Re: Rig Interfacing (was: [RTTY] Yaesu FT-920 setup - HELP!!)

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

George Wrote:

> I think I will "volunteer" to write up a more complete summary and 
> picture of what might be happening.

Good idea!

> Ian is somewhat on the right track.  The devil in the "details" is that 
> you can run the rigs through the CW keying line and the rig's own PTT 
> will be internally actuated in most rigs.  So that does not establish 
> the your own PTT circuit is working (separately).  Ian may have more 
> info on this than in his recent post mentioned so I can't go much 
> further with it.

Since we're now getting down to cases, I only posted information that I 
know for *certain*.  That only happens to cover CW keying and FSK, and 
only the MP.

You may reasonably suppose that the right kinds of optocouplers could be 
made to work for PTT as well, and/or for other Yaesu rigs... but we 
don't have that data yet.


> The "current transfer ratio" is a good measure of it but Ian is quite 
> correct in that something like 5 to 10 mA is needed to meet those 
> specs.

That isn't quite correct. The CTR spec includes the input current at 
which the opto is designed to operate. Most are designed to operate at 
round 10mA, but others are designed to operate around 1mA.

The CTR peaks at around the spec current, and typically falls away quite 
rapidly at lower currents. Where the available input current from the 
RS-232 lines may be quite limited, we should *not* be using optos that 
were designed for 10mA - the performance will definitely be poorer than 
expected from the CTR spec (and also quite variable between batches, 
I've found).

For reliable performance, we should only be using optos that are spec'd 
at 1 (one) milliamp, for example:

> > MCT5211 (transistor, CTR 110% @ 1mA) with 1k input resistor
> > FSK :
> > SFH618-A3 (transistor, CTR 100% @ 1mA) with 2.2k input resistor

I've had good results with the MCT5211 in both digital and analog 
applications at low currents. Since it also works at higher currents, 
and comes in the industry-standard 6-pin DIL package, the MCT5211 is my 
'universal opto' and I always have a few in the component drawer.

The SFH618-A3 is again spec'd at 1mA, and comes in a compact 4-pin DIL 
package that fits nicely inside a DB9 connector shell.


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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