Also, in the pic there is a register that has a receive overflow bit.
You should have a routine to check this bit, if it is set, do to an
overflow(I think it's set) you'll need to read your rx register twice to
empty it and the clear the overflow bit. Your termination resistor
should be aprox 110 ohm at both ends of the cable. Also I think it would
be wise to put in 2 resistors for critical biasing (a aprox 600 ohm
between pin 5 and 7 and another between 6 and 8 of the max chip), alot
of people have said to me that they have had it working fine without
them but I am yet to be able to get my setup to work faultlessly without
them.
Regards
Stuart O'Reilly

Dr. Imre Bartfai wrote:
>
> Hi,
> I know only SN75176 but I guess it is virtually identical. Here is what I
> suspect:
>
> do not tie RE to gnd. As RS-485 is a half-duplex device, this idea is
> definitely bad. At that chip I mentioned one of RE and DE is inverted, so
> it is a good practice connect them together and so control them. Such way,
> the device will either receive, or transmit.
>
> I hope this helps.
>
> Imre
>
> On Tue, 14 Dec 1999, Ali GOKSU wrote:
>
> > Hi All,
> >
> > I have designed an RS-485 capable device with PIC16F877's and MAX485CPA's. PIC16F877 has a built-in USART. I used CCS C PCM compiler for programming. My intent was to built an RS-485 network with these devices and interface them to a PC. Device is working well except RS-485 side.
> >
> > The connections between PIC and MAX485 is as follows :
> >
> > 1.RX pin on PIC connects to RO pin on MAX485.
> > 2.TX pin on PIC connects to DI pin on MAX485.
> > 3.RE pin on MAX485 connects to GND.
> > 4.DE pin is driven by an output from PIC.
> > 5.A termination resistor of 220 Ohm  is present between A and B pins of MAX485.
> >
> > For the transmit of data, I am using built-in RS232 function of CCS C Compiler like, printf, putchar, etc.
> >
> > I don't know what is wrong here. Any ideas are wellcome.
> >
> > King Regards,
> >
> > Ali GOKSU
> > Software Engineer
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> >