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!