RS485 communication problem with GHI module

Hello everybody, I have an issue with the rs485 module from ghi (RS485-GM-479).

I bought it hoping that it would be the simple solution to all of my RS232-RS485 problems. Turned out to be a headache.

On my current test setup I am using an USB-RS485 FTDI chipset based adapter connected to the GHI module and I have it connected back to my PC’s serial port.

When I open two terminals with putty, I can only communicate one way RS232->RS485 but not the other way around. Observing the voltages on the RS485 bus with an oscilloscope, it looks like the GHI module can drive the bus, but the USB-RS485 cannot. While the GHI module can change the voltage from ~0.6V to 4.8V in both A anb B pins, the USB-RS485 only changes the voltage in the A pin from 0.6V to 1.96V and the voltage in the B pin from 4.8V to 3.2V. Because of this, there is no change on the logic state of the RS485 bus (A-B is still < 200mV).

The RS485-USB adapter works fine if I plug it directly to any RS485 device. I have 3 GHI RS485 modules and they all behave the same way.

I wonder if I am missing something here, because I can’t make it work! Does it have anything to do with the pull-up/pull-down resistors on the GHI module?

Thank You

@ insanis - Look at the schematic for the module. Terminating resistors are mentioned.

I will try it, but I didn’t put them because I thought it wouldn’t be necessary (

Putting the terminator resistor made no difference at all. I tried replacing the pull-up pull-down resistors with 1K resistors and it still didn’t work.

Running out of ideas here =/ plz help

Maybe check the rs485 chip datasheet for some hints?

The autocontrol feature of the max13487 logically works fine. It just happens that any RS485 device I plug on the board cannot switch to the right voltage levels on the RS485 bus. It switches, but not enough. I already looked at the datasheet and couldn’t find anything helpful. Maybe I missed or misunderstood something?

I can’t post the link or the datasheet here, but if you google ‘max13487’ you should be able to find it pretty easily.

Turns out that the gate IC (74lvc1g57) was not working out properly. I had some problems with the cable I assembled for the test setup, but after I repaired it the MAX13487 reciever was working fine. Most likey what happened is that I got confused by the gadgeteer socket pinout diagram on the website (which is confusing in fact) and burned up the gate IC by reversing the power supply polarity. I have already ordered some samples from the ti website to replace it! Luckily the MAX13487 is still working fine.

@ insanis - You released the magic blue smoke! Poor magic blue smoke. It went before its time. At least you found the problem. What was the confusion about the socket that lead to this error?

@ Mr. John Smith - The pins of the socket U (gadgeteer) on the webpage are not correct, or at minimal they diagram is confusing (is that the jack or plug diagram? is that chanfer correct?). I had to use a multimeter and test the continuity on the IC’s to get the pins right.