At 2:20 PM -0700 2/25/2003, Andrew Kershaw wrote: >I suppose this is getting OT... But to beat a dead horse some more... > >>It's because the serial controller chip on a Mac does more than just >>pass bytes, it handles the address and data stream encoding and >>decoding. LocalTalk processing on the Mac is CPU intensive. Once a >>packet transfer begins the CPU is dedicated to processing bytes >>to/from the serial port. > >Ah, so the serial ports act as actual NICs then, eh? But which is >it? The CPU that does the encoding/addressing and writes the bytes >out the serial port, or the serial port that does the >encoding/addressing? Serial ports aren't like ethernet hardware with >a separate MAC/physical layer/etc, are they? I thought they were >pretty much just dumb pieces of I/O equipment - send whatever bytes >you want to them, they'll write it out - the CPU handles all the >addressing, encoding, etc. It shouldn't matter to the port what is >passing through it. And, similarly, that should be why you can only >hook up one device per port (as opposed to USB, which is a bus) - the >serial hardware doesn't do any addressing itself, so multiple devices >would never know which bytes belong to which device. Or is it that >the serial ports go from dumb to smart when you use LocalTalk? As >in, with LocalTalk inactive, the ports just read/write bytes. With >LocalTalk active, the ports do all sorts of network packet stuff like >addressing, ARP, RARP, etc?
Note that the serial controller chips on Macs are either a Zilog 8530 or variant or a ASIC incorporating the guts of an 8530. LocalTalk uses several unique features of this chip to work. LocalTalk was pretty much designed around these features. The serial controller does the data/clock encoding / decoding using a Digital Phase Locked Loop. It also handles the serial incoding/decoding. The serial transfer itself is fairly standard but it is synchronous, not async as are most serial communications (RS-232 type). The controller reads the address on incoming packets and ignores those not addresses to it. This means the processor isn't tied up reading in packets destined for other machines, a big reduction in processor load. LocalTalk really has more in common with Ethernet than with ordinary serial (Enet and LT are both serial of course). The LocalTalk packet incorporates it's AppleTalk address. The Node address is read directly by the serial controller as mentioned above. LocalTalk is designed exclusively for AppleTalk where as Ethernet allows multiple protocols with a byte in the header indicating which. The MAC address determines to which Ethernet interface a packet is to/from. The node address in the LocalTalk packet amounts to the same thing. In both cases the hardware detects if it is a destination for an incoming packet and processes it itself. In the case of Ethernet the controller stores it in it's own memory. In the case of LocalTalk the controller interrupts the CPU so it can do the processing. All other functions of LocalTalk are handled in software (AARP...) Mac serial port interface is a hybrid of RS-232C, RS-422 and RS-485. It is the RS-485 part that LocalTalk uses, it is a multidrop bus configuration with line drivers that can be set to Hi-Impedance on software command. This is what lets multiple device use the bus. > >What about the GeoPorts and Serial/LocalTalk DMA? They just have additional hardware to move the data without processor supervision. I'm not certain how they work with LocalTalk, whether they transfer all the packets under hardware control or if the CPU is still tied up during packet transfer. > >>Huh! I've used MacIP over LocalTalk on my PowerBooks w/ OS 9.1 >>plenty of times. Both with IPNetRouter and with a FastPath. > >I doubt that. Apple even states this in the Knowledge Base. (And >I'm pretty darn certain that this time the KB is correct.) ><http://docs.info.apple.com/article.html?artnum=31211> Heck, even >Sustainable Networks says it can't be done. ><http://www.sustworks.com/site/prod_ipr_troubleshoot.html#macos9> > >So, let me get this straight: The localtalk cable is connected >directly to your PowerBook's serial port, you've got AppleTalk on the >PowerBook set to "Printer/Modem Port", and you've got TCP/IP set to >"Connect via: AppleTalk (MacIP)"? All this under OS 9? I'd be >fascinated if this were true, because I've been living without having >my 68k non-ethernet-equipped Macs not on the Internet for some time. Yup. I just did it the other day, using my Airport equipped PBG3 running IPNR to a DUO. The PBG3 was doing NAT behind another router supplying NAT. Double NAT with Airport in the middle, very geeky I know. I didn't know you couldn't do it so I went ahead and did it, it worked. >MacIP _does_ work over ethernet, though. If you've got that FastPath >set up to provide an ethernet cable connection to/from your PowerBook >and a LocalTalk connection to the rest of the network, that is >entirely different than having the LocalTalk cable connected directly >to your PowerBook. Nope, LT direct from the FastPath to the PB. If a machine has Ethernet, I use that directly, not MacIP over EtherTalk. > >This I've tried with IPNetRouter & OS 9.1. It doesn't work. > >Explain please! I'd love to be wrong here! Explaination - It worked! One thing I can think of is that you have to manually specify IP addresses when using IPNR, it doesn't act as a MacIP server. -- Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting "I'm a designated driver on the Information Super Highway" -- PowerBooks is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Enter To Win A | -- Canon PowerShot Digital Cameras start at $299 | Free iBook! | Support Low End Mac <http://lowendmac.com/lists/support.html> PowerBooks list info: <http://lowendmac.com/lists/powerbooks.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/powerbooks%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
