Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!asuvax!hrc!xroads!wiz
From: wiz@xroads.UUCP (Mike Carter)
Newsgroups: sci.electronics
Subject: Re: Yet another request for Opto-Iso. recommend
Summary: MacIntosh 422???
	 WHat model are we talking about?
Message-ID: <762@xroads.UUCP>
Date: 14 Jul 89 08:43:02 GMT
References: <20380@cup.portal.com>
Organization: Crossroads, Phoenix, Az
Lines: 45

In article <20380@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes:
> I want to receive an RS422 signal from a 26LS30 with 40-60 ohms of series
> resistance.  I think isolation between the host and the peripheral would
> be a good idea.  Can anyone recommend a good cheap opto-isolator for use
> with RS422 from a Macintosh?  What should put across the RS422 signal,
> other than the opto-isolator leads?  (e.g. back-to-back Zener diodes or
> a MOV for overvoltage protection?  This thing will be for >25 feet, and
> must be cheap.)


Mark:
If you're talking about (for instance) the Mac 512K(E) etc the 9-pin
communications port is NOT RS422. It's RS232C ..just has a mini DB-9
connector (after all, why waste a DB-25 when all you need is a DB-9?)
I have the Mac ---> DB25 pin change (a small cable easily made) if you
would like me to dig it out.
 
As far as opto-isolation, that all depends. "Good idea" often gets shoved
aside for necessity and project cost trimming. If you are controlling
a circuit with a separate power supply and in a real-time environment
then opto-isoltaion is wise. Especially in projects involving robotics or
automation. If you're talking about a printer or modem..putting
opto-isolators on the serial out of the Mac is a overkill and a wate of time.
Also, you should consider that DIODES make great noise makers and can pick up
RF from nearby sources. 
When you run a communications program thru an opto, careful selection is
required at speeds of 19.2K baud and above. SLew rate being the all important
deciding factor.
 
If you're still bent on putting an opto on, use some 4N33's or some 4N28's.
All that is required here that you place the input cathode to logic ground
on the tranmitter side (Mac) and the signal line straight into the Anode.
 
(A 4N23 and their counterparts are single-channel opto's. Depending on
exactly how many lines you require will dictate which of the opto-isolator
family you will need. Quad's most likely.)
 
Hope this helps.
 
	-Mike
-- 
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= Mike Carter  N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road  ?  =
= hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu .              =
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