rs485 real example basics

[Guest_X44]

I've searched this forum for how I should start designing a basic pic-based circuit with rs485.
By basic I mean to provide me the least functionality of it. What 485 drivers should I use, and how should this be addressed?
I'm not after fancy features, just down-to-earth basics.
Unfortunately, I couldn't find anything in this forum that's just basic enough for me to learn.
I know C and CCS and have limited experience in designing circuits based on 16f87x devices and rs232, but not 485.
Maybe someone can provide some leads.
Thank you.

[Guest]

[Haplo]

I've used the DS75176 RS485 driver extensively and I'm very satisfied with it. To drive it you need to define the #use RS232 statement with the XMIT,RCV and ENABLE pins, connect the PIC's RX/TX to the driver's RX/TX and connect the ENABLE pin to the both Transmit_Enable/Receive_Enable pins of the driver.

[Guest_X44]

Thank you. This forum is giving me leads.

Does anyone have a driver to share for the DS75176 RS485 driver?

Also, do I need any level converters? Any more devices to connect to to complete a basic circuit? What about the crystal? What freq. should I use?

A _simple_ working circuit and code would be of great help. Even just sending/receiving a character or two just to demonstrate the basics is enough.

[Haplo]

The whole thing is very straight forward. Yo don't need any drivers (code) or level converters. As for the speed, it depends on the baud rate, whether you are gonna use a hardware or software UART, and any interrupts you might have in your code.

[Guest_X44]

Haplo, et. al,

On the DS75176 what do the RO, RE, DI, DO, RI mean? The specs I have don't say much.

Which pins are used and which of these connect to the PIC and, which of these connect to the outside world?
Is there a standard connector for a rs485? Is it a 9-pin d type male connector? I've seen pc motherboard with such. Which of the driver's pins connects to the d type connector?

Thanks.

[Haplo]

DORI and /DORI go to your +/- RS485 lines, RO goes to the PIC's receive, DI to the PIC's send and DE and /RE to the 'enable' pin you have defined in your #use RS232.

[Guest_X44]

Thank you so much, Haplo.
Another question, there are two (2) / DORIs there. The inverted /DO /RI and the DO RI.
hich of these go to the + and which one to -?

[Haplo]

The inverted one goes to -.

[Guest_X44]

Haplo, et. al.

Neutone's web site includes a schematic (http://home.houston.rr.com/neutone/Assorted%20circuits.pdf)
What are those diodes for connected to the MAX483?
Is it at this port that other rs485 circuits (external) can directly connect to?

Are optocouplers necessary (advisable) to isolate the PIC?

Thank you.

[Haplo]

Since that is Neutone's circuit I'll let him comment on it, but in general poeple use opto-couplers/diodes for protection. It depends on your application. For example I usually use two 5.6v zener diodes from the DORI and /DORI lines to the ground, along with two 24ohms series resistors.

[Guest_X44]

Hi Haplo,

Could you, perhaps, tell me (and maybe for benefit of other newbies) how the protection diodes and resistors and couplers are wired up, acc to your circuit.

[Haplo]

As I said, I usually use two 5.6v zener diodes from the DORI and /DORI lines to the ground. I also use two 24ohms resistors in series with the RS486 +/- lines to my driver chip (similar to R23 and R39 in Neutone's design).

[Haplo]

And again, the amount of protection you'll need depends heavily on your application. If you want very strong protection, I suggest you to follow Neutone's Design. His circuit is protected against just about anything...over-voltage, spikes, noise, lightning strikes, famine, pestilence,...

[Neutone]