LED on 08M serial out line


New Member
I've been peeking at Stan's website and noticed some breadboarded circuits that have an LED on the serial out programming line (pin 7 on an 08M)--no jumpers, just both connected at the same time.


Is this known to work? or do you need to disconnect one or the other.

I should be more specific.

1) can you program without disconnecting the LED?

2)can you use SERTXD without disconnecting the LED?

3) can you light the LED without disconnecting the serial line?

Other than stray characters , are there any consequences?
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Senior Member
Larry- light duty loads such as an LED or piezo can indeed be left in circuit when programming. Rev. Ed themselves pointed this out to me ~2002 & I've rarely since found a need to use a pin0 jumper. In fact the recent bare bones stepper => www.picaxe.orcon.net.nz/stepdemo.jpg called for a switched pin 0, but I got around that by just pulling out the stepper header pin when need be. Stan


Senior Member
1) can you program without disconnecting the LED?
2) can you use SERTXD without disconnecting the LED?

Yes to both, and it's very handy because it makes it possible to see the PICAXE responding to the download and when it is sending back data when executing. If SERTXD is sending data in a loop during debugging it's frequently handy to see the LED flash without having to open the Terminal window to see what it is sending.

3) can you light the LED without disconnecting the serial line?[/i]

Yes. The only issue is that a short pulse of the LED may be seen as a corrupt character if SERTXD is also being used to send serial data. Providing the LED is turned on for 3mS or more ( it usually will be in order to see it ) most terminal programs will ignore that, allowing SERTXD to be used as a LED and for serial data 'at the same time'. The LED will flash while SERTXD sends data, but it's often possible to live with that.

I always add LED's to Serial In and Serial Out and to show the power is on. Noting the Serial In LED doesn't come on when download fails is usually an instant reminder that the wrong COM port has been selected, the cable is connected to something else, or power hasn't been turned on. That can save a lot of head scratching.


New Member
Thanks to both you guys. The corrupt character thing doesn't bother me, and I see the diagnostic aspects as a plus. Soldering up a board now---it's going in.

Thanks, Larry