Msg: 6664 *Conference*

07-11-96 19:31:09

From: RON WIESEN

To : JEFF LABENZ

Subj: REPLY TO MSG #6661 (MODEL 200/PC FILE TRANSFER)

Null-modem adapter eh?  If you're using the adapter Radio Shack sells, then
that's the problem.  It ain't wired anywhere near to right!  I recommend you
get the DB25M-DB25F so-called "shielded" connection adapter.  It comes with
many jumpers.  The small two-sided circuit board inside this tiny adapter has
all 25 pin-outs from both ends (Male and Female) one each side of the board. So
it's actually a duplicated 25 to 25 cross connect circuit.

The "like numbered" pin-outs from the two ends trace toward each other and they
end on hole-through solder pads. These solder pads are D shaped and their
straight edges face each other.  So where you want a thru-connect of like
numbered pins, you just bridge solder across the narrow gap between the pads.
Where you need either a loop-back toward one end or where you need a
thru-connect of unlike numbered pins, then you insert one of the jumper wires
into the hole-throughs and solder it to the pads.

It's a real flexible device.  Great for making all kinds of strange adapters
--  even the generic 3-wire Null-modem adapter.  Of course you can use diodes
and transistors rather than jumpers and get some "extra" adaption.  Theres
enough room on both sides of the circuit board with the plastic shell in place.
It's silver colored plastic so Radio Shack claims it's "shielded" -- no way
Jose.

Here's one "extra" for your Null modem adapter.  You need pin 7 (circuit
common) of both ends to be thru-connected.  Rather than use a jumper, use a
pair of back-to-back germanium diodes (1N914, etc) for a "fused" circuit
common.  They only insert a 0.3 volt drop so operation isn't impaired.  If you
attach the adapter to some "strange" or "damage producing" device, you blow out
a 5 cent diode - no big deal.

For a "smart" Nul modem adapter, use a small NPN transistor (2N2222, etc) with
a 100 Kohm base resistor to act as signal gate.  Some positive signal from the
"Female-side" gates your laptop DTR (Data Terminal Ready) signal back to the
laptop as an incoming DSR (Data Set Ready).  In other words, the laptop asserts
its DTR and where the adapted device on the "Female-side" supplies positive
voltage (some pin wired via the resistor to base of the transistor) then and
only then does the laptop sense DSR.  On the "Male-side" where the laptop is
attached, pin 20 is DTR (wire it to transistor emitter) and pin 6 is DSR (wire
it from transistor collector.  Now add the typical loop-back of pins 4 (RTS)
and 5 (CTS) with a jumper wire and you have a real smart Nul modem adapter. The
laptop can tell whether it has the adapter attached (assert RTS, sense CTS) and
where it "detects" that the adapter is attached the laptop can then sense the
operational state of the far end device (assert DTR, sense gated DSR). Cool!