Hi All, A buddy of mine got a new computer, replacing his old 486. He uses his computer's serial port to "talk" to older equipment and now he's having problems. His new computer has a PCI card with a serial port and I told him it may be that the PCI (who's bus doesn't support -12volts) has less of a voltage peak to clearly send the data like his old 486 machine. His cables are kinda long and I figure he was pushing it even with his old computer and now that his serial is only +/-5v he can't quite get the data across. Is there a clear line in history of computers when this change from -12v to -5v was crossed in serial ports on PC's? I told him that any computer with ISA slots, even if the serial port was on the motherboard would support the -12v serial port he needs. Am I way off base? Thanks for any input, Bart
On Jun 1, 7:38 pm, "Bart" wrote: > Hi All, > A buddy of mine got a new computer, replacing his old 486. He uses his > computer's serial port to "talk" to older equipment and now he's having > problems. His new computer has a PCI card with a serial port and I told him > it may be that the PCI (who's bus doesn't support -12volts) has less of a > voltage peak to clearly send the data like his old 486 machine. His cables > are kinda long and I figure he was pushing it even with his old computer and > now that his serial is only +/-5v he can't quite get the data across. > Is there a clear line in history of computers when this change from -12v > to -5v was crossed in serial ports on PC's? I told him that any computer > with ISA slots, even if the serial port was on the motherboard would support > the -12v serial port he needs. > Am I way off base? > Thanks for any input, > Bart
I believe the original spec for RS-232 levels was minimum +/- 3v. Calling Jim Thompson for enlightenment. I'm surprised about the +/- 5 thing. I thought most everybody used a MAX232 (or knock off) to make
+/- 10 from the 5 volt supply.
ISA slot? Where would you find one of those? I needed an ISA slot for an EPROM programmer 4 or 5 years ago and the best I could do was a late 90's Biostar board on eBay for $35. I also keep a Pentium 166 machine and a K6-2 550 'just in case'
I keep ISA-based machines around, mostly because it is very easy for me to design and wire up a custom ISA board that works and ... very difficult for a hobbyist like me with reasonable tools at hand to do that for PCI. I just don't have the stuff I need for reflection wave technology. So ISA is important to me.
The ISA, once PCI came about, was supported by the south bridge. The simpler ISA transactions can be popped over to the PCI bus okay, but ISA DMA in particular has no equivalent on the PCI bus and there is no way to map it correctly without "sideband" channels to let the main chipset know what in the hell is going on. A PCI-to-ISA adapter, if it doesn't also have access to sideband wires, won't be able to do all DMA transactions, for certain. And if the motherboard wasn't designed to handle ISA at the start, it's likely those lines aren't available in a special connector and it's also likely the BIOS can't set things up. If a BIOS is included in the adapter, it will itself need to contend with a wide variety of possible motherboards, which also seems "hard" or "unlikely," too. I'd just assume "no ISA DMA" in that case.
My vague memory included that figure, but also a few others, too. Like
3k-7k ohm for the receiver resistance. But I also remembered a minimum 5V thing and I was confusing it in my mind with the 3V you mention above. So I looked up the RS-232-C spec. Turns out it is more interesting than that:
(1) The open-circuit output voltage of the driver cannot exceed 25V. (2) The driver must be able to sustain a short circuit to any other wire in the cable without damage and the short circuit current to any other wire cannot exceed 0.5 amps. (3) Signals are MARK, "1", when the cable pin voltage is < -3V and SPACE, "0", when the cable pin voltage is > +3V and everything else in between is "not defined." (4) The receiver's load resistance must be < 7k ohm when any voltage from 3V to 25V is applied BUT also > 3k ohm for any voltage < 25V (which includes the 0V to 3V range, as well.) So my recollection of the 3k-7k seems about right. (5) While it is true that the MARK and SPACE have the voltages defined in (3) above, a driver hooked up to a receiver meeting the loading in (4) above must present at least +5V and no more than +15V (for a SPACE) or at least -5V and no more than -15V (for a MARK.) (This is to provide the minimum of 2V noise margin.) (6) The slew rate cannot exceed 30V/us.. except when transitioning through the "not defined" region where it must take no more than 1 ms of time or no more than 4% of the bit time, whichever is smaller. (7) The shunt capacitance of the receiver shall not exceed 2500pF -- and that includes the cabling from the driver, too. (8) The impedance of the driver, powered off, shall exceed 300 ohms.
I think the -D spec is about the same, except that the timing through the "not defined" region has a few extra cases -- none of which materially affect much.
I tossed most of my reference material a few months ago, but as I recall, Mark is negative and Space is positive. Would you please verify this in your document?
RS-232 was +- 20V. It was used as a Standard, but was always a Recommended Standard, thus the RS.
12+/- was very common voltage to use and many devices used the power from the port to power them self. Many devices were made to force the output to 12+/- line level drive voltage from a serial port to ensure proper operations normally they spec it as 2.5 Volts +/- as the threshold for on/off with
0 volts as undefined. If you look up the spects on RS-232 line levels for older equipment, you may find voltages higher than 12 volts. 15 volts rings a bell to me. at work, we have a interface box that must be placed between the serial port and PLC to boost the voltage when used on a laptop. These old units required 12+/- volts like found in most PC's that had real serial ports. When using some PC's this box isn't needed. There are some Laptops that have higher voltage serial ports for industrial use. they work fine with out the aid of this helper unit.
P.S. the output is normally inverted and this is where the level of output was generated. You may want to test the voltage on it.
--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Outside of my parts box, I haven't seen a 1488 or 1489 in a PC for quite some time. They are though, if nothing else, nice little space heaters. I forget which of the two, but one of them runs almost too hot to touch long.
I don't have the actual spec, itself. So I'm interested.
I think I might. You can write to me at the email address that goes out with my posts, if you want to let me know what else there may be or otherwise work out details.
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