I integrated a Siemens S7-200 system not long ago and had a very interesting experience with an S7-200 RTD Input module (Siemens P/N 6ES7 231-7PC22-0XA0). I had an S7-214 CPU with one (1) digital input card, one (1) digital output card, and the RTD input module. I used a separate 24VDC, 5A power supply for my DC power. The problem was that whenever I energized the assembly, the Siemens RTD input module would startup showing fault. I thought the problem would be a software configuration error or perhaps an incorrect dip-switch setting on the module itself, but after careful examination, all of the software settings and dip-switch settings checked out.

After several hours of banging my head against this problem, I finally broke down and called my supplier for technical support. I was hoping for an easy resolution, as I was sure that tech. support could guide me to an easy resolution very quickly; however, I soon found that the problem that I was having with my RTD input module would prove very difficult to resolve. I was gratified to learn that my problem with the RTD module wasn't so simple to fix because I didn't want to feel like an idiot for calling tech. support.

After discussing the problem at length with tech. support and trying a host of different problem solving recommendations, the call from Siemens was that the module was bad. Siemens UPS'd me a new module overnight, free of charge. Problem solved....or so I thought. I pulled the new RTD module out of the box and inserted it into the S7-200 rack and powered up my unit. I was hoping to see the "green" light on the module light up, indicating all good; however, the now familiar "red" blinking light appeared on the bottom of the module and I knew I had some real trouble here.

I called Siemens tech. support back and informed them that the new module was not working. The Siemens tech. support crew that I was talking with arranged a conference call with several product specialists. So now I had about five (5) Siemens engineers round-tabled together to try and resolve this problem. I forwarded schematics of my controls to Siemens for them to examine, kind of hoping that they didn't find anything wrong....while also hoping that they would find a problem with my design so that we could resolve this problem and move on. The Siemens engineers didn't find a problem with my design and now I was getting real concerned here about hitting my looming delivery deadline with these controls.

During one of the many hours of working with this problem and trying to resolve the issue, I did observed that the 24VDC, 5A power supply that I was using took about 1-second to power up after power was initially applied. Not really anticipating a good outcome, I decided to power the RTD module off of the S7-214 base unit power supply. I wired it up and crossed my fingers as I turned the controls on. I was delighted to observe the "green" configuration light, which indicated all good. The problem seemed to be the slight delay in applying power to the RTD module from the stand-alone 24VDC, 5A power supply. Although the CPU and the discrete modules were not affected by this slight delay, the RTD module apparently needed to power up at the exact same instant as the S7-214 CPU. I relayed this information to the Siemens engineers and we all agreed that this was a definite learning experience.

After dealing with that issue, I now - as a standard practice with Siemens S7 systems, use the CPU rack 24VDC power supply to power all analog, temperature, and specialty modules. I really have to give credit to the Siemens tech. support professionals who really stepped up and tried to get this issue resolved for me. Awesome tech. support!