warack@aero.ARPA (Chris Warack) (08/23/85)
[ouch] I have been trying to hook my Mac up to various modems and computers for a while now. I have been totally unsuccessful. Today I was able to use some diagnostic tools and try to narrow down the problem. It seems that the Mac's signals are too weak to be listened to by other computers! Has anyone else had this problem??? I am trying hardwire connections using a printer cable [w and without Apple's modem eliminator]. I know other people who have used their printer cables to connect both with and without modems. I have tried both ports and I'm using MacTerminal. [Though I also tried MacKermit and Red Ryder briefly to see if that helped -- it didn't]. Finally after hooking up the Mac to a breakout box and comparing it to a normal terminal [HP 2621] I noticed that the LED's showing the signals for the Mac were awfully dim. In fact, Terminal Ready would sometimes disappear when the computer end was connected. I later surmised that it was being pulled-down by the computer. Next, I tried jump-starting the connection by substituting full-strength signals for Terminal Ready [pin20] and [pin4]. The computer responded like it was ready. But, it still wasn't picking up transmitted data [pin2]. I could see the light when data was sent, but the computer never responded. Again, the light was also dim. Am I correct in thinking there is something wrong with the power level of my comm ports? or am I doing something else wrong? I'd like to hear any other 'war stories' or advice before I take it in for servicing. Thanks, Chris -- _______ |/-----\| Chris Warack (213) 648-6617 ||hello|| || || warack@aerospace.ARPA |-------| warack@aero.UUCP |@ ___ | seismo!harvard!talcott!panda!genrad!decvax!ittatc!dcdwest! |_______| sdcsvax!sdcrdcf!trwrb!trwrba!aero!warack || || \ Aerospace Corporation, M1-117, El Segundo, CA 90245 ^^^ ^^^ `---------(|=
rick@sara70.UUCP (Rick Jansen) (08/26/85)
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The problem may be that the Macintosh serial ports
are RS422 and not RS232. RS422 signals swing from
ground to +5V, RS232 signals from -12 to +12V.
Now, if your modem has an input/output circuit
built with genuine modem ic's everything works fine.
But, some (very few) modems use a simple OpAmp
comparator as driver/receiver. In that case the modem
MUST be driven with signals BELOW ground, which
Macintosh does not supply.
Rick Jansen
{seismo,decvax,philabs}!mcvax!sara70!rick