I started a new job and was assigned to help a technician bring up a new system. There were a lot of problems with this system and major parts of it did not work. The system was very complex. There were multiple PC boards that needed to communicate with each other. There were also multiple system voltage buses, both analog and digital.
During the troubleshooting process I asked the technician to connect an oscilloscope to a particular signal that looked to me like it would supply us with important information. He connected up the scope and I saw a large sine wave on the screen.
I asked him, "Are you sure it's connected?"
He said, "I'm sure it's connected."
I said, "It looks like the ground lead of the scope isn't connected." He checked the ground lead and assured me it was indeed connected to ground. I suspected a break in the ground lead, so we checked continuity and it was intact.
I studied the schematic and noticed that the design engineer had used different ground symbols for the various grounds. For his own convenience in keeping track of the grounds, he had them labeled AGND, 5VRTN, 12VRTN, etc. All of these grounds were supposed to be connected, but the PC board layout software considered them to be separate signals. I asked the technician to do a continuity check between the various grounds and found that they were not connected. After the technician ran some jumpers between the grounds the system started working much better.
This entry was submitted by Cliff Harris and edited by Rob Spiegel.
Cliff Harris received formal electronics training in the US Army and later went to California State College at Fullerton to get his BSEE degree. He is now semi-retired and still involved in electronic projects at home.
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