VemsFrontier/NetWorkHardWare (2005-05-05 09:33:11)

Developer corner: find a good way for networking the AVR (mega88) boards

This is rather hardware related. VemsFrontier/Networking is about software/logical issues.

While the big ARM guys ( VemsFrontier/ArmEfi and IonSense ) will work nice with CAN, CAN is not feasible for ATMEGA88.

ATMEGA88 has an UART (ATMEGA128 also, except it has 2):

While FBUS requires no transceivers, the RS232 and RS485 transceivers are the same cost (very cheap) and availability.

Neither of the above can be used to talk to most notebooks directly: since majority of notebooks have neither of these ports, only USB.

Fortunately, [bridging ]either of these to USB is simple and cheap. It is normally just a "cable" with some circuitry inside.

RS232 and FBUS not only needs bridging to notebook, but also needs a HUB when more than 2 nodes are used. [converters]

The RS-485 is basically an update of the RS-422 specified for multipoint buses to be constructed. This standard meets and improves all the requirements of the RS-422 allowing, in addition, up to 32 drivers and 32 receivers can be connected to a single bus. [ST] site. TI has many cost efficient transceivers.

[modbus] goes hand in hand (as a practical upper layer) with RS485, although RS485 is just the physical layer and modbus is not a requirement.


RS485 on VemsFrontier/ArmEfi

Conclusion

We probably want CAN on as many boards as possible, including the 9343 display.

I'm afraid that 2 networks both CAN and RS485) would be costly to support originally. When we sell 10000 boards, we can maybe rethink this and invest in some development and use 2 networks from than on. The only difference this makes is that smaller sensor-controller boards (made from LPC2129) will have more IO than if they were atmega88 based. But the connector (and supply-connection!) costs dictate the same anyway.

Also, CAN is a sales pitch


LIN protocol - an alternative

For above reasons, RS485 seems to be the most widely used interconnection standard in the industry for low cost controllers and sensors.

I'd (sloppily) say that


CAN stuff

MCP2551 seem to be cheap as for as CAN goes $0.86

it looks like it it is ok for low voltage on the CPU side it

input:

ViH to 2V

ViL 0.8v

power:

Vdd 4.5V to 5.5V


See also