nqdaq-nbus over a CAN 2.0b bus. This concept is not part of this specification as it is no longer recommended nor used.

3.2.2. VBUS_LP and VBUS_LP2 low power bus

VBUS_LP buses carry 5 V power to plug-in units, 4 A maximum per unit. The total aggregated backplane current is limited to 8 A. The bus uses its voltage to communicate the current available amount of power in total. It is called power-availability, where pa-- state means the shortage is long lasting, all backup resources are deprived and power failure is imminent, pa- is the common state when eg. running on batteries, pa0 means the energy sinking/sourcing is in balance, pa+ means there is a surplus of energy available (eg. to charge backup batteries) but the amount of energy is limited, pa++ means there is a power source with virtually infinite capacity (an AC connection for example).

pa0 is always at the nominal bus voltage level, 5 V for VBUS_LP. pa- and pa-- are 5% and 10% less tha nominal, respectively. pa+ and pa++ are 5% and 10% more than nominal. There is a +-2.5% tolerance on the voltage levels, that is, pa- being -7.5% to -2.5% lower than the nominal voltage, for example.

Voltage levels lower than 7.5% less than the nominal voltage are considered bus undervoltage and must trigger plug-in units UVLO protection disconnecting the units from the bus.

Voltage levels higher than nominal + 7.5% are considered overvoltage and all units must deal with it individually by cutting the bus off (an OVP protection).

Additionally, all sinking units must limit their current sink to the value specified in the corresponding unit datasheet, no more than 4 A, which is the maximum current possible for the VBUS_LP bus. This limit protects the backplane connector and wiring and maintains availability of the bus power in case of a misbehaving unit. The same applies to sourcing units which are required to source power in a CC/CV manner, limiting the current to 4 A per VBUS_LP. This protects the unit itself from possible bus shorts.

Fig. 3.1 Graphical representation of power availability levels for VBUS_LP and VBUS_HP

Note

For practical reasons, VBUS_LP voltage levels are compatible with USB 2.0 and USB 3.0 voltages and it can be interfaced with them using minimum amount of components. The power availability feature together with the sinking/sourcing regulation on individual units help sink the power without overloading the USB port.

3.2.3. High power bus VBUS_HP

VBUS_HP bus provide the same functionality as VBUS_LP, with higher voltage levels. The nominal VBUS_HP voltage is 26.4 V, power availability levels remain the same. The bus voltage is usually referred to as 28 V despite not being strictly true. Maximum current per unit is 4 A for standard units, yielding 105 W of available power. When appropriate measures are taken on the unit, the maximum current per unit can be 10 A (264 W of power).

The total current permissible to be carried over the backplane is 40 A and is limited by the backplane design and thermal management. Refer to the backplane datasheet for the exact figure.

Note

Again, for practical reasons, voltage levels on VBUS_HP are compatible with 12-cell lead-acid batteries and with 8-cell LiFePO4 battery packs. Under specific circumstances and using appropriate protections, such battery packs can be connected to the VBUS_HP bus directly without any further regulation.

3.2.4. Ethernet data connection over 10Base-T1S

3.2.5. Ethernet data connection over 100Base-T1/1GBase-T1

3.2.6. Application specific differential lanes

3.3. Backplane connector pinouts

Backplane contains slots for 8 connectors named JM0 to JM7. Not all of them need to be used. These are the usual combinations:

• JM0 only - for simple units, no requirement for redundancy

• JM1 only - not possible

• JM0+JM1 - redundant low power bus and redundant communication bus

• JM0+JM1 - simple units with fast data transfer requirement (100Base-T1/1GBase-T1)

• +JM2 - ethernet switching boards, USB hub boards, etc.

• JM7 only - high power units, photovoltaic input units, battery packs

• JM0+JM7 - power supplies, external power bus interfaces

3.3.1. JM0

JM0 connector provides power and data for simple and low power units usually in the 1-3 power class. No power or communication redundancy is available if only JM0 is used. Signal placement on the connector is optimized to allow using PCBs made with simple processes, 2 or 4 layer.

Fig. 3.2 Backplane connector pinout and suggested routing - JM0

Table 3.2 JM0 backplane connector signal description

Signal	Description
CD	Card detect. Connect resistor to GND or VBUS_LP. See card-detection section.
CH	Chassis ground. See grounding-concept.
GND	Plug-in unit main ground connection.
VBUS_LP	Lopw-power bus connection
T1SP	10Base-T1S bus, positive
T1SN	10Base-T1S bus, negative
NBUSP	nqdaq-nbus bus, positive
NBUSN	nqdaq-nbus bus, negative

3.3.2. JM1

JM1 connector provides redundant VBUS_LP power and nwdaq-nbus data communication. For units with higher data transfer requirements, 100Base-T1/1GBase-T1 interface is available.

Fig. 3.3 Backplane connector pinout and suggested routing - JM1

Table 3.3 JM1 backplane connector signal description

Signal	Description

3.3.3. JM2

Fig. 3.4 Backplane connector pinout and suggested routing - JM2

Table 3.4 JM2 backplane connector signal description

Signal	Description

3.3.4. JM7

Fig. 3.5 Backplane connector pinout and suggested routing - JM7

Table 3.5 JM7 backplane connector signal description

Signal	Description

3.4. Unit power sinking/sourcing considerations

Note

Describe requirements for power filtering, maximum power levels, hot-plug/unplug management, etc.

4. Communication protocols

5. Part cross reference

6. References

7. List of abbreviations

8. Contributing and feedback

9. Document revisions