12 thoughts on - RS485 Audio Device

• Ok, I’ll bite. How does one do audio over RS485?

  I’ve never worked with RS485, but from some brief googling it looks like it’s a fancy version of RS232. I’m not sure where you’d get
  (analog) audio from on RS232.

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  Why RS485? Whats wrong with a simple 3-wire connection (monospeaker, monomic, ground) where you short monomic to ground on button press?

  Then you could use a simple usb device + device server to convert fron “smartphone headset” to usb then to network.

  On the server, you use a SIP phone client, who use this device as mic/speaker, which is configured to lift the hook on headset button press. In asterisk dialplan, you have logic which automatically dials where the doorphone should call upon hooklift.
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  Why RS485? Whats wrong with a simple 3-wire connection (monospeaker, monomic, ground) where you short monomic to ground on button press?

  Then you could use a simple usb device + device server to convert fron "smartphone headset" to usb then to network.

  On the server, you use a SIP phone client, who use this device as mic/speaker, which is configured to lift the hook on headset button press.
  In asterisk dialplan, you have logic which automatically dials where the doorphone should call upon hooklift.
  ——M7WMYZKAJFUBVPGQRKD4V21B9HPYJ4

• This one caught my interest too…more out of curiosity! Keep in mind that RS485 max speed drops to 100kbps after a relatively short distance. And,
  100kbps is RAW speed. If you encapsulated your audio stream in that you’d lose another 10%.

  So why are you doing this? If you are running a 100m cable (4 wire +
  shield) why not just pull at cat5/6 cable instead? Or just send analog audio over 2 of the wires with the shield to keep out hum. If there is a need to use rs485 you could stream your audio over that connection – but I’m curious why first.

  We did some work for broadcast (radio station) doing AoIP and converting some analog feeds but this seems unusual.

  Jason

  —–Original Message—

• It caught my interest for the same reason! It’s such an obscure query.

  My guess was that the desire was to run it over an existing shared RS485 bus, which means the maximum data rate available would be even less because it would be shared with other devices.

  The only way I could imagine a solution working in this scenario would be some sort of ATA attached to a router which supported PPP and had an RS232 serial port on the WAN side (eg like an old modem router), which could then be converted into RS485. You could possibly do all that in a Raspberry Pi with a USB-RS232 adapter (or even a USB-RS485 adapter directly!). Then you’d run SIP over IP over PPP to a similar device on the other end of the bus. But you’d obviously have to be sure to use a low bandwidth codec, and you’d possibly suffer audio quality issues when there was (RS485) bus contention.

  But that was all guess work. It would be great if OP Jerry could expand a little more on the application scenario, even if just to whet the curious appetites 🙂

  Pete

• Hi All,

  The reason for the question was simply that the customer desired some solution called an “AOR” or Area of refuge – I think it was. Basically a call button, microphone and speaker to hear back with the kicker being “a long distance” the solution has to run. RS485 is like 4000 feet.

  There are solutions our there apparently that are not built on asterisk –
  so I was just trying to find a solution that potentially worked with asterisk.

  Thanks!

  Jerry

• Unless there is already RS485 in place, forcing the use of that type of bus, this sounds to me like something that would be more easily achieved using one of those 2-wire SIP doorphones that puts standard analog audio over a copper pair between the handset and the base.

  I don’t have any specific model to reference to, but they’ve definitely been discussed on-list before.

  Pete

• I have one of those. I call it my ‘man cave.’

• Forget RS485 at that distance (your throughput will be too low). I would suggest you pull a fiber and just create an LAN connection on the end.

  I’m sure you would have had fun getting some of the old IP over Serial drivers working J

  From: asterisk-users-bounces@lists.digium.com [mailto:asterisk-users-bounces@lists.digium.com] On Behalf Of Jerry Geis Sent: Wednesday, November 02, 2016 3:46 PM
  To: Asterisk Users Mailing List – Non-Commercial Discussion Subject: Re: [asterisk-users] RS485 Audio device

  Hi All,

  The reason for the question was simply that the customer desired some solution

  called an “AOR” or Area of refuge – I think it was. Basically a call button, microphone and speaker to hear back

  with the kicker being “a long distance” the solution has to run. RS485 is like 4000 feet.

  There are solutions our there apparently that are not built on asterisk – so I was just trying to find

  a solution that potentially worked with asterisk.

  Thanks!

  Jerry

• If cable can be pulled , you have a couple of options.

  Long Reach Ethernet from Cisco is rated for 5,000 feet. Multi-mode fiber with fiber/ethernet media converters on each end would work and electrically isolate the two ends of the cable. Both are way overkill from a capacity standpoint, but sometimes there’s nothing wrong with overkill. Put an ATA on the far side.

• Hi,

  We are using Westermo products for this case. Westermo DDW-120 is rated up to 15 km (@ 192 kbit/s), higher speed is indeed achieved for shorter lengths.

  It’s not cheap, but it works well, even in harsh industrial environments.

  Sylvain

• Before walking down this path, take a moment to think critically: How far away is the AoR from the attendant station? Does there need to be local rescue/fire service access to the communications? How reliable does the link need to be? Will power always be available when the AoR pone is required to function?

  I did a 27mA powered intercom system at a chemical plant a bit over decade ago, the old analog circuits are dead simple – just a battery (or a double battery-backed power supply in my case), a current regulator and an audio bypass cap to make a simple audio loop.

  If that’s too much hardware, there are still sound-powered phones produced
  – very common in mining and maritime. An SIP phone can be wired into a sound powered phone circuit, where the preferred usage would be talking via SIP phone, but if all powered failed the sound-powered devices on the line would still make a complete audio circuit.

  Just a thought…

  -Tim

• We do something like this, however we have two pairs of wires.