🚀 go-pugleaf

> The following four connections will make your Mac RS-422 connector into a
> standard RS-232 connector.  Building a cable with a female 9-pin D connector
> on one end and a female (or male, depends upon your modem) 25-pin D connector
> on the other is the easiest way to do this.

> 	RS-422			RS-232
> 	(Mac 9-pin)		(Modem 25-pin)

> 	3			7
> 	5			2
> 	7			20
> 	9			3

I have just finish building an RS-232 cable for my Mac/modem, and it works
beautifully.  But I didn't use the diagram above, and unfortunately, the
diagram above doesn't work with software that looks for the RS-232 signal
called DCD (Carrier Detect).

The problem is the way it uses the pin 7-to-20 connection.  Pin #7 on the Mac
is a "listen" line called HSC (High-Speed Carrier?) in RS-422 lingo.  Pin #20
on a DB-25 is a "talk" line called DTR (Data Terminal Ready) in RS-232 lingo. 
The problem is that a modem doesn't send a DTR signal.  (It sends a
DSR (Data Set Ready) on pin #6, but this is irrelevant, because the Mac is
looking for a "Carrier Detect.")  The right connection for a DCD is pin #8 on
a DB-25.

Now that the two "carrier detects" are hooked up, this is what happens.
The Mac tells the modem to dial, and when the modem gets the carrier from a
remote modem, it makes true the DCD line.  When the Mac "hears" a true from
its HSC line, then it knows that a distant modem has been hooked-up.  Of
course, software that ignores the DCD will work, but it might get you sometime
in the future.

I used the diagram found in the "Open Window" section in March's issue of
MacWorld.  The article there shows how to hook a Mac up with a Volksmodem. 
So, I simply matched the signal *names* from the Volksmodem with a D-25
connector.  This was my end result:

	Macintosh	Modem		RS-232 Signal Name
	(9-pin DTE)	(25-pin DCE)

Pin #	3	<--->	7		Signal Ground (GND)
	5	---->	2		Transmit Data (TxD)
	9	<----	3		Receive Data (RxD)
	7	<----	8		Data Carrier Detect (DCD)

Note that a cable built this way will also work for a simple RS-232 line to a
mainframe.


John Lam______________________________________
{allegra,seismo,inhp4}!rochester!ur-univax!jnl

PS. No flame intended. (:-)