Re: 1-wire to USB converter that can use 1820s directly

["Rod Speed" <rod.speed.aaa@xxxxxxxxx>]

Marc F Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote

> Here's one of the pertinent Application Notes
> " Guidelines for Reliable 1-Wire Networks" :
> http://pdfserv.maxim-ic.com/en/an/app148.pdf

Read that LONG ago thanks.

> It begins:

> "INTRODUCTION
> The 1-Wire® protocol was originally designed (many years ago) for
> communication with nearby devices on a short connection-a way to add
> auxiliary memory on a single microprocessor port pin. Customers soon
> devised unique applications that involved extending the bus and
> moving the slave devices farther and farther from the master.

> Problems came up as the bus lengths exceeded both the capabilities of
> the bus masters and the limits of the protocol. 1-Wire device designs
> responded to the call with added features and protocols, multidrop
> (networking) capabilities, durable steel containers (iButtons®), and
> mechanisms to assure valid data transfers even in severely
> intermittent contact situations.Despite this evolution, 1-Wire
> components still perform poorly when bus masters are improperly
> designed or implemented, or when masters intended for short line
> use are pressed into service with greatly extended buses.

And that last isnt true with what is used in USB/1-wire
converters/adapters.

> A 1-Wire network is a complex arrangement of devices, wire, and
> connections. Every network is different, often in both topology
> (layout) and hardware. Various claims have been made about 1-Wire
> network length and loading limitations, but the context may have often
> been unclear. As a result, users have sometimes been surprised when
> topologies fail despite the assumption that they are within
> specification.

And Pete Anderson and others have found that strings of 1820s
work fine at MUCH greater distances than I am going to use.

> Special bus masters have been devised and tested
> with a single network design only to find that they
> are unsuitable for use in other layouts."

And Pete Anderson has found that strings of 1820s work fine.

> A significant proportion of folks wiring
> up a whole house full of 1-wire devices

Didnt say anything about doing that.

> with a single channel will find that they would have saved
> time, money and aggravation by reading and understanding
> the words above and designing and implementing accordingly.

And anyone with a clue can see that Pete Anderson has found
that the sort of strings of 1820s that I want to use work fine.

I was JUST asking if anyone had any experience with particular
USB/1-wire converters/adapters in that situation because the
devices that Pete Anderson uses the strings of 1820s with
have real downsides in my particular situation.

And I later proposed a different approach when no one was
able to suggest a particular USB/1-wire converter/adapter
which was known to do the job in my situation, doing something
on the 1wire side of the USB/1-wire converter/adapter, like
using one of the spare wires in the RJ11 cable to provide
say 5V power to the 1820s, instead of using parasitic power.

Pete says that parasitic power works fine. The main problem
is that I cant see what he is doing with the strings of 1820s
at the inner end, because I cant find schematics for any of
his devices that use strings of 1820s.

Presumably that sort of thing must be out there on the web
and its just a matter of finding it. Which is why I asked the
question in the first place, given that statement that what
may appear to be fine from the datasheet may not work
out that well in practice. I was essentially asking if anyone
had actually implemented it and found it worked fine.

One other obvious approach is to get one of the cheaper
devices of Pete's that does use strings of 1820s fine,
basically to get the schematic and to use to test the
string of 1820s and then modify the 1-wire side of a
USB/1-wire converter/adapter to do the same thing
electrically. That would provide a much cleaner end result
than using one of Pete's devices with a USB/RS232
converter, for hardly any more money,