Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!spool.mu.edu!snorkelwacker.mit.edu!apple!ntg!dplatt From: dplatt@ntg.com (Dave Platt) Newsgroups: comp.sys.mac.system Subject: IIfx Serial Switch cdev, System 7, flow control... Message-ID: <422@goblin.ntg.com> Date: 14 May 91 18:14:14 GMT Article-I.D.: goblin.422 Organization: New Technologies Group, Palo Alto CA Lines: 61 A question or two for the Powers That Be: what's the actual truth regarding the need for the IIfx Serial Switch cdev under System 7? Background: The IIfx serial ports are normally accessible only via the serial IOP, via the serial drivers. In the usual "faster" mode, the SCC isn't accessible via the bus, and any attempt to access it generates a bus error. This causes problems for software which really does want to access the SCC registers directly... in particular, sound-digitizer interface software (for the MacRecorder, SID, CSD, etc.) will bomb if run on a normally-configred Mac IIfx. I've also noticed another characteristic of "faster" mode... it's actually slower in one important case! It seems that the serial IOP takes a significant amount of time to "see" that an XOFF character has been received, and to stop sending data. This causes problems when one is sending serial data to devices that use XON/XOFF flow control, and which have only a limited amount of buffer space remaining when they send an XOFF. If the IIfx port is in "faster" mode, it will continue to send data for some time after the XOFF is received... thus overrunning the buffer in the serial device and causing a loss of data. I own two devices which get nailed in this way... a Paralink serial-to-Centronics- parallel converter, and a USR HST Dual Standard modem. Apple released a cdev which apparently sets a bit in the PRAM, so that (the next time the machine is rebooted) the serial drivers and SCC are configured to use a "compatible" mode, in which the SCC is accessible via the bus, and (apparently) the serial drivers access the SCC directly rather than via the serial IOP. This permits the MacRecorder software (and similar packages) to drive the SCC. It also eliminates the latency in XOFF processing (both of my serial devices work fine when my IIfx is configured in "compatible" mode). I've heard from a couple of sources (including, indirectly, the Apple Answer Line) that the IIfx Serial Switch cdev "is unnecessary, as its functions have been rolled into System 7." I've also heard, however, that it may be necessary to keep a copy of this cdev around "until sensitive software is updated." So... for those folks who know: - Just how has the functionality of this cdev been rolled into System 7? Is there a serial-driver call that an application can use, which means "Permit me to have direct access to the SCC"? - Has the problem with XOFF latency in "faster" mode been fixed in System 7 Golden? [It still exists in 7.0FC1] - If I continue to use my copy of this cdev to configure my IIfx in "compatible" mode, will my SCC turn into a newt once I install System 7 Golden? -- Dave Platt VOICE: (415) 813-8917 Domain: dplatt@ntg.com UUCP: ...apple!ntg!dplatt USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303