Electrical – rs232-ttl converter problem

arduinoconverterrs232ttl

I bought this RS232-TTL converter
https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/101990008/1597-1306-ND/5487765

and I'm using this along with an Arduino Mega 2560

I'm feeding the Arduino using this dc-dc converter directly to a 5[V] pin (i know this is not recommended and using a dc-dc is advised)
https://www.digikey.com/products/en?keywords=pyb10q24s5u

The DC-DC is feeded by a 12[V] – 12[A] DC power supply.

The same DC-DC is used to feed the rs232-ttl module

Testing configuration:

-2400b 8 bits 1 sb np.

-PC > USB cable > Arduino(usb powered) > RS232-TTL module(powered by the 
Arduino)>RS232-USB>PC(com8)

->sometimes it works:

Vcc = 5[V]
V+ = 8 [V]
V- = -8 [V]
C1 and C2 oscillate.
communication** works

-> and sometimes it doesn´t.

Vcc= 5[V]
V+= 4[V]
V-=0.6 [V]
C1 and C2 not oscillating.
Communication** not working.

EDIT: if i send a character from the PC, it starts to work, which is not a desired behavior.

EDIT2: same behavior with the Arduino out of the picture)

Now, using the intended connections .

(usb cable not connected)
PowerSource>DC-DC>Arduino(powered by dc-dc)>RS232 module (nothing 
connected)

Symptoms:

Vcc= 5 [V]
V+= 4 [V]
V-=0.6 [V]
C1 and C2 not oscillating.
Communication not working
heats

Same connections, using loopback:

Vcc= 5 [V]
V+= 4 [V]
V-=0.6 [V]
C1 and C2 not oscillating.
-Rx at 3 [V]
-Tx at 3 [V]

Same connections, connected to pc

Vcc= 5 [V]
V+= 4 [V]
V-=0.6 [V]
C1 and C2 not oscillating.
Rx out: at 3 [V], when a signal is sent from PC, it goes from 1 [V] to 
4[V] then goes back to 3 [V]
Tx out: nothing

i've been trying everything, i bought 6 RS232 modules and tried 3 so far, all of them have the same symptoms

EDIT:
well, the RS232-TTL works IF a signal comes from the RS232 side first (ie:PC or Modem), if the TTL part does Tx first (ie: Arduino), the max3232 will heat and it becomes unusable, it seems to be a problem with the low power mode.
And that is all, no solutions, just a work around.

Best Answer

So it was a bad design in the end, the unused Tx can't be left floating, yet all of the modules I tried had the unused Tx floating (MAX232 has 2 channels, but the modules use just 1 channel) .

If they are floating, they will act as antennas if there are RF signals, and will latch the IC, thus heating in the process.

Used the TI galvanic isolated TTL-RS232 transceiver instead, made a PCB with it and now the communication works flawlessly. http://www.ti.com/lit/ug/tidu298/tidu298.pdf

In this place it's explained why it is bad design, and it's about the same problem I had, just in a slightly different situation http://www.edaboard.com/threads/352767.html

Extra advice: if you are planning to transmit data through RF and you will be placing the modem relatively close to your transmitting TTL device, by all means try to get a modem that has an interface at TTL levels, adding a RS232 interface in the middle also adds a possible point of failure, plus it costs more.

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