Macintosh RS-422-to-RS-232-C Adapter



I am trying to attach some Macintosh computers to a AT&T ISN network. The ISN
requires an RJ-45 connector. We are having two problems.

1) According to the article "Macintosh Mini-8-to-RJ-45 Connection", "To
do this, you need some kind of adapter." Is there a way to build a
RS-422-RJ-45 cable without an RS-232 adapter in the middle?

2) Is there a consistent difference between computers in the Macintosh II
family and Macintosh SE computers in terms of distance tolerance for RS-422?
We get a Macintosh SE to work in situations where a Macintosh IIci will
not. My thought was similar to that in another article on serial port cable
length specifications. That is, we were a little over the distance spec and
the Macintosh SE seems to be able to handle that. Word on the Internet
is that people have to make different cables for the Macintosh II than
for the Macintosh SE in these situations.


Because the built-in Macintosh serial ports are based on RS-422, which uses
differential signals, a simple rule should be followed to assure
compatibility with RS-232-C devices.  Ground the RD+ line and leave the TD+
line disconnected.  This converts the port to RS-423, which is compatible
with almost all RS-232-C equipment.

The adapter spoken of in the article you referenced merely adapts the
different connector types: RJ-45 to DB-25.  The grounding of RD+ was done
by the Mini-8 to DB-25 adapter.  The following diagram shows the proper
cable configuration for connecting a Macintosh serial port to an RS-232
device:

 Macintosh Mini-8 Port                          25 Pin RS-232 DTE Device
 Signal Name    Signal  Pin                     Pin* Signal  Signal Name
 -------------  ------  ---                     ---  ------  -------------
 Handshake Out  HSKo    1  ---- if needed --->   6   DSR     Data Set Ready
 Handshake In   HSKi    2  <--- if needed ----  20   DTR     Data Term Ready
 Transmit -     TD-     3  ------------------>   3   RD      Receive
 Receive -      RD-     5  <------------------   2   TD      Transmit
 Transmit +     TD+     6  Not connected
 Signal Ground  SG      4  -----|-------------   7   SG      Signal Ground
 Receive +      RD+     8  -----|

          *Note that the pin assignments shown may be wrong if the
           RS-232 device is wired as DCE instead of DTE.  Since we
           don't know how the RJ-45 connectors are wired, we can't
           provide pin assignments for them.  However, the signal
           assignments will be the same, so you can translate pin
           numbers from the signal names.

Although both Macintosh ports usually work with RS-232-C equipment without
the considerations mentioned above, their maximum cable lengths and
resistance to noise and interference are reduced.  The approximate 50-foot
length limitation may be reduced drastically and reliability can be
impaired.

We have noticed that the Macintosh SE and earlier models are slightly more
tolerant of cable design.  While the Macintosh II family serial ports are
capable of the 50-foot maximum cable length RS-232-C specification, this
requires the cable to be designed as shown above.  If RD+ is not grounded,
the maximum length will be reduced.  The Internet community discussions you
mention are probably based on a partial understanding of these factors.


Published Date: Feb 18, 2012