RS232 LED status indicator circuit

[philipdj]

rs232 led

Hi,

I am using the XBIB-R-DEV board for a project.
https://ftp1.digi.com/support/images/XBIB-R-dev schematic.pdf .

I referred to the schematics because I'm doing a custom design for a similar board. I have a doubt regarding the LED status indicator circuit used there. Can anyone please explain as to how it works? There were some concerns raised by some peers regarding it's proper functionality or whether it actually works.

This is the circuitry. I'd really appreciate any help. Thanks.

 

[BABESH]

rs232 led indicator

i think when Dout or Din are low then you will get respective LED ON.

 

[philipdj]

xbib-r-dev

Thank you Babesh. That is exactly what is required of the circuit. But during a design review a few peers raised concerns about the complexity and the presence of capacitors a major power consumer. So I need to give a proper explanation. I'll try explaining what I understood from the circuit. Could you or someone correct me.

Let's take just the case for DOUT. DOUT is high during idle state. So in this case, Q5 is ON. This provides a discharging path for capacitor C26. So C26 doesnt get charged enough to turn on transistor Q3. But when DOUT turns high Q5 turns off and C26 charges through the path VCC-Q3-R26-C26. When the voltage across C26 becomes high enough to turn on Q3, the LED lights up. Again when Q5 turns off, C26 discharges. So basically, because it is hard to see transitions in an LED for serially transmitted (because it is too fast), this circuit slows the transitions(by the capacitor charging and discharging time) so the LED shows some kind of activity when data is being transmitted. And it's the capacitors that play the most important role. Is this right?

Please note that I dont need to see each and every bit. The purpose is just to see if my serial transmission is working. Any help will appreciated. Thanks.

 

[BABESH]

rs232 status led

Hi philip,
May be the analysis is different from what u described,
Q3&Q5 will be ON when emitter to base voltage >0.7V because it is PNP transistor.
in case of npn, base to emitter voltage should be >0.7V.

In this case during initial condition cap will be charged to almost Vcc, (in real vcc-0.7V)
this will make emitter to base voltage difference <0.7V hence it will make Q3 OFF,

The voltage at Q5 EMITTER is vcc and base is vcc hence the difference is zero so Q5 OFF,
when din is low emitter to base voltage is >0.7V hence Q5 ON, this will discharge capcitor and
also make voltage at emitter of Q3 almost zero by which the Veb>0.7V so Q3 ON making LED ON,

The purpose of capacitor may be for following reasons:
1)To maintain voltage at base equal to vcc by which Q3 will be off,
2)to delay the LED OFF time by 50mSec (1uFx10K=0.05)x5=50mSec.
3)(Wild Guess)Since when controllers make logic high the exact pin voltage may not be vcc instead will be less by 1v.
so when given directly to base of transistor then it may drive thansistor ON so in order to reduve volatge by 0.7V it might have emloyed.

--Babesh

 

[philipdj]

rs232 indicator

oops.. PNP! I got confused.. sorry.. that makes a lot of sense. thanks.