henry@ZOO.TORONTO.EDU (07/15/90)
>>absolutely not the case. In fact it is a TTL port where the signals >>vary from 0 to +5V and not -6V to +6V as RS-232 stipulates. This is no > ^^^^^^^^^^ >The last time I looked at the EIA RS-232 spec, it gave a range of >-3V to -20something and +3V to +20something... Let us be precise about this. RS-232C requires that a transmitter send at least +-5, with an upper limit (when connected to a receiver) of +-15. Receivers are required to function properly with an input anywhere between +-3 and +-25. The 2V difference on the low end is for noise margin; the 10V on the high end is because a very obsolete version of the standard (RS-232B) permitted transmitters to send as much as +-25. RS-232C receivers are required to read 0V on the input as negative, so that an unconnected input will look idle. This is usually done by placing the "positive/negative" breakpoint at +1V or so. Under no circumstances, except while in transition between positive and negative, is an RS-232C transmitter to send anything between -5 and +5. Unfortunately, because of the unconnected-input provision, most RS-232C receivers will read a TTL low (typically a few tenths of a volt positive) as negative. And although the usual TTL power supply can be anything between +4.75 and +5.25 -- the lower half of that range not being a legal RS-232C voltage -- most RS-232C receivers will read anything in that range as positive. So if cables are short and noise is low and the phase of the moon is right, you can use TTL levels to talk to an RS-232C receiver. Of course, if cables get long or other complications intervene, forget it. (One complication that can intervene is RS-423, a successor to RS-232C used by Sun and others. It is pretty well compatible with real RS-232C, but not with kludges like this.) Alas, generating real RS-232C voltages used to be a considerable pain in digital systems that otherwise ran on just a TTL power supply, so some manufacturers -- especially in the cost-sensitive and quality- insensitive PC market -- cut corners. Now there are transmitter chips that take a TTL supply voltage and do the voltage generation right, praise Allah, but there is a lot of old hardware still being sold. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry