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How to apply a SPDT to RS232 signals?

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tony_lth

tony_lth

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Hi,
I want to switch a RS232 signal to TWO terminals with a SPDT.
I noticed RS232 level is up to +-15V.
So I selected this SPDT DG470EQ-T1-E3, and attachment is its datasheet.
But I have some questions about it:
1. Because of positive level signal and negative level signal of RS232, should I use +-15V as the power supply for DG470EQ-T1-E3 IC?
2. I have found some single positive power supply can switch on RS232 signals, so is it OK to use +12V as the power supply for DG470EQ-T1-E3 IC?
3. For the control logic 0/1 of the DG470EQ-T1-E3 IC, I have two options, which should I use?
3a. GND as logic 0, and +5V as logic 1
3b. +5V as logic 0, and -5V as logic 1, which meets the RS232 definition.
Many thanks.
Best,
Tony Liu
 

Attachments

  • DG470EQ-T1-E3 dg469-241128.pdf
    131.2 KB · Views: 76

A dual supply is required for positive and negative input signals for DG470.

Gnd is logic 0 and +5v for logic 1.

tony_lth said:
I have found some single positive power supply can switch on RS232 signals, so is it OK to use +12V as the power supply for DG470EQ-T1-E3 IC?..
Click to expand...


Probably you are referring to RS232 level circuits which operate from single supply , to convert from 5v input to RS232 output. Not applicable here for the DG470 , because of the switching signal voltage range involved.
 
Hi,

What signal level sees the "not selected" circuit?
Are there still true RS232 levels? If not....expect malfunction.

I recommend to use two RS232 translators and switch TTL signals.
Maybe AND both RxD signals.

Klaus
 
Hi, KlausST and srizbf,
The main signal may be routed to Route1 or Route2 by the SPDT.
The main signal is the RS232 side from MAX3237, so the main signal should be the standard RS232 true signal.
The Route1 is the RS232 side of MAX232, so this Route1 should be the true RS232 signal.

But Route2 is the USB/RS232 cable, I am not sure whether the Route2 RS232 is true signal or not.
Could you comment if DG470 could be applied in this situation without any malfunction?
Best,
Tony Liu
 

Attachments

  • MAX3237EIDBR.pdf
    813.3 KB · Views: 71
  • MAX220-MAX249-67423.pdf
    445.5 KB · Views: 74

As others have mentioned, the basic rule of thumb with analog switches is:
that their supply's voltage (both) should be equal or greater than the signal's voltage range you are attempting to switch.
 
Hi, Schmitt trigger,
Yes.
But MAX3237 only uses +5V, because it has some voltage boost circuits, and it should have some voltage inverter circuit.
So MAX3237 can support RS232 output, which means +15V voltage and -15V voltage signals.
Best,
Tony Liu
 

Hi, prairiedog,
Good comments.
But RS232 signals may be positive or negative, so can't drive photo in negative voltage.
Best,
Tony Liu
 

Hi,

Photomos can switch AC, so positive and negative voltages.
But they can't "drive" the signals.
True RS232 signals are LOW or HIGH, not high impedance.

I recommend to read the basics about RS232 signals...voltages, currents...

Klaus
 
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