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What's wrong with my RS232 switch ON/OFF circuit?

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tony_lth

tony_lth

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Hi, All,
For switch on/off RS232 signals, I used CD4066, and because my control signal is TTL level, I have to use CD4054 to convert 0V/5V TTL signal to +5V/-5V signal to control CD4066 On or OFF.
But unfortunately, the circuit doesn't work, and the total current is about 0.8A and CD4054 is fired.
I wonder what's the reason for the burn CD4054?
1. Is it because that CD4054 GND is nor directly linked to main GND, in fact, through a think wire about 10mm length? I think RS232 is very low freq, maybe this is not the root cause.
2. Is it because that STR4/STR1 directly linked to +5V? Should it be by a 10K?
3. Other comment?
many thanks.
Best,
Tony Liu
 

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  • CD4054BM_CD4066BM.JPG
    CD4054BM_CD4066BM.JPG
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  • cd4054b.pdf
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  • cd4066BM96.pdf
    1.3 MB · Views: 82

Hi,

with ON/OFF you mean high_impedance, low_impedance?

What is wrong:
* signal input voltage range (RS232 signals may go up to +/-15V)
* no ESD protection
* floating inputs
* maybe more...

I didn´t check all signals. It seems very over-complicated to me. I don´t see the use case.

Klaus
 
Hi, KlausST,
When control=5V(H), hope 4066 low impedance, ON status.
When control=0V(L), hope 4066 OFF status.
the floating input, you mean 4054 IN2 and IN3?
Thanks.
Best,
Tony Liu
 

Hi,

the floating input, you mean 4054 IN2 and IN3?
Click to expand...
General rule:
Dont´t leave any (unused) input floating. This is true for digital as well as analog inputs.
In many cases floating inputs cause increased supply current, oscillation , EMC problems, EMI problems....
(In best cases it just makes the input value unpredictable.)

Klaus
 
Need a switch for RS232 signal with TTL as control signal

Hi, ALL,
I had designed a switch with 74LV4066, which can switch on/off a 5V/0V signal with a TTL(5V/0V) control.
Now I want to switch on/off a +5V/-5V signal with a TTL control, is any IC available to do so?
Pls recommend.
Many thanks.
Best,
Tony Liu
 

Attachments

  • 74LV4066.pdf
    254.3 KB · Views: 80

Re: Need a switch for RS232 signal with TTL as control signal

Many analog switch and analog multiplexer parts will
do, provided you have the higher voltage supplies
handy to cover the larger signal range. You may
want to give some thought about overvoltage and
other types of fault protection, which are special
features of some switch and mux products - power
off signal blocking, etc could be handy and so on.

Another option is to give each RS-232 line its own
receiver input pair, and select / mux the 5V-domain
digital signals. This might give you cleaner line match
and overvoltage / fault protection features that are
meant specifically for the serial interface application.
 
Re: Need a switch for RS232 signal with TTL as control signal

Hi, Dick_freebird,
Could you recommend some IC part-types?
I am a newbye for analog IC, thanks.
Best,
Tony Liu
 

Re: Need a switch for RS232 signal with TTL as control signal

Hi, ALL,
I found 74LV4066 can also be used in +/-5V situation with TTL control, although 74LV4066 datasheet recommend in +5V/0V situation with TTL control.
So now what's the risk for such design?
Best,
Tony Liu
 

Hi,

I found 74LV4066 can also be used in +/-5V situation with TTL control,
Click to expand...
I can't find this information.

You still refuse to accept that RS232 levels are up to +/-15V.

On the transmitter side the absolute value of the levels must be >5V at a load of 7kOhms (or less). This can't be guaranteed with +/-5V supply.

It seems the switch is directely connected to a RS232 connector. So what's the use of such a switch?

Klaus
 
Hi, KlausST,
Yes, you are right.
In its datasheet, it can only be used in +5V/0V supply.
But in fact, I found it can be used with +/-5V.
74LV4066 can work in +/-5V, and 1pcs is good with little current consumption. Another pcs can also work, but current consumption is very big.
Is the big current consumption caused by power supply?
Best,
tony Liu
 

Hi,

In its datasheet, it can only be used in +5V/0V supply.
But in fact, I found it can be used with +/-5V.
Click to expand...

You can't be serious!

Klaus
 

Hi, KlausST,
Sorry, I designed a PCB with CD4066, which is +/-5V supply.
And the circuits have problems, because RS232 and RS422 want to multiplex the same connector.
Which caused many trouble for me.
I just want to find a quick solution, although it is dirty.
I found two IC share one trace on PCB is terrible, many troubles are unexpected.
Do you have some comments?
Best,
Tony Liu
 

Hi,

to switch between RS232 and RS422 I recommend dedicated circuits like: MAX3160E.

****
A 4066 won´t work because the logic input threshold is referenced to the negative supply.
--> You need to look for switches where the logic input is referenced to GND and there are dedicated analog switch supplies. Like with DG411.
(DG411 is not suitable to switch RS422 signals because of R_ON, nor is it suitable to switch RS232 signals with +/-5V supplies.)

Additionally there are some analog switches that are able to switch analog signals beyound the supply rails. Maybe they can switch RS232 signals with 5V or +/-5V supplies.

Klaus
 
Hi, KlausST,
Many thanks.
I should use some IC as MAX3160 to do so.
My original design is wrong.
Best,
Tony Liu
 

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