mmm@cup.portal.com (Mark Robert Thorson) (07/13/89)
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.)
wiz@xroads.UUCP (Mike Carter) (07/14/89)
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 -- ============================================================================= = 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 . = =============================================================================
jimc@iscuva.ISCS.COM (Jim Cathey) (07/18/89)
In article <762@xroads.UUCP> wiz@xroads.UUCP (Mike Carter) writes: >> 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, > >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?) The Macintosh is indeed RS-422 -- that's how they're able to get 230.4 kbits out for Appletalk. For very short cables you can just hook 422 to 232 and get away with it, but not always. Most folks don't seem to have problems with the fake RS-232 of the Macintosh. I for one applaud their decision. RS-232 is a hunk of #^#$% that should have died out long ago. +----------------+ ! II CCCCCC ! Jim Cathey ! II SSSSCC ! ISC-Bunker Ramo ! II CC ! TAF-C8; Spokane, WA 99220 ! IISSSS CC ! UUCP: uunet!iscuva!jimc (jimc@iscuva.iscs.com) ! II CCCCCC ! (509) 927-5757 +----------------+ "With excitement like this, who is needing enemas?"