Antonio D. Posted March 3, 2011 Report Share Posted March 3, 2011 Hi everyone! I'm having problems with serial connection (RS232) to a M91-2-T1 in debug mode. The PLC's serial connection was working fine in a test environment but after the assembly of the device in a production environment the serial connection became unreliable. It drops the connection after 10-20 seconds (variable) of running in debug mode and I cannot reconnect until the process has stopped. The ladder program consists of a welcome screen where you choose three tests. While the PLC is in the welcome screen, the connection is fine and I can debug without problems. When I select any test and start it, everything starts ok but after around 15 seconds a "Resyncing" message appears and the connection times out generating a "cannot connect" message. Every subsequent reconnection try is rejected and the port appears to be blocked by the PLC. All the hardware is the same (serial cables, computer, power supply), the only thing changed is the fact that the PLC is now mounted in a cabinet instead of a testing table. Any ideas/suggestions? Thanks in advance. AMD Link to comment Share on other sites More sharing options...
MVP 2014 Simon Posted March 3, 2011 MVP 2014 Report Share Posted March 3, 2011 It sounds like electrical noise. What is the PLC controlling? Link to comment Share on other sites More sharing options...
Antonio D. Posted March 4, 2011 Author Report Share Posted March 4, 2011 It sounds like electrical noise. What is the PLC controlling? It is controlling 6 pneumatic valves (isolated by relays). The valves circuit is powered by 110V. I thought the same and tried getting closer to the PLC (shortening the cable, putting the cable as far as I could from the 110V circuit) with no luck. Do you recommend replacing the stock (RJ11) cable with a shielded one? Thanks, AMD. Link to comment Share on other sites More sharing options...
MVP 2014 Simon Posted March 4, 2011 MVP 2014 Report Share Posted March 4, 2011 Using a shielded cable may work, by keeping the noise out. Also try to reduce the amount of noise * make sure the solenoids have RC snubber networks on the coils, to reduce switching transients * use solid state relays rather than mechanical relays. The latter provide some magnetic coupling between the coil circuit and contact circuit, allowing the nasties to conduct back into the PLC. * check that all grounds and commons are connected in a star formation, with no loops to allow noise currents to circulate. Link to comment Share on other sites More sharing options...
Antonio D. Posted March 7, 2011 Author Report Share Posted March 7, 2011 Using a shielded cable may work, by keeping the noise out. Also try to reduce the amount of noise * make sure the solenoids have RC snubber networks on the coils, to reduce switching transients * use solid state relays rather than mechanical relays. The latter provide some magnetic coupling between the coil circuit and contact circuit, allowing the nasties to conduct back into the PLC. * check that all grounds and commons are connected in a star formation, with no loops to allow noise currents to circulate. Well, tried with a shielded cable with no luck. About the other recommendations to reduce noise: - According to the vendor, the solenoids are "noise isolated" so they do not need RC snubber networks. - Sadly, I cannot change the mechanical relays but they have circuit protection to avoid transient currents going back to the PLC. - All grounds are very well connected with no loops. I ended up going back to pen & paper and writing some test cases to debug my program... finally found the bug but I know that it would have been much faster with a working debug mode or a simulator (so sad the latter is not available for M91). Thanks for your help! Link to comment Share on other sites More sharing options...
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