Hi John,

>From the PC side, I have a design for RS232 to RS485 convertor, uses only MAX232
(RS232) and 75176 (RS485) driver chips.I am in the process of
documenting it.

I have one more product called SMART SWITCH communicating at 2400 baud.
It uses
a 12C508 with one input and one output line, one line for TxRx and one
line for
direction control and 2 lines for crystal. The RS485 address is embedded
into
each device. To have a variable address for field programmablity, the
crystal
will have to be eliminated and a 24LC00 to store the device address
interfaced.
I have not tested the stablity of the internal oscillator for 2400 baud
or at
any lower baud at industrial temperatures. If communication speed is not
a major
problem, try a lower baud like 300 or 600. Even with the internal
oscillator
frequency varying with temperature and from device to device, the
percentage
error might be covered. Well this is only a theory and not practically
tested.

For simple I/O control a 12C508 will do the job instead of 12C509 ( even
with
24LC00 interfaced).

For more number of I/O's use a 18 pin device like 16C54 and use 74165
(parallel
in serial out) and CD4094 (for serial in parallel out) for I/O
expansion.

Line used

2 for RS485 communication
2 for 24LC00 interace
5 for two shift register interface
4 lines free for other interfaces like ADC MCP3201, etc.

Regards

Pailoor

John De Villiers wrote:

> I have a need for an RS-485 Network.
>
> The network will have one master , a pc with an RS-232 -> RS485 converter.
> Most of the slaves will be minor function nodes ( one or 2 I/O lines ) and a
> few will need to be a lot bigger.
>
> I've taken a look at the spec sheets for various PIC's ( those with buit-in
> USARTS and others ) and i've also gone through just about every one of
> MAXIM's RS-485 chips.
>
> I'd like to be able to use something as small as a 12c509 for the smaller
> slaves (Cost is a major factor here), im also prepared to go for the bigger
> PIC16C63/6 types for the larger nodes.
>
> Doing this im going to end up lots of different mixes of hardware and just
> about every chip is going to need special tweaking.
>
> What ideas do you have for trying to keep the network as uniform as possible
> whilst also keeping the node cost down to a minimum?
>
> Should i look at other RS-485 drivers than the maxim ones?  I know theyre
> pricey, but where i am u gotta work with what i can source locally.
>
> John