Forcing - PLC Programming & Automation

One Photo-Eye, Two Signals: Light-Operate, Dark-Operate and the Setting That Inverted Your Zone Logic

One retroreflective 42EF with its reflector, beam made on an empty zone and beam broken by a carton, and below it the output and LED state of a light-operate J model and a dark-operate K model in each case

Why a replacement photo-eye inverted your zone logic: light operate dark operate on a 42EF, which one to fit, and the rung that normalises it.

HVAC Point Names, BACnet Objects and Overrides That Expire

by PLC

Ladder rung for stage 2 cooling: cooling demand in series with stage 1 running, the interstage delay timer done bit, and a normally closed contact on the minimum off timer, driving the stage 2 command

PLC building automation that a stranger can maintain: naming, sequences of operation in the code, staging, BACnet mapping, trends, alarms and overrides.

Remote PLC Monitoring and Control Without Open Ports

by PLC

Ladder rung for a remote write permissive: XIC on Remote_Session in series with XIO on the Local_Only key switch and XIO on WDog_Fault, driving the Remote_Write_OK coil

Remote access to a PLC that security will sign off: outbound VPN routers, read-only web HMI, an access rights table, a permissive rung and an audit trail.

PLC Troubleshooting: A Method That Finds Real Faults

by PLC

Ladder rung that latches the first I/O connection fault so it survives the recovery, with a reset pushbutton

A repeatable PLC troubleshooting order: module LEDs, controller major faults, I/O connection status, forces, cross-reference, trends and Ethernet checks.

Studio 5000 Watch Window: Monitor Tags While Online

by PLC

Timing chart: a 40 ms bit pulse in the controller against a 500 ms Watch window read, showing that three of four pulses are never sampled

Build watch lists in the Studio 5000 Watch window, monitor and write tag values online, and know when the sample rate hides what you are chasing.

ControlLogix Ladder Troubleshooting, Rung by Rung

by PLC

Ladder rung with a bug: XIC Conv3 Start in series with XIO Conv3 EStop driving OTE Conv3 Run, with no seal-in branch around the start contact

A rung by rung ControlLogix ladder troubleshooting method: read the highlighting, use cross reference and search, find double coils, read the fault record.