Re: LocalTalk/AppleShare between IIgs and Mac LC 575 - how?

[dempson@actrix.gen.nz (David Empson)]

Rodney Hester <rod3@mindspring.com> wrote:

> "David Empson" <dempson@actrix.gen.nz> wrote in message
> 1eunn3p.110ft3vayx3fqN%dempson@actrix.gen.nz">news:1eunn3p.110ft3vayx3fqN%dempson@actrix.gen.nz...
> >
> > Are you using the modem port on the IIgs?  If not, then you can set the
> > IIgs to use the modem port for AppleTalk, and leave the ImageWriter
> > connection intact.
> 
> I'm currently using the modem port for my Focus hard drive and have the
> printer port dedicated to AppleTalk, for complex reasons I won't bore the
> group with.  =)

I've never heard of a Focus hard drive being plugged into the modem
port.  Perhaps you mean that you have the card installed in slot 2?  :-)

If you have a ROM 1 IIgs, this won't be a problem - as long as you have
slot 1 set to "Printer port", slot 2 to "Your card" and slot 7 to
"AppleTalk", you will be able to plug the AppleTalk network into the
modem port, and you can leave the ImageWriter connected to the printer
port.  Your Focus card will continue to work perfectly OK.

If you have a ROM 3 IIgs, you won't be able to use the modem port for
AppleTalk unless you move the Focus card to a different slot.  (Slot 2
must be set to "AppleTalk", not "Your Card".)

> > (b) For a permanent LocalTalk network between the Mac, IIgs and
> > ImageWriter II, you will need a proper cabling system.  There are two
> > standard cabling systems, which cannot be mixed.  In both cases, you
> > need a "drop box" for each device on the network (three in total, in
> > this case), as well as two cables to connect the network in daisy-chain
> > fashion.
> 
> I'm a bit confused - if I don't need drop boxes for direct-connect, why are
> they necessary for each node?

The serial ports use RS-422, and electrically they operate in a
push/pull fashion, i.e. sourcing or sinking current depending on whether
the output voltage is high or low (it uses signal levels of about +4.5V
and -4.5V).

You cannot use a simple "wire-or" technique to connect multiple
push/pull devices together, because if the lines are outputting
different signal levels they will source or sink more current than they
are designed for.  This may damage the driver chip (they are usually
designed to cope with it) but it nearly always prevents communication.

The computer is able to control whether the transmit output is active,
but when inactive I think it just clamps the output to ground, which
won't help.

The other part of the equation is that there are separate transmit and
receive data signals.  The printer cable is actually a cross-over
arrangement, so the transmit output of each device goes to the receive
input of the other device.

If you are connecting two devices on your network, then they can only
communicate with each other, so the cross-over cable arrangement works
fine.

If you want to connect more than two devices, you would need to come up
with some arrangement whereby the transmit output of each device goes to
the receive inputs of all other devices, while at the same time not
directly wiring the transmit outputs together for electrical reasons.

The drop boxes achieve just this.  They contain a transformer which
provides an electrically isolated connection between the transmit output
of each device and the LocalTalk/PhoneNET network.  The network itself
is just a neutral electrical loop, in the form of a twisted pair with
termination resistors connecting the pair at each end.

Whenever the transmit output of any device undergoes a voltage
transition, it will induce a signal on the network, which is then
distributed to the receive input of all devices.

An inactive transmitter has a steady state output, so it won't interfere
with the network.

(The transformers also provide the benefit of avoiding issues such as
requiring a common ground and related issues such as ground current
loops.)

When the serial ports are being used for AppleTalk, and are actively
transmitting, they use a data encoding technique called FM, which has a
clock transition for each bit, and an additional transiting for a
particular bit value (I think it is for a zero bit in this case, but I'd
have to look it up to confirm this).

An active transmitter therefore causes one or two transitions on the
network for each bit cell.

The maximum rate of signal transitions is twice the data rate, i.e.
LocalTalk/PhoneNET runs at a maximum data rate of 230400 bps, but is
actually 560800 baud.

There is still a problem with collisions between transmitters, which
will result in corrupted data arriving at the receivers, but it doesn't
run the risk of electrically damaging anything.  The LocalTalk protocol
layer within the AppleTalk protocol stack deals with filtering out any
corrupted data (avoiding collisions as much as possible), and higher
layer protocols are responsible for retransmitting any data which might
have been lost.

> > Back when all this was current, LocalTalk cabling was rather expensive,
> > and PhoneNet was relatively cheap.  Nowadays you could probably get
> > either type second hand without paying too much.
> 
> So far the pricing I've gotten is around $7 for a single LocalTalk connector
> kit.

That is an improvement.  When I bought my PhoneNET connectors (actually
another brand which is compatible with PhoneNET, and has LEDs to
indicate network activity), they cost about NZ$50 each, while a
LocalTalk connector kit would have been more like NZ$150.  (Halve these
numbers to get approximate US dollars.)

> > Incidentally, with a networked ImageWriter II, you will also be able to
> > print to it from the Mac.  You may need to locate a copy of the
> > AppleTalk ImageWriter driver, as it hasn't been updated since System
> > 7.1, and was no longer installed by default some time around System 7.6
> > or 8.0.
> 
> Ah - any pointers on this?  I'm running System 7.5.5 (started at 7.5, then
> upped to 7.5.3R2, then 7.5.5).  Will do some web searching.

If it isn't installed as part of the system, I'd expect to find it as an
optional item in the System 7.5.3 installer (I'm pretty sure it wasn't
removed until at least 7.6).  If you get hold of the "TomeViewer"
application from Apple, you can extract it manually from the
installation tome without having to run the installer.

You may also be able to download the driver directly from Apple's
software updates site.  (TomeViewer is there somewhere as well, though
it might be on the developer site rather than the general software
update site.)