RS232 - RS 485 converter

application note 75176

i'm trying to communicate pic16f877 from the pc via an adam232/485 converter and 75176. but there seems to be a problem somewhere. i've connected pin1 of 75176 to rx of pic, pin4 to tx, shorted pins 2&3 of 75176 and connected it to rc4 for dataflow controlling purpose. is any termination resistor is necessary at A&B? also can u tell me whether the pins of 75176 are open collectors?

Re: isolated rs485 schematic

ljmnunes said:

Hi, Proton!

I have a schematic of a RS232-485 converter.
I always used without troubles.

It is in PCad 2002 format.

Good luck

Laercio

Click to expand...

can u give the jpeg format or pic sort of thing

Re: isolated rs485 schematic

We have designed and assembled USB to RS485 Isolated Adapter. The adapter schematic is attached. We are using FTDI FT232. It is used for motor induction control from RS485 bus of motor inverter. The test configuration is also attached.

We have tested the adapter and it works fine in regular environment with desktop PC and also with laptop. It is also work fine when it is connected to the inverter with the motor is not running and the laptop charger is not plugged to main power supply. However when it is connected to the inverter we found problem with the adapter. The problem is occurred the motor which is connected to the inverter is running. The problem is the USB and FT232 is erronous and disconnected. The RX and TX LED are blinking without pattern. We need to plug and unplug the USB connector to get the FT232 connected again.

For the desktop PC, which is connected to the main power supply trough power supply cable, we also found the same problem.

Anyone, please help us solving the problem. Thanks

Re: isolated rs485 schematic

I find this problem in FT232R ,please contact FDTI for the problem,I think with better driver for FTDI chip the problem can be removed.

Sometimes we need to re-plug the USB connection!!!!The pcb dos not recognize FT232R

Re: isolated rs485 schematic

Hi, Lercio!
Interesting schematics of rs232-485 converter.
Disadvantage is that modification is needed when transfer speed is need to be change. That can be help with set of jumpers of course.
Does anybody know how works those commercial converters like Adam's etc. with automatic data flow control?
-grid

Re: isolated rs485 schematic

Thank you for your feedback 7rots51.

We have found the problem, it is about a defect in PCB manufacturing which causes the USB ground (0 V) connected to isolated ground. Removing the shorted grounds, we could remove the problem.

The next problem is how about non-isolated FT232R devices? How to remove the non-isolated USB problem? Optically isolated should help, off-course, but is there other simpler solution? Is connecting the USB shield to the PCB ground, as suggested by FTDI support, will help? Is it good to connect the USB shield to PCB ground directly?

7rots51 said:

I find this problem in FT232R ,please contact FDTI for the problem,I think with better driver for FTDI chip the problem can be removed.

Sometimes we need to re-plug the USB connection!!!!The pcb dos not recognize FT232R

Click to expand...