Bus 485: Communication problem

[leolib2004]

Hi! I am making a 485 bus to connect a rabbit (RCM 3710) with 4 pics (16f628A). I use 7lb176 transceiver and no resistors for the line because it is a short line (less than 30 cm).

The problem is that i send 5 bytes from Rabbit to PIC and the first bit of the first byte sometimes changes (1 to 0 or 0 to 1).

Communication from PIC to Rabbit is ok. All 5 bytes are received ok from the rabbit.

What do you suggest?
I think it is strange because it not always happens and it is just one bit...

PD: I use PKT libraries for the Rabbit and UART library from "MICRO C" for the PIC.

 

[siongboon]

Maybe the coding for your RCM 3710 device did not enable the 485 bus correctly.
You can try to use oscilloscope to look at the signal.
It will help you further to narrow now the root of the problem.

Check out this website,
**broken link removed**

It illustrate how a RS485 signal should look like.
From there you will have a better understand of where your problem might be.

Hi! I am making a 485 bus to connect a rabbit (RCM 3710) with 4 pics (16f628A). I use 7lb176 transceiver and no resistors for the line because it is a short line (less than 30 cm).

The problem is that i send 5 bytes from Rabbit to PIC and the first bit of the first byte sometimes changes (1 to 0 or 0 to 1).

Communication from PIC to Rabbit is ok. All 5 bytes are received ok from the rabbit.

What do you suggest?
I think it is strange because it not always happens and it is just one bit...

PD: I use PKT libraries for the Rabbit and UART library from "MICRO C" for the PIC.

 

[leolib2004]

Thanks! I will check out that site! The strange thing is that just one bit changes... Maybe I need some resistences or capacitors somewhere...

I have put a capacitor between ground and the enable of each transceiver because communication was not ok...

It bothers me because i have to put a delay (5 ms) between each byte sent from the PIC... I think that is too much...

Maybe the coding for your RCM 3710 device did not enable the 485 bus correctly.
You can try to use oscilloscope to look at the signal.
It will help you further to narrow now the root of the problem.

Check out this website,
**broken link removed**

It illustrate how a RS485 signal should look like.
From there you will have a better understand of where your problem might be.

 

[siongboon]

I think probably that might be the reason.
There must be other reason why communication is not ok, such that you want to put the capacitor.
Maybe it is not the correction solution.

Thanks! I will check out that site! The strange thing is that just one bit changes... Maybe I need some resistences or capacitors somewhere...

I have put a capacitor between ground and the enable of each transceiver because communication was not ok...

It bothers me because i have to put a delay (5 ms) between each byte sent from the PIC... I think that is too much...

 

[leolib2004]

Thanks! I will read more about 485! I will read the site that you reccomend me and use my oscilloscope again to find the problem. I will tell you what happens.

Maybe i need some resistences or capacitors somewhere...