Bust-Out

Wouldn't it be nice to have a little docking unit which plugs into the R20 and provides immediate support for external connection? Happily, I've teamed up with Normand who shares similar aims to make this a reality.

The top jack on the R20 carries audio out as well as CI-V control. A chip from the MAX232 series is just the ticket for converting the voltage levels used on the R20 to the correct voltage for your PC serial port.

Now what's really interesting about this chip, is that it has a series of built in charge pumps which generate the split plus and minus 12v needed for the RS232 protocol. It just so happens that one of the difficulties with designing a slicer is just that. The op-amp needs a +/- split rail.

So what we are proposing here is to design a box which plugs into the EP/CI-V jack sockets and deals with both tasks. It will connect to an RS232 port for rig control, and another port will give decoded data from a slicer.

Now it may at first appear as a trivial detail, but the whole power supply concept is pivotal in its usefulness to the end user. As it happens, the R20 already has a 6v power adaptor supplied with it. There are wide variations in the different market variations supplied throughout the world, the most distinct variation is that some are analogue, and some are switch mode.

The overhead in additionally running an op amp and MAX 3232 chip would be entirely acceptable. A 2 level slicer could be powered from the 3232. A 4 level slicer would require considerably more current and the viability of this will have to be evaluated.

Here's my suggested connection layout. The box is powered by the original 6v supply. This goes straight through to the R20. The MAX3232 chip takes a maximum of 5.5v so a couple of IN4001 diodes could be used in series with its supply to drop it down to within this. A zener based shunt regulator would be even better.

A larger DC input socket could also be provided so that the whole show can be run off a battery or cigar lighter socket. It's crossed my mind that if the USB lead is taken through the box, 5v could be gained from that thus doing away with the need for a power supply. This is at first an attractive option but may be problematic. The USB spec allows currents of up to 500mA but in order to do this it has to tell the USB controller to allow this by a process known as enumeration. By default, the available current is just 100mA which is enough to power the box but not the R20 as well. There are other limitations placed on us such as a maximum smoothing cap size of 10µF in order to prevent excessive inrush currents.

As the action of plugging the CI-V cable in mutes the speaker, we need to provide a jack socket in order to listen to the audio. A 3.5mm stereo socket would provide a useful output to a speaker / headphones or via a jack to jack lead to the PC for recording, soundcard decoding or just simply to monitor via the PC speakers.

I've shown just one serial cable although it would be possible to have one dedicated to CI-V and another for slicer decoding. I don't know of any instance where you would want to do both at once so the best alternative for most would be to use a three pole 3PDT switch to allow the serial socket on the box to perform either function. Looking at the table below, it is even faintly possible that the two devices could co-exist on the same port. I'll run some tests and see if this works out.


Pin Assignments and Schematics


	PIN	FUNCTION (RS232)	CI-V	2 - LEVEL	4 - LEVEL

	1	DCD			DATA 3
	2	RXD	RXD
	3	TXD	TXD
	4	DTR
	5	Signal Ground	Ground	Ground	Ground
	6	DSR			DATA 2
	7	RTS	{ LINK }
	8	CTS	{ LINK }	DATA	DATA 1
	9	RI




	The CT-17 Schematic



	I'm a big fan of twinkling LEDs, not just because it looks pretty but it does make a black box look more interesting and alive. Most importantly, it's reassuring and informative to see the data on the move plus you get a better idea of what's going on. Low current LEDs can draw just 1mA but it's always better I feel to use a hex buffer so there is no significant loading and therefore no chance of data corruption.

	If this unit were used for programming certain devices as opposed to being used for rig control, there is the possibility of turning a perfectly good device into a paperweight.




	The diagram below shows the method I have used to add LEDs to a typical bi-directional interface.



To strip it down to a bare minimum, the top LED is most useful as it shows traffic in both directions. The bottom LED shows only data which is coming from the PC and is useful to confirm that the host app is trying to communicate with the client device.

A break-out box for the R20 and other devices