Optical isolation of serial interface with DB-9 isolator dongle - Page 1
Forum section: electronics / projects

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 04, 2017, 02:46:38 pm
content: I found a cheap DB-9 isolator dongle on AliExpress:https://www.aliexpress.com/item/Serial-Adapter-RS232-TO-RS232-DB9-DATA-Converter-Photoelectric-isolator-Lightningproof/32334176147.htmlI would like to understand however where the DB-9 dongle gets its power from. Most DB-9 dongles take power from the Handshaking signals.But will this work for my PCB board, where I am using a MAX232 chip to convert between TTL and RS232 levels?Has anybody on this forum used such a DB-9 isolator dongle?Could you check this adapter with a MAX232 chip?It will help me to understand the power transfer. According to description isolation is present in three line TXD(2), RXD(3) and GND(5).

author: eliocor
author_rank: Unknown
timestamp: May 04, 2017, 10:00:42 pm
content: For the price it has...Buy it and take it apart!!!

author: HoracioDos
author_rank: Frequent Contributor
timestamp: May 04, 2017, 10:30:54 pm
content: Quote from: pascal_sweden on May 04, 2017, 02:46:38 pmThat's a good question for customer support.I guess this is the product: Soarland & Hexinhttp://www.hexin-technology.com/HXSP-148_3-lines_RS-232_Optoelectronic_Isolator-Product-249.htmlContact InfoSkype: leslie-hexinTel: 86 755 28779909Email: sales@hexin-technology.com

author: jeroen79
author_rank: Frequent Contributor
timestamp: May 04, 2017, 10:44:06 pm
content: How much current does it take to drive an optocoupler's LED enough to fully open it's output's phototransistor?How much current can an RS232 port supply through it's TX line?

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 05, 2017, 08:42:20 am
content: I have contacted the company Hexin, and awaiting feedback from them.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 05, 2017, 09:21:01 am
content: How much current the opto-coupler LED needs, depends on the model. Usually the current from RS232 is strong enough to drive the normal to slightly more sensitive versions. A good choice would be something like 6N139, as they are also reasonable fast without needing too much current.The problem can be the receive side of the dongle: depending on the settings / program on the PC side, there might be no positive voltage on any line, when a positive voltage is needed. However there is a negative one. So a good circuit would include a kind of charge pump to always get the ideal +- supply.Just for an isolated RS232 interface on self build µC circuits, I have used simple opto's several times. The ouput signal is not 100% by the standard (only +6 V / 0.1 V - thus no negative), but usually good enough for a PC. Many of the computer mouses don't give a much better signal. So this type of circuit is usually in such a way that starting from a standard signal level (+-9 V and sufficient current) one can get a signal that most PCs will accept, though the output level will not fully meet the standards. So you can't cascade two of these. One might also run into a problem when starting with a cheap USB-RS232 adapter.

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 18, 2017, 03:59:25 pm
content: No feedback from Hexin yet on my questions.Does anybody here know about another company who has a similar low-cost solution?This way I can contact them instead.

author: H.O
author_rank: Frequent Contributor
timestamp: May 18, 2017, 04:54:32 pm
content: It's $10 delivered, just buy one and try it, if it works take it apart and figure out how it works, if it doesn't try finding another one.It'll be faster than trying to find a manufacturer who'll answer your questions on an item where the time it takes them to answer your question is worth more than the item itself.Or are you looking to buy thousands of them? Then you REALLY should buy a couple of different ones and find out which one will be the best for your application.

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 25, 2017, 12:30:24 pm
content: Let's assume that it takes the power from the Handshaking signals on the RS-232 interface.How come that the MAX232 does not provide these Handshaking signals?Are there variants of MAX232 that provide Handshaking signals?There are special variants of MAX232 that have additional ESD protection.Ideally there is variant that has both ESD protection and provides Handshaking signals.This way the DB-9 port on my PCB board has ESD protection, and can be optically isolated through the external DB-9 isolator dongle.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 25, 2017, 01:01:18 pm
content: Quote from: pascal_sweden on May 25, 2017, 12:30:24 pmThe MAX232 is just a level shifting buffer with a charge-pump power supply. If you want handshakingsignalsyou have to provide them. The purpose of the handshake signal is not to power the interface - it is serendipitous that in some situations it is possible to steal some power from these signals.If the bare opto solutions given above are not adequate for your needs and you require a more robust solution that does not rely on using the handshake power you will have to employ a DC-DC convertor, or use something like the MAX3250 (if you can get it!)