6 Nov
2014
6 Nov
'14
2:06 p.m.
hp and Karl Hammar, we already have sketch of control chain here:
http://wiki.portalmod.com/wiki/Control_Chain
On 6 November 2014 10:56, <karl(a)aspodata.se> wrote:
Gianfranco (The MOD Team):
...
The physical interface of the Control Chain
devices is an RS-485
full-duplex serial line running at 1Mbit/sec. Currently up to eight
devices can be daisy chained and by use of the RJ-45 cable we also
power the devices ( 1 pair for input,1 for output and 1 for power
supply).
...
How are you handling "collisions" and idle bus (rs485 does not
specify that kind of things) ?
Idle bus can be taken care of with a resistor devider (as described
in e.g. [1]), but that solution draws power when the bus is idle.
Why not a "active" kind of termination by connecting "A" to +2, 3.3,
or 5V and "B" to GND (or vice versa), both throught 60 Ohm resistors ?
"Collision" handling can be avoided by using a one master and letting
the other nodes to be slaves, and polling the slaves.
Or one can use something similar to J1708 [2] and CAN by driving the
device enable pin instead of the transmit pin and let the bus "float"
(by setting the output driver to hi-z) up/down to the "recessive" state
by a "idle bus"/failstate circuit. You have to have some software to
detect any collisions then.
I drafted some thoughts in [3], but havn't implemented it "yet".
Ideas ?
Regards,
/Karl Hammar
[1] http://www.ti.com/lit/an/slyt324/slyt324.pdf
[2] http://www.ti.com/lit/an/snla038b/snla038b.pdf
[3] http://turkos.aspodata.se/git/openhw/serial_bus/Readme
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