RS232 null modem to straight

[Firecs]

Hi there!! complete noob, google/youtube trained. Electrician / Fire alarm technician

I am currently designed a board/ widget. First prototype works completely.

Now I am looking to see if I can replace the relays in my design.
This works to mechanically change over the rs232 ports pins so you only need one cable for any rs232 db9 equipment
Is there a solid state IC that could be used to replace the 2 relays that can switch the rs232 voltages? or is the relays the best option? they are small omron SMD relays anyway

Also would Dave be interested in doing a PCB layout review video of my board and the traps for young players/ EMC/ EMI mistakes?
Consists of Lipo charger, rs232 port, boost converter, buck converter, PS2 port, USB, Micro controller module using Pycom GPY
All made from datasheets and Daves videos.

 RS232 sheet.pdf (53.15 kB - downloaded 115 times.)

[Kleinstein]

There are CMOS switch chips that could be used in theory. A DG403 or similar could replace 1 relay. However the CMOS chips are not that much cheaper than a cheap relay and they are more sensitive to ESD and similar problems. They may not like a signal when not powered. There may be special UART-RS232 drivers with an disable one could use as kind of send and receive in parallel instead of switching.

[Mario87]

What about an optoisolator? Basically a solid state relay, can be very small & cheap.

[pqass]

This thread discusses using a pair of rs232 chips and selectively disabling one of them; an extra chip could remove two relays from your BOM.  However, your SP3232EEY has /SHDN and /EN control inputs but only on the SOIC/SSOP/TSSOP packages.
https://www.eevblog.com/forum/projects/rs232-nullmodem-3-3v/msg3576993/#msg3576993

Just in case, I would still use a 47R resistor on the RS232-side TX line to the outside world and logic-side TX-to-MCU.  ie. no two transmitters directly connected without some resistance between.  Although, I'm not sure how much that would affect your top transmission speed.

EDIT: Although, the datasheet says "Short-Circuit DurationTxOUT.............Continuous."  It also says that output short circuit current is +60mA max.  So, I'd still like to keep the resistance in there; maybe make it 100R or 220R.

[luca0074]

you can use just one relais to switch rx and tx. see attached schematic: https://www.eevblog.com/forum/projects/rs232-nullmodem-3-3v/msg3576237/#msg3576237
this is tested and works reliable.

[fordem]

This works to mechanically change over the rs232 ports pins so you only need one cable for any rs232 db9 equipment

My temptation is to ask WHY?

And to explain where I'm coming from - let me ask a second question - how often will you need to switch from one to the other?  You're either going to use the RS232 connection for initial setup or for some sort of monitoring - either method is essentially a "set & forget" scenario, after the initial connection has been made and the task completed, how often will you need to touch it?

For what it's worth, I do UPS support, I do have to use RS232 quite frequently, I carry a straight through cable and a "null modem" dongle, it weighs few grams, unlike the bag of other cables that I also lug around in my laptop case because some UPSs require custom cables.