To enable connectivity and compatibility between MoCoBus devices, each device must implement a standardized interconnect strategy. If a device is to claim compliance with MoCoBus, and not use the standardized interconnect, it must provide an adapter to the standardized interconnect, otherwise it will not be considered to be compliant.
Connector and Cable
The cable chosen for the MoCoBus interconnects is the standard CAT-5 cable with an RJ-45 connector (e.g. standard 100BASE-TX cable), it is chosen because:
- It provides a positive locking mechanism
- It provides the minimum required contacts for MoCoBus
- It meets the flexibility and durability requirements for field use
- Meets the pair-twisting requirements of RS485
- It is readily available world-wide
- It is low-cost
One may, provided complete compatibility, substitute a higher grade of cabling, such as Category 6, as long as connector and pin-out compatibility is preserved.
Additionally, for any CAT-5 cable, it must meet the either CMP or CMR environmental rating for outdoors use, or any situation where one or more devices receives power from the bus alone.
It is worth noting that only standard (non-crossover) CAT-5 cables are supported, one should never use a non-standard cable wiring. These types of cables are rare these days, often referred to as an "Uplink" or "Crossover" cable are to be avoided.
Buses
MoCoBus is actually comprised of two distinct buses, and a shared power rail. The two buses are:
- Command and Control
- Two-wire, RS-485 bus
- Simple Synchronization Bus
- Three-wire +5v TTL bus
The Simple Synchronization Bus gets its ground signal from the power rail, which uses the three remaining wires on the CAT5 cable: 2x 12V and 1x Ground.
Power Rail
MoCoBus provides a +12V power rail, with the intent that low-current devices may be powered entirely from the bus. However, the total load capacity of 600mA (miliAmps) should never be exceeded, and each node device is required to break away from the power rail in the case of over-draw. This is to reduce the possibility of fire from overheated cables when power draw exceeds the maximum specification.
See the Reference Designs below for how to safely connect to this power rail.
Example Bus Connectivity
The following diagram shows how three nanoMoCo devices and one master device would be wired together using these buses:
Connectors by Device Type
Each Node device shall have two RJ-45 connectors, wired such that the individual bus connections make direct connections (keeping a bus topology). No device may have more than two connectors, which would allow for other topologies.
Each Master device shall have only a single connecter per bus, and shall terminate the Command and Control bus according to requirements below.
RS-485 Bus Termination Requirements
The final Node on the bus shall always have a terminator plugged into its free RJ-45 port to terminate the bus and prevent signal reflections.
Every Master device must terminate the bus at its end.
Termination resistors chosen shall be valued at 120 Ohms.
The total limit of devices per bus is 32 devices.
Wiring Diagram
The following indicates the standard wiring for MoCoBus on an RJ45 connector:
|
RJ45 Connector |
Bus Line |
|
1 |
RS485-A |
|
2 |
RS485-B |
|
3 |
COM1 |
|
4 |
COM2 |
|
5 |
GND |
|
6 |
+12V |
|
7 |
+12V |
|
8 |
COM3 |
Node Reference Design
To aid in system development, the following reference design is provided to show all features of bus connectivity, ESD protection, and the ability to regulate consumption from the power rail.
Master Reference Design
To aid in system development, the following reference design is provided for a Master device, showing ESD protection, termination, and protection for the power rail.
Reference Design Files
The reference design files for Eagle, including Bill of Materials can be downloaded here:
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