Micros - Relays Logic ?

[Wp100]

Hi,

Just interested to know why many of the mechanical and SSR relay boards now coming out of the Far East feature Active Low Logic as against the more usual Active High Logic.

Thought there must be some commerical reason for this change around , though as many micros ports seem to power on default to zero, a bit puzzled why ?

 

[Pommie]

Aren't most µCs able to sink more current than source?

Mike.

 

[schmitt trigger]

What Pommie said...
.......and it is also easier to drive from a different voltage level (i.e. 12 volts) with a simple NPN open collector. There are several IC drivers which feature arrays of NPN open collector outputs, allowing significant integration.

But you are correct.... With the default to zero of microcontrollers, one of the first instructions to be executed would be to set the ports high.

 

[Pommie]

Don't most µCs initialize with all I/O as inputs? Hence the need for pull up/down resistors to prevent inadvertent activation of peripherals.

Mike.

 

[ronsimpson]

With the default to zero

Many IO pins on micros come up set to input or to tri-state. (no current flow in any direction)
I usually set the pin to 1 or 0 which ever I want then set the pin to output.

 

[Wp100]

Hi,

Many of those relay modules come with transistor or opto coupler inputs so current drain from the micro is minimal.

Just that after so many years of driving relays/SSR High, these board have changed to Low, surely for a reason ?