RS232 null modem to straight  - Page 1
Forum section: electronics / beginners

author: Firecs
author_rank: Newbie
timestamp: July 01, 2021, 01:21:51 am
content: Hi there!! complete noob, google/youtube trained. Electrician / Fire alarm technicianI 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 equipmentIs 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 anywayAlso 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 GPYAll made from datasheets and Daves videos.RS232 sheet.pdf(53.15 kB - downloaded 93 times.)

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2021, 04:15:53 am
content: 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.

author: Mario87
author_rank: Frequent Contributor
timestamp: July 02, 2021, 11:14:53 am
content: What about an optoisolator? Basically a solid state relay, can be very small & cheap.

author: pqass
author_rank: Super Contributor
timestamp: July 02, 2021, 01:19:22 pm
content: 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/#msg3576993Just 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.

author: luca0074
author_rank: Contributor
timestamp: September 13, 2021, 09:45:50 am
content: 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/#msg3576237this is tested and works reliable.

author: fordem
author_rank: Regular Contributor
timestamp: September 14, 2021, 01:25:03 pm
content: Quote from: Firecs on July 01, 2021, 01:21:51 amMy 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.