Radial bus at low baud rates

[e100]

Thinking of abusing the CAN bus spec to create a star network for home automation.
The slew rate of a CAN bus transceiver such as the TI HVD230 can be set to a max of 160 ns which allows unterminated radial stubs of up to 5 meters.
See http://www.ti.com/lit/an/slla270/slla270.pdf

How to lower it even more so I can run run longer stubs at very low baud rates such as 1200 bps?
Can I add series resistors like they do with USB, then adjust the 60 ohm termination resistor at the hub?

The chip makers application notes don't talk about baud rates or signal integrity below 50k.

Mike

[e100]

Hmm, no responses, that that indicate that this idea is impractical, won't work or is unlike anything anyone has ever done before?
I know that the phone line to my house has a number of unterminated radial branches that go off to several rooms and it still works OK, at least in the < 5 kHz range.

Mike

[SeanB]

Sounds like an ideal use for the venerable RS422 bus, just use either 2 twisted pairs, with one providing a ground reference, as you really want to keep in the middle of the CMRR of the differential receivers. Or use a microprocessor at the ends that has an ethernet PHY in it ( 10/100M will be fine, and ultra cheap from the switch side) and POE for the system, using a central POE switch to power them.

That way you have both easy connection ( cheap cables and connectors) and with ethernet and POE you have some power at the nodes, at least enough to power sensors and such, even small displays and perhaps even small low power actuators, without needing power cabling.

Even going RS422 use cheap Cat5 cabling and connections, so you have redundant pairs and can transfer power down the spare pairs as well, if you can live with the cable loss.

[magetoo]

Could one perhaps use multiple transceivers, diode-ORed together, to create a sort of repeater/hub for multiple segments?  That way each segment could be done according to spec electrically.

[T3sl4co1l]

You can put RC or LC filters on the devices, to slow down their edge rate.  Make sure the receivers are okay with that, and don't use more than e.g. the system ACK delay, or baud rate, can withstand.

This also increases immunity to RFI and EMI.

CAN has higher immunity than RS-485, though the latter is probably more easily filtered due to its simple design.  Isolated adapters aren't uncommon for both, though beware if they have low capacitance isolation as well.

Tim