User manual

Preface

LT Series User Manual

(2) When the LT generates a “Watchdog Timer Error,” LT

operation will halt. Also, when Errors occur in Input/

Output control areas that the LT cannot detect, unex-

pected movement may occur in those areas. Therefore,

and to prevent unsafe machine movement, a “Failsafe

Circuit” should be created which is completely external

to the LT.

Section 3.2.5 – “Installation Cautions”

(3) If a problem arises with an external unit’s relay or tran-

sistor that causes an output (coil) to remain either ON

or OFF, a major accident can occur. To prevent this, be

sure to set up external watchdog circuits that will moni-

tor vital output signals.

• Design a circuit that will supply power to the LT unit’s

I/O unit before starting up the LT. If the LT unit’s internal

program enters RUN mode prior to the I/O unit’s load con-

trol power turning ON, an incorrect output (signal) or mal-

function could cause an accident.

• Design a user program that ensures the safety of the user’s

system, in the event of an LT display or control error, or

either a data transmission error or power failure between

the LT and a connected unit. These types of problems can

lead to an incorrect output (signal) or malfunction, result-

ing in an accident.

• Do NOT use the LT as a warning device for critical alarms

that can cause serious operator injury, machine damage,

or production stoppage. Use stand-alone hardware and/or

mechanical interlocks to design alarm indicators and their

control/activator units.

• Do NOT use LT touch panel switches to perform operator

safety-related or important accident-prevention operations.

These operations should be performed by separate hard-

ware switches to prevent operator injury and machine dam-

age.

DANGERS