RS232 Question - Page 1
Forum section: electronics / projects

author: andersendr
author_rank: Regular Contributor
timestamp: June 01, 2010, 02:54:13 am
content: Hello everyone, I have a quick question about an RS232 interface I am working on. I am wondering your thoughts on transferring a 5v power and GND over the RS232 cable using unused pins like RTS and CTS. I do not have to go a long distance say maybe 3 to 4 meters at the max before another UC would be there to retransmit the data. I thought it would be ok to do since the data is not critical or fast updating. It will be user supplied via buttons, switches, and pots.

author: Unknown
author_rank: Unknown
timestamp: June 01, 2010, 03:10:37 am
content: Since it's just DC, I wouldn't worry about signal integrity issues. Make sure you don't draw much power (probably not an issue for just a micro and some buttons). I would worry about compatibility with other RS-232 devices, eg. someone plugging one end in their computer. Your power/ground pins could be connected to +/- 25V (although most are more like +/- 12V or 5V). I would make sure to use a non-standard connector and clearly label them.

author: andersendr
author_rank: Regular Contributor
timestamp: June 01, 2010, 03:19:16 am
content: alm, ok sounds good. No this will not be connected to a computer in any way shape or form. It will just be used for com between two micros one being the input side, with the buttons and such and the other being the output side, SSR's and other outputs.

author: migsantiago
author_rank: Frequent Contributor
timestamp: June 01, 2010, 07:13:24 pm
content: HelloJust remember to use good supply filtering because there may be some noise at the end of the 4m cable. A 0.1uF between Vdd and Vss on the far away circuit should be enough.

author: Unknown
author_rank: Unknown
timestamp: June 01, 2010, 08:58:03 pm
content: Quote from: andersendr on June 01, 2010, 03:19:16 amI was mainly worried about someone doing it in the future. Make sure it's clearly marked and documented, and I'd prefer to use something other than DB-9. Equipment should be as foolproof and intuitive as possible.And I agree with the filtering, I'd put at least a decoupling cap and probably also a ferrite bead (it'll probably low current, so a cheap 0603 shunt would be fine) against HF noise (both pickup and radiation).

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 04, 2010, 03:44:39 am
content: Put power only on the pins that are logic outputs on a normal port. And use current limiting.Power over RS232 already exists.http://www.startech.com/item/PCI4S650PW-Dual-Profile-4-Port-RS-232-Powered-16650-Serial-PCI-Card.aspx

author: toftat
author_rank: Newbie
timestamp: June 04, 2010, 02:44:34 pm
content: I did this for a project, I put 12v power on a couple of pins and extra grounds on another couple. I used a 7805 on the uC board to provide the isolation I wanted...Most importantly, I used male DB9 connectors on both ends of the cable ensuring I wouldn't plug it into a regular serial port on accident.

author: andersendr
author_rank: Regular Contributor
timestamp: June 14, 2010, 02:27:01 am
content: I like all the suggestions, this will not be power from a computer so a simple drop in PCI card will not workalthough I wish it would. This is going into model trains, 1/8th scale so I think I'll do both ends mail or both female. Here is a link to see the size of the trains it will be going into. (www.dellslivesteamers.org)