INTRODUCTION

Application and function

The MTL454x and MTL554x are isolator modules which enable an analogue 4/20mA control signal to be

passed to a device located in a hazardous area from a safe area. The output current available to the hazardous

area is limited to comply with the requirements of the process explosion hazard. The modules are also

designed and assessed according to IEC 61508 for use in safety instrumented systems up to SIL2 when

the required function is to repeat the loop current within ±2%, and the safe state of the output is <3.6mA

(downscale). Higher integrity levels for a SIF can be achieved by using the modules in a voting architecture.

For ‘smart’ valve positioners using the HART protocol the units allow bi-directional communications

superimposed on the 4/20mA signal current.

There are no configuration switches or operator controls to be set on the modules – they perform a fixed

function related to the model selected. The MTLx546 models are single channel while the MTLx549 models

are dual channel, although both channels must not be used in the same safety instrumented function.

These modules are members of the MTL4500 and MTL5500 range of products.