Hello,

I have a third party RS485 device (possibly using MAX485 transceiver), I want to interface with S7-200 CN PLC to control it in Freeport mode.

Setup is:

PLC port 0 pin 3 connect to RX/TX+ of the device

PLC port 0 pin 8 connect to RX/TX- of the device

pin 3 and pin 8 are connected with a termination resistor 120 ohm.

But the setup doesn't work.

I checked the device can communicate with a PC using a RS485/232 converter, and the PLC can communicate with the PC using a USB/PPI cable.

So is my setup is correct? What is the use of pin 4, pin9? Shall I need to use special cable?

Thanks!

Best Regards,

Danny

Hello,

if it's close connection, try unplug termination resistor. What is that 3-rd party device? What connection pins are available on that device completely?

Hello,

The device will transmit out some status periodically in RS485, baud rate 9600, 8N1, there are 2 terminals for wiring (+), (-) for connection. I can see it is using MAX485 IC with 120 ohm terminating resistor. Then I connect to the PLC Port0 with a twisted pair cable with 120 ohm terminating resistor also at the PLC side, the remaining pins on Port0 are unconnected.

I write a simple test program in the PLC including changing to Freeport mode, setting baud rate, turn on Q0.0 when receive 6 bytes of data (end character is 0x0A) and send back some data. The PLC program is attached.

When tested with the RS485 device, I can see the RS485 data sent to PLC correctly using a CRO, but the PLC has no response.

So I want to know my hardware setup is OK? Or there is something more to configure in the Freeport mode?

Thanks!

Best Regards,

Danny

Hello,

SMB Freeport def looks ok to me. What did you see in your RECV area table? Look to Online table in VB199-VB205, there you can see received values. I don't understand one thing, that device cyclically transmit data by it's own system or you need ask that device with specific message? This looks to me strange in your code, why you send back what you have received, simply I don't expect that correct. Look also to help how correctly set RECV table. You can also better not set recv end character, try leave settings only via some timeout and then chcek recv message consistency. Also you can try set your output to TRUE not only set via true condition, only if it turn to TRUE possibly.

Hello,

Thanks for your information.

This is test software only, I just echo back some dummy data.

I will follow your ideas to check the software first, I am new to the PLC software. 

So the hardware setup seems correct?? so that I can focus to debug the software.

Thanks!

Best Regards,

Danny Chan

Hello,

Also, what is the functions of pin 4 (RTS) and pin 9 (11 bit input)? We can just ignore them?

Most important is the make sure the hardware is correct first.

Thanks!

Best Regards,

Danny

Hello,

you need use only pins 3,8 for user serial communication.

Hello,

I managed to get it work, because of some software mistakes.

Thanks for your help!

Best Regards,

Danny

Hello,

you're welcome...