RS232 to TTL custom board, no output from MAX232

Thread Starter

Tommaso

Joined Apr 3, 2021
11
Hi everyone, i'm kind of a beginner in pcb making, i needed a RS232 to ttl and tought it would be fun to just design one... Bad choice apparently! So, essentially i followd the reference schematic in the MAX232 datasheet to create a prototype, but i can't get it to work. Essentially i get no signal at all on the MAX232, no data in either way. I tried probing the ttl side with an oscilloscope but it seemed like no signal was getting through at all! I'm posting my schematic in the hope that some of you experts could give me a hand.
1617462078618.png

That is all, for testing sake the transistors and LEDs were not connected. I'm not sure what kind of information you need so please ask me and i'll post everything i can, Thank you in advance!
 

MrChips

Joined Oct 2, 2009
23,073
Show us the rest of the connections.
What is connected on the TTL side?
What is connected on the RS-232 side?
What signals (sources) are you sending on both TTL and RS-232 side?
 

DickCappels

Joined Aug 21, 2008
7,158
you show PNP tranisitors. I think think you want NPN.

Why are you connecting the base of Q1 to an input on the Max233?

Are you aware that RS-232 signals are supposed to voltages above and below ground?

What are you trying to make happen?
 

upand_at_them

Joined May 15, 2010
760
Remove your LED/transistors. Don't complicate things before you even have it working.

On my RS232-serial cable I have pins 1 and 4 connected. Been that way for over 15 years...don't remember if I needed that, but my stuff works.
 
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Thread Starter

Tommaso

Joined Apr 3, 2021
11
Show us the rest of the connections.
What is connected on the TTL side?
What is connected on the RS-232 side?
What signals (sources) are you sending on both TTL and RS-232 side?
To the TTL side i have a TTL to USB adapter and on the rs232 i have a PC.
 

MrChips

Joined Oct 2, 2009
23,073
What PC are you using that has RS-232 port?
Maybe you should draw a complete picture of what you have connected, from device #1 to PC.
 

Thread Starter

Tommaso

Joined Apr 3, 2021
11
you show PNP tranisitors. I think think you want NPN.

Why are you connecting the base of Q1 to an input on the Max233?

Are you aware that RS-232 signals are supposed to voltages above and below ground?

What are you trying to make happen?
Sorry the PNP transistor are my fault, i took the first symbol i found, those are supposed to status indicatos, i read that using a led in parallel can drag down the data line so i used a transistor instead.
 

BobaMosfet

Joined Jul 1, 2009
1,502
Hi everyone, i'm kind of a beginner in pcb making, i needed a RS232 to ttl and tought it would be fun to just design one... Bad choice apparently! So, essentially i followd the reference schematic in the MAX232 datasheet to create a prototype, but i can't get it to work. Essentially i get no signal at all on the MAX232, no data in either way. I tried probing the ttl side with an oscilloscope but it seemed like no signal was getting through at all! I'm posting my schematic in the hope that some of you experts could give me a hand.
View attachment 234374

That is all, for testing sake the transistors and LEDs were not connected. I'm not sure what kind of information you need so please ask me and i'll post everything i can, Thank you in advance!
@Tommaso

This isn't how you do this:

1617464138174.png

Most of your connections are correct. If you want to do LEDs on TX/RX, you do them this way:
TTL side:
1617464974276.png

RS232 Side:
1617465021199.png

You need pin 10 tied to ground; 9 floating; 7 floating; 8 tied to ground.

Here is my version (I created a daughterboard for my projects):

RS232_Module_B.jpg

Note the tantalum caps for the polarized connections.
 
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Thread Starter

Tommaso

Joined Apr 3, 2021
11
This isn't how you do this:

View attachment 234381

Most of your connections are correct. If you want to do LEDs on TX/RX, you do them this way:

View attachment 234382

You need pin 10 tied to ground; 9 floating; 7 floating; 8 tied to ground.

Here is my version (I created a daughterboard for my projects):

View attachment 234383

Note the tantalum caps for the polarized connections. I'm using a MAX202 for this, which uses 0.1uF caps- You're using MAX232 which uses 1.0uF caps. Polarized. Same exact schematic, otherwise.
Thank you i'll try those connections. I used transistors because i read that something having led in parallels like that can bring down the data line voltage, but if it works like that i'll remove those. Essentially you are saying that the only problem is those pins not tied to ground? Oh and would you mind posting your schematic? Thanks
 

Thread Starter

Tommaso

Joined Apr 3, 2021
11
Next questions.
What Cisco router are you using?
What is the purpose of the Cisco router?
I work with router often and those have rs232 ports, this project was destined to work with those kind of devices. This one in particular is an old 3825 i had laying around.
 

BobaMosfet

Joined Jul 1, 2009
1,502
Thank you i'll try those connections. I used transistors because i read that something having led in parallels like that can bring down the data line voltage, but if it works like that i'll remove those. Essentially you are saying that the only problem is those pins not tied to ground? Oh and would you mind posting your schematic? Thanks
@Tommaso
Re-read my above entry, I updated it for the MAX232, which as did use. I've posted the parts of the schematic you need for LEDs, the rest of have right except for what needs grounded. LEDs in parallel don't drop voltage, they require more current- which it will supply (you're only talking a few mA)
 
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Thread Starter

Tommaso

Joined Apr 3, 2021
11
@BobaMosfet I tried fixing the circuit with your advise but with no luck, still no signal at all, at this point i don't know what the problem could be, i'm using a max232rd so maybe is in some way different? It has no white dot to indicate pin one so i assumed it the little white line that indicated pin 1, maybe is that that the problem?
 
@Tommaso If there is no dot, then the end of the IC with the divot out of it is the end is the 'top' with the pin 1 at the top left.

  1. Is there a possibility you've damaged your IC? Just a thought.
  2. Can you post an image of your circuit showing exactly how things are connected.... maybe your schematic isn't perfectly matching?
  3. Are you using polarized caps (non-polarized caps won't work)
 
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