I cut a RS232 cable to make a serial cable for myself, but didn't work out, weird letters

For Developers
1

I bought some RS232 to ethernet cable, I cut the ethernet side. Although RS232 side has 9 pins, inside there was only 4 cables. By measuring the cables with multimeter and with the help of this photo:

rs232-pinout-dsub-female
rs232-pinout-dsub-female1042×621 71.3 KB

I did what this photo says, I soldered them up exactly like that. And then I used the jumpers on my router, but it didn't work. It sends some letters but inaudible letters. Then I hook up my PL2303, everything is normal. Is it not possible to cut RS232 to make PL2303 like serial cable? I also changed the name to ttyS0 accordingly, still no use. Why it doesn't work?

2

What are you connecting this cable to??

Most devices (like all-in-one routers) that have an internal serial port are using TTL serial (3.3V) and are not compatible with RS232 voltages.

3

MAX232 is the IC typically used to adapt RS-232 and TTL levels

4

I am connecting this to a Asus router, which uses 3.3V . I was told this RS232 works between -15V and +15V. Maybe it's a voltage issue??

5

MAX232 does the trick and uses +/- 7,5 V in RS-232 side. It always worked for me

6

Only if you want to fry your Asus.

7

Yes... you cannot use RS232 on your device.

To be clear, your Asus router expects 3.3V TTL, but it might be 5V compliant. TTL does not use negative voltages, but RS232 does. There is a risk of damage to your Asus router if you use RS232. But even if it doesn't break it, it won't work.

You need a TTL serial device.

For some further reading:
https://www.sparkfun.com/tutorials/215

8

So it gives much more voltage then just 3.3V's? Ok, I'll just buy another PL2303 then. My electric store run out of PL2303's so I thought maybe I could do it like this :confused:

9

Not just 'more voltage' -- it's also an inverted signaling standard.

  • TTL
    • logic high = positive voltage
    • logic low = zero voltage
  • RS232
    • logic high = negative voltage
    • logic low = positive voltage
10

Oh I get it. Actually I don't get it too much, I barely passed my electronics class haha :smiley: Just didn't like the professor at all. Otherwise electronics seems like more fun to me than in colege. Thanks though.

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