Introducing the ZX24e

[zbasicandy]

There is still a problem ... with the switch in the "TTL position" the Rx input is not pulled up and garbage characters will show up in the COM1 input queue. This caused a lot of problems for me.

From another link ...

[mikep]

[mikep]

Thank you to everyone who gave me input on the ZX24e.

[JC]

I like the concept and design a lot.

One question...
Would it not be better to have the + end of the Red/Green LEDs tied directly to +5, and a separate current limiting resistor in series with each LEDs minus lead to the cpu's ports?

I suspect that the forward voltage drop across the red and green diodes is different, sometimes significantly so.
If each LED has its own resistor this does not matter, they each can have whatever current through them you desire, which need not be the same, and they can have their individual forward voltage drops.

As currently designed, if the red led has a 0.7 V forward drop, and the green LED has a 1.4 V forward drop, then the red wins, and the green will be dim, if both are on simultaneously. I suspect.

My eye sight is too poor, and my soldering iron tip to large to play with SMDs, but it may be worth bread boarding just the LED(s) to test this. A data sheet for the part will also list both forward voltage drops.
Nit picking, and perhaps not an issue, but I thought I'd mention it before you lock down the design.

I like the overall concept and design, and hope to play with one soon!
JC

[JC]

Never mind the above LED posting.

The data sheet notes that the Vf for BOTH the Red and the Green LEDs is the same, 2.1 v, (Max 2.8 v). I have some non-SMD green LEDs that are actually two diode junctions stacked, hence the different Vf.

JC

[JC]

My last LED comment...

As designed, with one current limiting resistor, the current through the resistor is the SAME whether only one, or both LEDs are on. If both are on then the current is split between them, and they each get 1/2 of the current they get if they are on alone.

Hence, each LED is much brighter if it is on alone, and much dimmer if it is on simultaneously with the other LED.

Turning on both LEDs does not double the current drawn.

I = V/R, = (5-2.1) / 220 = 13.2 mA, current through the resistor, with either one or both LEDs on, as the voltage at each end is fixed, regardless of how many LEDs are on.

If each LED has its own current limiting resistor, on its minus lead, then each draws 13.2 mA, and has uniform intensity, regardless of the status of the other LED.

Lastly, who cares. As a simple visual cue to the programmer, either design is fine.

This can be easily verified. Turn on either LED, (recall the negative logic... Setting the port to 0 turns ON the LED). Then FLASH the other LED at 1/2 Hz, (ON Delay (0.5) OFF Delay (0.5)...) The LED that is always on flickers bright/dim/bright/dim...

Sorry for beating this to death.
JC

[mikep]

[JC]

5/15/06 Hi Mike,
I wondered what the current status is on the ZX-24e? I'm looking for a few more I/O pins, and it would be much easier to drop your e version in a protoboard, rather than go order a separate XTal, 10uH coil, specific eeproms, etc. How is the project coming along?
JC

[mikep]

Thanks for asking JC.

I have an improved circuit and PCB layout based on input from this group. I'm in the process of sending that to manufacturing and then I can start ramp up for production. My best guess is 8 weeks before availability given that I also have a 2 week vacation and a day job