Comparing REAL Values on a ControlLogix: Why EQU Is the Wrong Instruction Page 882 of 1756-RM018A carries a worked example nobody quotes: subtract 10 from 10.1 on a Logix controller and the result could very well be 0.10000038, rather than the 0.1 you were expecting. Two pages of guidance later the same manual puts it … Read more
Automatic tag generation against one 1756-L83E produced 5537 server tags in about ninety seconds, and the clients on that site reference 684 of them. The other 4853 are array elements nobody reads, structure members nobody reads, and a full PID_ENHANCED nobody has looked at since commissioning. They cost project load time, they cost browse time … Read more
One-Shots on a ControlLogix: ONS, OSR and the Rung That Fires Twice A batch counter on a reel changer reads 412 when the shift log says 11 reels ran. The routine sits in a 100 ms periodic task, so 401 spurious counts is forty seconds of something firing ten times a second — and the … Read more
RMPS and RLIM on a ControlLogix: Ramping a Setpoint Instead of Stepping It A sequence step writes 75.0 into the speed reference of a PowerFlex on a packing line that was sitting at 20.0, and the drive comes back with an overcurrent on acceleration about 400 ms later. Nothing in the controller did anything wrong. … Read more
A SCADA client stores one string for every value it reads, and on the line I am about to describe that string is CLX_Line1.Bagger.Infeed.Belt1_Run. Four names, each chosen by a different person on a different day, and 684 more strings on that one device built the same way. Rename the tag group on a Friday … Read more
TON, TOF and RTO on a ControlLogix: What Each One Does When the Rung Goes False The prescan row for a TOF in 1756-RM018A sets .ACC equal to .PRE. Not to zero, the way a TON’s does — to the preset, which is the value that means the timer has already finished, so the first … Read more
SQO and SQI on a ControlLogix: the Sequencer That Will Not Step SeqCtl.POS sat at 1 for three days on a 1756-L83E while the indexer kept running on step one, and the rung holding the SQO had nothing in it at all — left rail, instruction, right rail. That is the entire fault, and the … Read more
JSR, SBR and RET: Passing Parameters Into a Routine Without a Controller Tag Type 4, Code 31 on a 1756-L83E, and the routine it pointed at was three rungs long. The general instructions reference gives three separate ways to earn that code and every one of them is about counting: a JSR with fewer input … Read more
FAL and FSC Over an Array: the Control Tag, the Position and the Scan It Takes An FSC with a Length of 10 found its mismatch at element 3 on a 1756-L83E and then stopped looking. .FD was 1, .POS was 3, .IN was 1, and elements 4 through 9 were never compared — not … Read more
The MSG Control Tag on a Logix Controller: What Each Tab Writes Into It Line_Msg.DN came true on rung 13 of a 1756-L71 and the forty DINTs the rung was guarding still held last shift’s numbers. The bit had been set by a different message — a four-element write to a capper down on rung … Read more
COP and CPS Instructions on a ControlLogix: Which One Copies What A COP with a Length of 20 moved five bytes. The source was Packer_Raw, a SINT[5]; the destination was Packer_Scaled, a DINT[10]; the rung went true on every scan of a 1756-L71 and nothing faulted, nothing latched, nothing appeared in the Minor Faults tab. … Read more
The scope says twelve bays, a MiCOM C264 in each cubicle, a gateway to the control centre, and an SCD file that the protection engineer will issue in week six. There is no ladder anywhere in it, no tag database you recognise, and no download button of the kind you have been pressing for fifteen … Read more