gandrews@netcom.COM (Greg Andrews) (05/24/91)
In article <1848@madnix.UUCP> zaphod@madnix.UUCP (Ron Bean) writes: > > I have an older Avatex 1200 which does not have a command-escape >sequence of any kind. The only time you really *must* have one is if you >don't have a DTR pin on your serial port (eg, Macintosh), and you need a >way to make the modem hang up. All other uses are optional. > > Most modems will hang up if you drop DTR, but I ran into one piece of >software that doesn't even try that, and insists that you must have some >kind of command-escape (although it doesn't have to be Hayes-compatible). >The program is Compu$erve's CIM ("Compu$erve Information Manager"), which > Hi Ron, several people have asked "why use the +++ escape when you can use a Break or DTR instead?", and I'd like to bring up a point about that. Your article was just the latest one I've seen on the topic - I'm not really replying to your points directly. Probably the major reason for having a character-based signal to the modem rather than using DTR or a Break is two-fold, and it's apparent when you consider the state of modem communications when the Hayes modem was being developed (mid-to-late 70's?): a) The same reason async communications became so popular in the first place. You only need three wires in your cable and the circuitry for async is correspondingly simpler and cheaper to design in computer equipment. b) The entities who needed to grab control of the modem were people and not machines, thus the mechanism needed to be something a human could perform while using any of the popular terminals of the time. In short, if your terminal doesn't let you manipulate DTR or a Break signal comfortably, then those methods will be useless. That was a very common situation with terminals in those days. If you can't use an RS232 control signal or a Break signal, then you're stuck with the normal keyboard characters available on standard terminals. Therefore the choice of the plus sign as the default character, and the need for an unusual interaction to signal your desire to chat with the modem. >zaphod@madnix.UUCP (Ron Bean) -- .------------------------------------------------------------------------. | Greg Andrews | UUCP: {apple,amdahl,claris}!netcom!gandrews | | | Internet: gandrews@netcom.COM | `------------------------------------------------------------------------'
peter@ficc.ferranti.com (Peter da Silva) (05/24/91)
In article <1991May24.075247.11534@netcom.COM> gandrews@netcom.COM (Greg Andrews) writes: > b) The entities who needed to grab control of the modem were people and > not machines, thus the mechanism needed to be something a human could > perform while using any of the popular terminals of the time. I have never seen a system that was unable to generate a 250 ms break. I have seen few, if any, terminals that did not have a break key. Certainly the most common DTE devices of that period, the Lear Seigler ADM3 family and the 8080 and 6502 based PCs, had the capability. The only problem with the use of the break signal was terminal emulator software. -- Peter da Silva; Ferranti International Controls Corporation; +1 713 274 5180; Sugar Land, TX 77487-5012; `-_-' "Have you hugged your wolf, today?"
steve@pro-hindugods.cts.com (Steve Fenwick) (05/26/91)
In-Reply-To: message from gandrews@netcom.COM Is there any way to drop the DTR signal from BASIC (on an Apple) using a command or routine? Steve ------- CoSysop: pro-hindugods