Redpark Serial Cable Electrical Characteristics

I'm currently doing some contract work that involves developing hardware to plug into an iPod device.  Of course, we're inevitably having problems getting the MFi license needed to talk directly to the iPod, so for the short term, We need a different solution.

We decided to develop the hardware to talk over RS-232, which can be fed into an iPod using the Redpark Serial Adapter.  This is a pretty neat little device; selectable speeds up to 57.6k baud (a little slow, but fine for telemetry), comes with a whole SDK to easily integrate into your app, and comes with a loopback test dongle.

Of course, having our hardware talk over RS-232 instead of straight TTL serial has the disadvantage that RS-232 has no provisions for power.  Batteries were a no-go, but we thought it would be possible to maybe "harvest" the needed power for our 3.3V electronics off of the RS-232 data lines coming from the iPod.  This is risky, since many RS-232 adapters cut corners and take liberal interpretations of what +5-15V actually means, so having this vampire power supply work depends on really how well this Redpark cable implements the standard.

Turns out, they implement it quite well!
Scope image of a mark-to-space edge on the RTS line.  2V / 1ms per div. Don't you love my 70's-tastic Tek 468?
 An RTS edge again, but at 2V / 0.2μs per div to check the slew rate.
9600 baud loopback test. 2V / 1ms per div.

Looking at those images, and doing some tests with my volt meter, the Redpark puts out +6V open, 30mA dead-short, and -5.6V open, -22mA dead-short.  That's a lot of power!  I'm going to be able to run my electronics off the RTS line no problem.  The cable also appears to implement all eight data lines, which can also be rare in cheap adapters.  They even meet the slew rate spec of <30V/μs, with a slew rate of only 7.5V/μs.

So long story short, Redpark didn't cut any corners here; The signals are all implemented according to spec.  The iPod connector does feel a little cheapy light, and I'm a little disappointed that their demo app is just a loop-back and signalling line test, and not something at least remotely useful like a little hyperterminal app, but the BB-9 connector feels solid, and I'm certainly not longer apprehensive about building our project on this product.

Popular Posts