🚀 go-pugleaf

The 850 interface from Atari has 4 serial (cereal?) ports and one
Centronics-type parallel port.  One of the serial ports can be made into a
20-mA current loop to run old fashioned Teletypes if you wish.  I bought an
850 a couple of months ago from CSC, Inc., Hightstown NJ.  Their phone is
(609) 448-8889.  Their price for the 850 was the standard $109.00.
The Centronics port plugged in and ran a borrowed Epson printer with no
problems.  I bought an assembled cable, incidentally, from CSC for $25.00.
You can make one but you will spend at least $15 in parts unless you have
a source of connectors at very low cost.

INTERESTING NOTE for Atari fans... the 850 I bought came in the "new" color
of the 130XE/520ST.  This makes me suspect that a new production run has
started and that rumors of unavailability of this product are "greatly
exaggerated" to paraphrase Mark Twain.  I would emphatically NOT recommend
that anybody get suckered in to paying $149.00 for this interface as I have
seen some suppliers cheerfully suggest.

The P: R: connection looks like a reasonable alternative.  It has I believe
only two serial ports but that's probably more than most people will ever use.
It DOES NOT require a power transformer like the Atari one as it steals power
out of the Atari serial bus pins.  Presumably this will not greatly increase
the load on the Atari's transformer, but then again it sure won't decrease it!
This one sells for around $60.

I have not used the others you mentioned but I am sure they would work fine
assuming the cable from them to your modem was wired suitably.  However, with
the above two interfaces you get a parallel printer port as well as the serial
ports.

One other product should be mentioned: I think it is Astra Systems that makes
a very nice disk drive which also throws in I believe one serial and one
parallel port.  (Somebody else may have more details).  If you want another
disk drive also, this is a very reasonable thing to consider.  But it is not a
cheap way to get just the serial port!

NOTE that RS-232-C ports, while they have 25 pins, use only about 6 of them
in practice.  It is not too bad a job to wire up a cable, and this might be
a good place to save some money.  Also, the ribbon cable approach is even
easier despite the fact that it wires up all 25!

Hope the above is of help.