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Dear Readers,
I want to connect a Linux PC with a Siemens S7 200 CPU with RS485 instead of RS232+PC/PPI Adaptor.
The first devise is an embedded PC with Linux (TRM/816 from www.ssv.de). This PC has a RS232 and a RS422/485 interface (only one can be active - which can be selected by means of two jumpers). The RS422/485 has only 2 pins: TX+/RX+ and TX-/RX-.
The second device is a Siemens S7-200 CPU 222.
This works already:
I connected the RS232 of the PC with the RS485 of the PLC by using the RS232/RS485 PC/PPI adaptor from Siemens. Now, I can read and write strings to and from the PLC. On the PC I use posix functions and on the PLC I programmed something that I can answer to the commands. This works fine with 9600 Baud (8,n,1, no handshake). Using 38400 Baud, there are sometimes errors.
This does not work:
I replaced the PC/PPI adapter and connected the RS485 interfaces directly. I thought, this would work without changes. But it didn't.
One time I connected the TX/RX+ to Signal B of the PLC(+) and TX/RX- to Signal A. The other time I changed it (TX/RX+ to A, TX/RX- to B).
My questions:
Why does it not work? Do I have to use other communication protocolls and/or settings?
I hope, somebody can help me.
Many thanks in advance
Ernst Murnleitner
I want to connect a Linux PC with a Siemens S7 200 CPU with RS485 instead of RS232+PC/PPI Adaptor.
The first devise is an embedded PC with Linux (TRM/816 from www.ssv.de). This PC has a RS232 and a RS422/485 interface (only one can be active - which can be selected by means of two jumpers). The RS422/485 has only 2 pins: TX+/RX+ and TX-/RX-.
The second device is a Siemens S7-200 CPU 222.
This works already:
I connected the RS232 of the PC with the RS485 of the PLC by using the RS232/RS485 PC/PPI adaptor from Siemens. Now, I can read and write strings to and from the PLC. On the PC I use posix functions and on the PLC I programmed something that I can answer to the commands. This works fine with 9600 Baud (8,n,1, no handshake). Using 38400 Baud, there are sometimes errors.
This does not work:
I replaced the PC/PPI adapter and connected the RS485 interfaces directly. I thought, this would work without changes. But it didn't.
One time I connected the TX/RX+ to Signal B of the PLC(+) and TX/RX- to Signal A. The other time I changed it (TX/RX+ to A, TX/RX- to B).
My questions:
Why does it not work? Do I have to use other communication protocolls and/or settings?
I hope, somebody can help me.
Many thanks in advance
Ernst Murnleitner