Article 153263 of comp.sys.apple2:
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From: armagon@my-deja.com
Newsgroups: comp.sys.apple2
Subject: Re: Please help with //c to IBM PC connection
Date: Tue, 12 Oct 1999 04:20:22 GMT
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Hmm...  I've tried to figure what to do with info similar to that in the
past.  I do not know if the apple //c's strange handshaking method
caused grief.  For the //c side the pinouts look like this:  (Info is
from http://home.swbell.net/rubywand/R023PINOUTS.htm#011 , with the
entire section being quoted):

Quote Starts
------------


011 //c (5-pin) serial port and modem cable pinouts  F to top
(Most of this is from a post by David Empson.)


   Mini-Din 5
 //c Serial Port
 (Female) Socket

   5       1
    4     2
       3

The functions are:

1  Handshake Out (nominally DTR)
2  Data Out (TxD)
3  Ground
4  Data In (RxD)
5  Handshake In (nominally DSR)


   Mini-Din 5
//c Serial Cable
  (Male) Plug

   1       5
    2     4
       3

To wire up a IIc to a simple modem without hardware handshaking, use
the following pinout:

IIc     Modem
DIN-5   DB-25M
1 ----- 20 DTR
2 ----- 2  TxD
3 ----- 7  Gnd
4 ----- 3  RxD
5 ----- 6  DSR   (you might want to use pin 8, DCD in some cases)

The IIc cannot do hardware handshaking very well, but this is as close
as you can get:

IIc     Modem
DIN-5   DB-25M

1 ----- 4  RTS
2 ----- 2  TxD
3 ----- 7  Gnd
4 ----- 3  RxD
5 ----- 5  CTS

The IIc's handshaking lines have annoying side effects, which cause
problems with hardware handshaking:

1. The "Handshake Out" signal is implemented to mean "I want to send
   data" (the official and original meaning of RTS).  If you turn off
   the output handshake line, the IIc will stop sending data.  For a
   hardware handshaking modem, RTS is supposed to mean "You are allowed
   to send me data" (from the computer's point of view).

   If the computer tells the modem to stop transmitting, the computer
   will also be unable to transmit.  This will reduce the rate at which
   data can be transferred bidirectionally, but doesn't cause any other
   problems.

2. The "Handshake In" signal is implemented to mean "There is receive
data
   present" (the official meaning of DCD).  If the incoming handshake
line
   is disabled, the IIc will stop receiving data (ignore any data on
RxD).
   For a hardware handshaking modem, CTS is supposed to mean "You are
   allowed to send me data" (from the modem's point of view).

   If the modem tells the computer to stop transmitting, the computer
will
   also be unable to receive, and will discard any data sent by the
modem
   while CTS is not active.  This can cause screen corruption and loss
of
   data blocks or acknowledgements during a file transfer, which will
   require retransmission.   It is only likely to be a problem while a
   lot of data is being sent, so is more likely to cause problems
   during a file upload than a download.

   If the comms software is quick enough, it can drop RTS immediately
when
   CTS is lowered, which will prevent the modem from sending any more
data.

You will need the //c System Utilities disk to set up the serial port
speeds, or a comm program that overrides them anyway.

Max speed is 9600.

-----------------
Quote Ends


In article <2mrM3.2848$Ck.6996@news1.mia>,
  "Gus Vilomar" <gvilomar@bellsouth.net> wrote:
> I was checking out Apple Oasis for Windows and I found the following
text in
> the help section.  I hope this helps.
>
> Null Modem (Modem Eliminator)
>
> The null modem connection is a short cable and two connectors. The
standard
> serial connectors are DB-25 and DB-9. Following are their pin
descriptions:
>
> DB-9 Connector
>
>     Pin   Signal                   Pin  Signal
> -------------------------------------------------------------------
>      1    CD   Carrier Detect       6   DSR  Data Set Ready
>      2    RXD  Receive Data         7   RTS  Request to Send
>      3    TXD  Transmit Data        8   CTS  Clear to Send
>      4    DTR  Data Term. Ready     9   RI   Ring Indicator
>      5    GND  Signal GND
>
> DB-25 Connector
>
>     Pin   Signal                   Pin  Signal
> -------------------------------------------------------------------
>      1    GND  Ground / Shield     14        {2nd TXD}
>      2    TXD  Transmit Data       15        Transmit Clock
>      3    RXD  Received Data       16        {2nd Rec. Clock}
>
>      4    RTS  Request to Send     17        {Receive Clock}
>      5    CTS  Clear to Send       18        {Unassigned}
>      6    DSR  Data Set Ready      19        {2nd RTS}
>      7    GND  Ground              20   DTR  Data Term Ready
>      8    DCD  Carrier Detect      21        {Sig. Quality}
>      9         {Reserved}          22   RI   Ring Indicator
>     10         {Reserved}          23        {Data Rate Sel.}
>     11         {Unassigned}        24        {Transmit Clock}
>     12         {2nd CD}            25        {Unassigned}
>     13         {2nd CTS}
>
> Here is the Null Modem connection:
>
> PC  (DB-9)                   Apple*(DB-25)
>
>  RxD (2) ---------------------- TxD (2)
>  TxD (3) ---------------------- RxD (3)
>  GND (5) ---------------------- GND (7)
>
>  RTS (7) --\                /-- RTS (4)
>  CTS (8) --/                \-- CTS (5)
>
>  CD  (1) --\                /-- DSR (6)
>  DSR (6) --+                +-- DTR (20)
>  DTR (4) --/                \-- CD  (8)
>
> * For Super Serial Card (SSC) put the TERMINAL/MODEM jumper pointing
up at
> 'MODEM'.



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