Gary T. Pepper wrote:

> Hi All,
>
> I would like to build a distributed PIC-based data acquisition and control
> system for home use.  Some uses might be environmental and security
> monitoring/control.  I'd like to use one master PIC as a main controller
> (mainly to obtain/display system status info) and a number of slave PICs.
> Each slave PIC has a simple function to perform, such as sensor monitoring
> and/or a simple control function.  The master PIC can issue simple commands
> to the slave PICs, whereas the slave PICs can only respond to the master
> PIC with status info.
>
> I was wondering if anyone could give my some guidance (i.e. opinions) on
> what might be the best method to "network" (e.g. RS485, I2C, SPI etc) a
> number of slave PICs to a master PIC, which would satisfy the following
> design requirements;
>
> 1.  There are a minimum of 8 slave PICs and a maximum of 32
> 2.  The master PIC is separated from the each of the slave PICs by no more
>     than 100 feet
> 3.  Bidirectional communication (transfer of data, commands) is
>     required to/from the master PIC and to/from each of the slave PICs.
>     Only one slave is allowed to transmit data to the master PIC at
>     any instant.  Obviously all the slave PICs have to listen to all
>     the commands issued by the master PIC.
> 4.  The maximum data transfer rate required is 1200 baud.
> 5.  The master PIC should be able to communicate with the slave PICs
>     in any desired sequence.
> 6.  Simple (cheap) cabling connections are desired.
>
> I hope that I've provided enough info so that someone can make some
> suggestions.  Thanks for your input!
>
> Regards,
> Gary Pepper


I am using RS485 to implement a half duplex 9600 baud system with one
master 16F84 and one slave 16F84 with plans for supporting multiple slave
nodes. The  specs  for the transceivers I am using (LT1483)  indicate that
32 nodes  are  possible with a transmission rate of 250K bit/sec. There are
transceivers that allow up to 256 nodes. Several hundred feet of cable
should not be a problem at 1200 baud. Some RS485 transceivers claim
several thousand feet at low transmission rates. I have not tried them. My
system is operated in an industrial  environment so I am using a shielded
 twisted
pair cable and an industrial  connector. However, in a more benign environment
an RJ11 and standard  unshielded twisted pair cable would work well and be
very cheap.


| PhD Electrical Engineer
|
| King of Prussia, PA 19406
| kirmse@netaxs.com
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