Friday, January 24, 2014

A lie can travel half way around the world while the truth is putting on its shoes.

Not that Mark Twain knew much about fibre optic cable. If he did he'd realise that he was only talking about seventy milliseconds...!

I often have to provide proposals and quotations in response to customer's tender documents and in the last six months I have seen three such tenders specifying tight-buffered fibre for their internal networks.  When I push their staff engineers for a justification they "um and err" and are easily persuaded to do the right thing (i.e. use spliced loose-tube fibre). If you need to read up then I've written a few things in the past.

Last week I came across the following from the Argosy website;
Tight buffered cables are intended for indoor applications. They are more hardwareing than loose-tube cable, as such they are well suited for long indoor LAN connections, burial or complete even submersion in water. Tight buffered cables have a special two-layer coating. The first layer is plastic, the other a waterproof acrylate.
I wonder if this mis-information is their doing?

Thursday, January 23, 2014

Friends don't let friends use stock firmware in their routers

Over the years the number of security flaws that come as standard with £50 plastic-box routers have been numerous. That 'free' router that came from your ISP probably suffers from one of these;
  1. UP & P enabled by default
  2. PING on the WAN side enabled
  3. Port 32764 left open
That last one is very serious as it allows a remote attacker to make a query of the router and dump out lots of diagnostic and configuration information. That may be of no consequence but it does allow a hacker to gain knowledge concerning your network and work on other attacks. The problem bedevils Linksys and Cisco models and SlashDot have a good write-up.

In a very real sense your router is the gateway between your network and the wild-west that is the public internet. If you can't even trust the little hardware device that sits in the cupboard under the stairs what can you do? Well, use an open source firmware in your router - Tomato is very user friendly and DD-WRT is very powerful. There are numerous others and since the source code is open it is regularly examined by the community that develops it and so many eyes spot any nasties (malicious or just bad programming) in the code.

I grabbed a couple of Buffalo models from eBay for when my eldest two went away to University and I wouldn't dream of letting my home network be based around a closed-source router.

Thursday, January 09, 2014

Chassis vs Signal earth on RS422 remotes

Grounding is essential to reliable operation of any RS422 connections. It is also the most overlooked and least understood. The easiest way to ground your RS422 equipment is to simply use "Earth" ground as your return path. Although easy this may not be the best method for grounding your application, because current leaking from equipment, electro-static discharge (ESD), and lightning all drive current through this path which results in high noise content. The reason for this increased noise level is due to the fact that "Earth" ground presents a relatively high resistance. RS422 is designed to operate normally with a ground potential difference of +/- 12 Volts. During normal operations this is typically not a problem, however during fault conditions or lightning strikes even within ½ mile the ground potential difference can reach hundreds and in some cases thousands of volts. This will most likely result in damage or failure of one or more devices on the RS422 router.

In TV facilities I most often come across three methods of earthing;
  1. No earth - assume that the mains return is good (all equipment is class-1 and bolted into it's bay and that signal and chassis earths will be close)
  2. Use pins 1 and/or 9 on the 9-pin D-type to couple the chassis earths together; please, don't get me started about intentionally connecting mains earths between different areas! Do you like dealing with induced hum between different areas or buildings?!
  3. The best way; using pins 4 & 6 - the signal screens (it's what they're there for).
The job I'm finishing at the moment had a problem with Digital Rapids workstation which wouldn't run through the RS422 router although patching around the router worked - the PC could control a VTR. My first port of call was to test the cabling/router patch by sticking an old Sony RM450 edit controller at the back of the Rapids and pretend the RM450 was the workstation - all good; VTR control and timecode return worked fine (so Tx and Rx doing their things). 
So my first thought was to measure the impedance between the signal ground on the router and the mains earth - high Z so no return patch for the RS422 via the router if it was relying on the mains earth (scenarios 1 & 2 above) and since the router is optically isolated on it's data inputs I wasn't surprised. That is the way it should be done.
Now then; most PCs that are running video apps and have to control a piece of broadcast kit use an RS232 port with an external RS232-422 adaptor. These essentially just balance the Tx and Rx pins and there are several models. None of them (in my experience) actually use a pair of rep coils to properly balance rather they use a pair of op amps in a differential input configuration. This is fine but doesn't have the noise immunity that you get with coils (common mode rejection). What it does mean is that all of the noise immunity of the circuit comes from the electrostatic shielding of the earth and so you better get it right!
I cracked open the cheap'n'cheerful '232-422 adaptors supplied with the rapids and they had the screen connected to the shield of the 9-pin on the RS422 side (so relying on scenario 1 above). Moving that to pins 4 & 6 (scenario 3) fixed the problem. 

As an aside the Adenda "Rosetta Stone" adaptors that Avid supply do the right thing!