Line Driver for 10-15 ft cable with RS232 COM - Page 1
Forum section: electronics / projects

author: rockbassist90
author_rank: Contributor
timestamp: October 10, 2014, 07:48:10 pm
content: With a 10-15 FT cable sending and receiving ascii commands in an electrically noisy environment, I figured to use a line driver as well as a bi-directional tvs diode for noise suppression and signal integrity for an RS-232 application.Any recommendations?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2014, 08:15:43 pm
content: Go cry in a corner?Idunno... how noisy are we talking? Can you set up a cable, grounded at the far end, and look at the noise to ground on the near end with a scope? What frequencies and amplitudes are we talking? Is it CW / radio, or random industrial switching and contact noise? High frequency, low frequency, spikes, etc.How fast are you looking for? If it's 9600 baud, you can filter the piss out of that and survive quite a lot of crap. Even at 115.2k you can get away with a lot. You're going to be looking for shielded cables pretty soon out there, though, especially if it's really noisy. And if it's really really noisy, and/or you can't tolerate / afford / change to shielded cables, you're better off with something differential and isolated, like RS-422/485 with an isolated RX/TX transceiver. Or even Ethernet (which changes even more of your circuit).Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 10, 2014, 08:16:01 pm
content: RS-232 already has quite high levels against noise, what problems are you having?Or just skip RS-232 and go to RS-422. This is all a solved problem and tons of parts exist.

author: SeanB
author_rank: Super Contributor
timestamp: October 10, 2014, 08:27:37 pm
content: 5m of cable, noisy environment just means the regular 1488/9 drivers on a +-12V rail will work up to 115k, with shielded cabling ( the regular industrial shielded cable you typically have there and locking Harting connectors or CX circular connectors) will do fine. If you are getting to 20m you will have issues, but dropping to 9600b will work up to 100m with little problems. Connect the shield to ground at the transmit end, and use a core to carry the ground through to both sides.http://www.onsemi.com/pub_link/Collateral/MC1488-D.PDFhttp://www.onsemi.com/pub_link/Collateral/MC1489-D.PDFThese are the drivers and receivers you will need to use, not 5v ones, or those with built in charge pumps. Place them in sockets just in case somebody applied 230VAC to the line, and they do not survive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2014, 10:25:15 pm
content: Quote from: SeanB on October 10, 2014, 08:27:37 pm*At both ends.If this causes ground loop problems elsewhere, terminate the shield with a capacitor (0.01uF or more?), or use isolation.Shielding from just one end is bogus, and defeats the purpose of the shield almost entirely -- don't be misled by conflicting here-say and unproven rules of thumb. Want to say a lot of that advice is from the olden days, where impedances were higher and that kind of shielding was more relevant.QuoteYeah, not a bad idea. A MAX232 from +5V will generate +/-10V supplies, which result in +/-8V output, but to truly push the max spec on RS-232, you have to have real, original, hard-core drivers like these (or, do it discrete..).Tim

author: ConKbot
author_rank: Super Contributor
timestamp: October 10, 2014, 10:31:21 pm
content: I'd be more impressed if you could make RS-232notwork in a 10-15 foot distance with a ~10v driver, i.e. MAX232. Ive seen it run ~100 feet at 9600 baud hooked upto a laptop, (most likely only 5V drivers), though not in a particularly EMI-harsh environment.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2014, 06:51:27 am
content: I once had a 10m run to a printer where the original installer had used speaker cable with 2 cores, run next to a cable with an airconditioner on it. Would give the odd single bit error when the compressor cycled. Could lead to wrong figures on an invoice, so kind of needed fixing.I replaced the cable with a 4 core shielded audio cable ( this was before 4 pair Ethernet cabling was around) so the printer could actually give back data and pause on errors like paper out, buffer full or offline. Very important as then I upped the link speed from 120 BAUD to 9600 BAUD, which gave an impressive speed boost in the print capability, and which actually was able to fill the 4k buffer in the added serial printer interface card, though not often as a full page was around 3k of data.