TTL serin on 28X1

[dj_trippn]

Hi everyone,

I am interfacing a EM-406A gps unit from Sparkfun with a 28X1
http://www.sparkfun.com/commerce/product_info.php?products_id=465

The gps unit outputs TTL 2400bps serial. I have searched the forum and discovered the 28X1 port c are all schmit trigger inputs. My question is can I use port A for serial in commands?

I have the serial in working on a 08M that has a TTL input, but I need it to work on a 28X1. What exactly is the difference between TTL and schmit inputs? The voltage level that classes a HIGH? Is it possible to add a hardware mod to allow the TTL signal on the schmit input?

If its not possible on a 28X1 will the 28X2 do it?

Thanks in advance.

 

[Andrew Cowan]

I think you can use an op-amp two switch between TTL and schmitt trigger. The difference is the voltage that it switches on at. It may work anyway without any adaptor - you'll have to try.

A

 

[dj_trippn]

Thanks for your response Andrew. I have tried it on the Port C inputs but it doesnt work. Ok so there is the op-amp mod.

Would adding a capacitor + resistor help by keeping the voltage up higher near the schmit voltage high level?

 

[hippy]

The datasheet indicates its Protocol, Output Level as -

TTL level, Output Voltage : 0V to 2.85V
RS232 level

There may be a selectable option to have it produce RS232 which should work with a 28X1 with the output via a 20K resistor in series with the output. That would be the easy option if it exists ( datasheet not displaying fully in my Acrobat Reader ).

You can use an op-amp to boost te voltage but it is more common to run the signal through a logic gate (74HCxx etc ) or transistor.

A pull-up R is only advisable where the output driver circuit is known. Used inappropriately it may damage the chip / module.

You may be able to connect the GPS module to the high-speed serial RX pin of the 28X1 and use HSERIN and operating the PICAXE at a lower voltage may help improve things.