Hi

I am using an alternate to Siemens 6ES5 734-1BD20 cable. Here is a link:
http://plcmasters.com/Product-Siemens-S5-6ES5-734--1BD20-Clone-Converter-Cable_7.aspx.

Its schematics can be found at:
http://www.scribd.com/doc/19282424/...-TTY-Interface-to-Communicate-With-STEP-5-PLC.

I built another one using these schematics and it works as well (using prolific USB-COM converter).

I am connected to S5 115U-944. So this cable works when my notebook runs on battery. It does not work when I plug in the power to my notebook. In fact it burns out the MAX232 chip in a minute. I change the chip and then it works fine on battery.

A P4 desktop with a real COM1 does not connect as well.

I would appreciate any help in this regard.

 

That would probably be why the Siemens cable provides isolation. When running on batteries your laptop is isolated, the other two scenarios would have the chassis grounded. They must add $200 isolators:^)

Regards
cww

 

I had built the isolated circuit as well (available on runmode.com), it did not work. PLC continually gave a DLE NACK signal.

Can you guide me on how can I isolate the current circuit? I tried to connect my notebook power with the same EARTH line as PLC, it sometimes works, sometimes PLC times out.