Date: Tue, 29 Oct 2002 10:55:17 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: <soralx@cydem.zp.ua> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: i am looking for a 5 volt signal Message-ID: <200210291855.g9TItHsq009943@apollo.backplane.com> References: <Pine.BSF.4.44.0210270918380.364-100000@blackbox.yayproductions.com> <3DBC42AF.C68F31B0@mindspring.com> <200210272207.g9RM76sX091839@apollo.backplane.com> <200210271935.12396.soralx@cydem.zp.ua>
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:The output signals don't go straight from the chip - do they? :I've seen few KOhms resistors on most boards for each output pin. : :27.10.2002; 19:32:00 :[SorAlx] http://cydem.zp.ua/ It depends on the chip. Most modern serial driver chips have series resistors on the outputs, inside the chip, and do not need any external current limiting (which is what the resistor does in effect). Parallel port chips typically do not have output resistors and instead depend on beefy TTL output stages, and a good motherboard manufacturer will put series resistors on those pins. All modern chips have diode protection on their outputs but you can still zap them (static shock it to death). Serial pins tend to be far, far more robust then parallel port pins due to having to go through level converters to get the right voltages on outputs and to be able to handle +/-12V on inputs. For example, most serial driver chips can handle up to +/-25V on their inputs before they blow up. Try putting even +12V on a parallel port pin and you will likely fry the chip, even with an external series resistor, and an external series resistor alone will not stop a big static shock, only a Zener diode is fast enough to do that. An external series resistor is there strictly for current limiting purposes. -Matt Matthew Dillon <dillon@backplane.com> To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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