Is the TTL level converter cable really needed?

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tomkinsc 12 Nov 2009 23:05

So I recently finished building the MakerBot, and the only thing left is to actually connect it to a computer. I understand that it interfaces over serial, but everyone online seems to insist that a logic level converter is necessary, which I find strange. I work quite a bit with Atmega 128L MCUs and just connect them straight to the computer. The MakerBot uses an Atmega 644P, which lists a Vcc of 1.8-5.5V. I don't really see why we would need a TTL level converter for it. Aren't most motherboards using 5V for the RS232 port nowadays? Why couldn't I just make a connection adapter, to go from the DB9 port to the MakerBot board?

Thanks,
Chris

Last edited on 12 Nov 2009 23:06 by tomkinsc + Show more

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tomkinsc, 12 Nov 2009 23:05

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Re: Is the TTL level converter cable really needed?

clothbot 13 Nov 2009 03:51

It's a USB-to-TTL interface that's provided with the (deluxe) kit, not an RS232-to-TTL. In addition to providing power, the (TTL) logic levels are bit0=0 and bit1=5V.

RS232 is bit0=-5V and bit1=+5V relative to the ground pin. You're still going to need to use something like a MAX232 to interface safely and reliably with the 644P.

Every time you drive the TTL logic below ground you're stressing the ESD protection diodes and related I/O circuitry. It's only a matter of time before those give out if you're driving them directly with your RS232 port.

See "Chapter 11. I/O-Ports" and "Chapter 25. Electrical Characteristics" of the ATmega164/324/644 datasheet for the details.

References:

Atmel: ATmega644P
Wikipedia: RS232
Maxim-IC: MAX232 Overview

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clothbot, 13 Nov 2009 03:51

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Re: Is the TTL level converter cable really needed?

Le_Garage 08 Mar 2010 18:45

Hi,
I'm trying to use my MakerBot the Sparkfun USB TTL convertor that one used for LilyPad and some other board.
I gess I'ts the same problem that tomkinsc ?
I'm still in the first test, I sucess some operation (checking and changing temperature, mean that I'm connected!) but I have a lot of bug and crash.
I would know if that come of my computer and some missing drivers [I'm as good in Linux and setup on than in English… And my english don't help me !] or if it's a problem with the convertor ?
I have to purchase a classic cable ?

I'm sorry for my poor english, I hope you understand what I would write.
Thanks.
Leo.

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Le_Garage, 08 Mar 2010 18:45

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