thad@cup.portal.com (Thad P Floryan) (10/22/90)
A few weeks ago a discussion concerning UPS and SPS (Uninterruptible and
Standby, respectively, Power Systems) appeared in this newsgroup.
I commented that one should NOT pull the wall plug (of the UPS) to test it,
and was asked "why?". I commented "because you lose the protective ground."
Some people disagreed with that comment. Please consider the following:
The purpose of a protective ground is to provide a safe circuit path as a
"crowbar" for any dangerous anomalies in electrical equipment. The ground
is connected to the metal chassis of equipment (e.g. the UNIXPC) and it, not
you, is supposed to provide the "path of least resistance" for stray volts.
Sadly, many people ignore that, operating their systems with a 3-to-2 prong AC
adapter, and also ignore pin 1 ("FG (Frame Ground) Protection circuit.") on
their RS-232 connections.
If you have RS-232 device(s) connected to the system, or even if you have a
phone line connected to the RJ-11 jacks on the UNIXPC, you have "other" stuff
in the loop which, if faulty, could present hazardous voltages OR voltage
(potential) differences, especially if the devices are connected to separate
AC circuits (e.g. different breakers).
I remember 12 years ago in a new office building that everytime during the
first several days when I turned on my terminal, the computer in the other
office building would crash; problem was traced to NO ground in the AC outlet
in ONLY my office. Thus, rather than the normal +/- 12V on the RS-232 lines,
there was some 70V on the line with respect to the computer on the other AC
power circuit; the RS-232 line length was some 500' measured with my TDR
equipment (and operated fine at 9600 baud after the AC outlet was grounded
(simply required the building maintenance guy to connect the conduit to the
in-the-wall box)).
For reference, the no-ground condition was discovered and located by my
purchasing a "SNAPIT Circuit Tester" Model 49662 at the local True Value HW
store. This little gem (approx $5, and about the size of a 3-to-2 prong AC
adapter) has three lamps which indicate:
R G G (R=Red, G=Green)
- - -
. * . open ground
. . * open neutral
. . . open hot
* . * hot/ground reversed
* * . hot/neutral reversed
. * * correct
Consider, too, the fact that SOCKETS have their contacts recessed for safety
because they present voltages, whereas PLUGS, with their exposed prongs, are
normally "dead" and receive voltage (~ server:client relationship :-)
HOWEVER, a UPS is generating AC independent of your local utility; I don't
want to be holding a plug thereof and, say, sneeze and drop the plug into my
lap or onto other equipment; can you fantasize "zzzzZAP! *P*O*W*" ? :-)
To be fair, a UPS is supposed to isolate itself from the power line (and the
plug), but suppose the UPS is defective? I simply don't want to take the
chance, and most mfrs of UPS/SPS systems DO state that one should NOT test
the unit by pulling its plug from the wall socket.
My systems are properly grounded at ALL TIMES, all my RS-232 connections have
pin 1 connected straight through, and my hands are NOT charred stumps. :-)
Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]dold@mitisft.Convergent.COM (Clarence Dold) (10/23/90)
in article <35147@cup.portal.com>, thad@cup.portal.com (Thad P Floryan) says: > My systems are properly grounded at ALL TIMES, all my RS-232 connections have > pin 1 connected straight through, and my hands are NOT charred stumps. :-) On other Convergent Systems designed in the 3B1 era, pin 1 was not a good place to attach the drain from the shield on an RS232 cable. The chassis and logic ground are separate, "stitched" together with a number of capacitors. All of that shielding metal is to keep the zaps on the outside. Tying the shield to pin 1 allows the zaps to sneak inside, even if only for an inch or two before they go to chassis ground (separate from logic ground). Much better to tie the shield to the metal body of the connector, which must always be secured to the chassis with screws. Some connectors have a separate tabe for attaching the shield. Otherwise, it must be tack-soldered. -- --- Clarence A Dold - dold@tsmiti.Convergent.COM (408) 435-5293 ...pyramid!ctnews!tsmiti!dold FAX (408) 435-3105 P.O.Box 6685, San Jose, CA 95150-6685 MS#10-007
murphyn@motcid.UUCP (Neal P. Murphy) (10/25/90)
dold@mitisft.Convergent.COM (Clarence Dold) writes: >On other Convergent Systems designed in the 3B1 era, pin 1 was not a good >place to attach the drain from the shield on an RS232 cable. >The chassis and logic ground are separate, "stitched" together with a number >of capacitors. All of that shielding metal is to keep the zaps on the >outside. Quite true. But I though that pin 1 was chassis ground and pin 7 was logic ground on RS232c. Granted, the little pin may not handle lots of current, but it is supposed to be part of the shielding connection. NPN
thad@cup.portal.com (Thad P Floryan) (10/26/90)
Regarding the discussion of UPS/SPS and RS-232 signals re: grounding, four (4)
people (3 in email, one publicly) have commented that PIN 1 on an RS-232
circuit should not be connected straight through, claiming that a shield should
only be grounded at one end (of the shield).
Wondering how THAT issue arose, I re-read TWICE my original posting, and
NOWHERE did I address shielding nor imply that RS-232 pin 1 was a shield.
OK, this is not April 1 and I'm positive my fingers didn't type something other
that what I consciously composed, so I figure this must be "Let's See How Far
We Can Confuse Thad Week". :-) :-)
Regarding RS-232, there are TWO pins that can be called "grounds." As I
clearly stated in the original posting, pin 1 is a protective ground, and pin
7 is a signal ground.
A shield, if used, connects the metal cover of an RS-232 connector housing to
the foil or braid shield of a data cable, and should be connected at ONE end
only.
'Sfunny, in nearly 3 decades of working with datacomm equipment, not once have
I used a shielded cable, and I operate 9600 or 19200 baud at distances that
would amaze you, especially considering the hostile environments in which I
typically operate. Consider, for instance, that RFI is such a problem for
radios at my "site" that I literally have to pull my car out of the garage some
20 feet before the FM car radio will work; lucky for me I operate in a location
where homes are not close together. :-)
One person's email commented about ground loops. Yes, that is a real
phenomenon, and when I worked for the Electronic Defense Labs ground loops were
something to avoid like the plague. But, with all the equipment I have
operating presently (such as V.32/V.42 modems, printers, etc.), ground loops
simply are not causing any problems, and I still connect pin 1 straight through
on ALL RS-232 connections to satisfy the requirements of a PROTECTIVE (for
humans) GROUND.
The documentation presented below is a flattened-from-WordPerfect copy of the
material from one of the Appendices of a manual describing one of my hardware
products for which I did all the hardware and software design and PC layout;
it looks much better typeset and printed on a laser printer, and I hope all
the control characters have been removed. And, yes, I also wrote the manual
and it's copyright 1987, but I feel the material (below) will benefit those of
you who are reading this now; the information is derived directly from AT&T
and CCITT documents and from the product itself (whose name is camouflaged
with XXXXXXX since this is not a commercial message).
I hope the following serves as a definitive reference for RS-232-C signals
when you're in doubt as to the meaning of some of the common pinnings. Please
note that I have NOT used the word "shield" and that RS-232 pin 1 is clearly
described as being a PROTECTIVE GROUND; see the descriptions for pin 1
(protective) and pin 7 (signal) grounds.
Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com ]
-------------------- begin included material --------------------
RS-232-C SIGNALS
NOMENCLATURE
DCE - Data Communication Equipment, typically a modem; also XXXXXXX's
HOST port which emulates a modem.
DTE - Data Terminal Equipment, typically a host computer or CRT terminal;
also XXXXXXX's MODEM port which emulates a host computer.
SIGNAL STATES
Transmitted and Received Data signals are considered in the marking state
when the voltage on the circuit is more negative than minus three volts with
respect to signal ground, and in the spacing condition when the voltage is more
positive than plus three volts with respect to signal ground. All control and
timing functions are considered ON when the voltage on the circuit is more
positive than plus three volts with respect to signal ground, and are
considered OFF when the voltage on the circuit is more negative than minus
three volts with respect to signal ground. The Data Terminal Ready [DTR](CD)
circuit is "fail-safe" in that a power off condition in the terminal equipment
or a disconnection of the interface cable is interpreted as an OFF condition.
The data, timing and control functions are defined below:
Voltage: NEGATIVE (-) POSITIVE (+)
Binary state: "1" "0"
Signal condition: MARK SPACE
Control and timing function: OFF ON
TERMINATOR IMPEDANCE
The terminating impedance at the receiving end of interface circuits has a
resistive component of not less than 3000 ohms nor greater than 7000 ohms over
the range of voltages for which the signal is defined.
RISE AND FALL TIMES
For control circuits, the time required for a signal to pass through the
transition region of minus three to plus three volts during a change of state
does not exceed one millisecond.
For Received Data [RD](BB), the time required for the signal to pass
through the transition region is greater than one microsecond but does not
exceed 20 microseconds.
OPEN CIRCUIT VOLTAGES
Open circuit driver voltage on any circuit does not exceed +/- 18 volts
with respect to signal ground. When the terminating impedance is between 3000
and 7000 ohms, and the terminator open circuit voltage is zero, the potential
at the point of interface is not less than +/- 5 volts or greater than +/- 15
volts. The terminator on an interface circuit is designed to withstand any
input signal within the +/- 25 volt limit.
Pin Name, Abbreviation, EIA Circuit, CCITT Circuit
1 PROTECTIVE GROUND, FG, AA, 101
A protective (frame) ground circuit is provided by means of the ground wire
on the power cord. This also provides grounding of the chassis. The Signal
Ground circuit (pin 7) is the common reference point for all circuits on the
interface. The Signal Ground and protective grounds are tied together
internally. This arrangement is intended to provide additional margin against
longitudinal power line noise.
2 TRANSMITTED DATA, TxD, BA, 103
Signals on this circuit are generated by DTE and received by DCE. A
positive signal is considered a binary "0" or SPACE and a negative signal is
considered a binary "1" or MARK. The transmitting terminal should hold circuit
[TxD](BA) in the marking state when no data are transmitted, including
intervals between characters.
3 RECEIVED DATA, RxD, BB, 104
Signals on this circuit are generated by a local DCE and received by a DTE
typically in response to signals transmitted by a remote DCE.
4 REQUEST TO SEND, RTS, CA, 105
Signals on this circuit control the data channel transmit function of the
DCE. The ON condition causes the DCE to assume the data channel transmit mode.
The OFF condition causes the DCE to assume the data channel non-transmit mode,
when all data transferred on [TxD](BA) have been transmitted.
5 CLEAR TO SEND, CTS, CB, 106
An ON condition of [CTS](CB) indicates to DTE that a local DCE will
transmit to a remote DCE any data present on the Transmitted Data [TxD](BA)
circuit. If [CTS](CB) is OFF, a local DCE will internally clamp [TxD](BA) to
mark and ignore signals presented on [TxD](BA) by DTE.
[CTS](CB) will be OFF whenever Data Carrier Detected [DCD](CF) is OFF.
6 DATA SET READY, DSR, CC, 107
Signals on this circuit originate from a local DCE to indicate its status.
The ON condition indicates that a DCE is in the data mode and is connected to
the communication channel (telephone line). The ON condition of this circuit
alone should not be interpreted to mean that a communication channel has been
completely established or used to determine the status of any remote DTE.
7 SIGNAL GROUND, SG, AB, 102
This circuit establishes the common ground reference for all interface
circuits. This circuit is normally connected to protective ground (pin 1) to
minimize noise into electronic circuitry.
8 DATA CARRIER DETECTED, DCD, CF, 109 (RECEIVED LINE SIGNAL DETECTED)
The ON condition of this circuit indicates that data carrier is being
received by a local DCE and has been received for at least 155 +/- 50
milliseconds. This circuit will go OFF if the received data carrier falls
below the receiver threshold for more than 17 +/- 7 milliseconds. During the
time that [DCD](CF) is OFF, the received data circuit [RxD](BB) is clamped to
the marking state.
9 ASSERTED POSITIVE (+) VOLTAGE
10 ASSERTED NEGATIVE (-) VOLTAGE
Pin 9 provides +12 volts through a 1KOhm resistor for testing purposes and
for asserting signals at the other circuits of the interface. Pin 10 provides
-12 volts through a 1KOhm resistor for testing purposes and for asserting
signals at the other circuits of the interface. These pins should not be used
by other equipment nor should any extraneous voltages be connected to these
pins.
12 SPEED INDICATION, SI, CI, 122
This circuit will be in the ON state when a local DCE is in the high-speed
mode (e.g. 1200 baud for 212A compatible modems). At all other times this
circuit is in the OFF state. Regarding 212A compatible modems, this circuit's
state does not necessarily correspond with the state of a speed select switch,
particularly with respect to an answering modem which automatically adapts to
the speed of the originating modem each time the data mode is entered (ignoring
its own speed selector).
14 SECONDARY TRANSMIT DATA, STxD, SBA, 118
This circuit is an auxiliary output channel, presently uncommitted, that
will be used by future XXXXXXX options. This circuit is connected only at
XXXXXXX's HOST port.
16 SECONDARY RECEIVE DATA, SRxD, SBB, 119
This circuit is an auxiliary input channel, presently uncommitted, that
will be used by future XXXXXXX options. This circuit is connected only at
XXXXXXX's HOST port.
20 DATA TERMINAL READY, DTR, CD, 108.2
This circuit provides a means for DTE to control the connection of a local
DCE onto the communication channel. The ON condition of this circuit is
necessary to maintain the connection of a DCE to the communication channel.
The ON condition must also be present before a local DCE will enter the data
mode either manually or automatically. If this circuit is turned OFF for more
than 50 milliseconds during a data call, the local DCE will enter an
irreversible disconnect sequence. For automatic answering applications, the
presence of an OFF condition on this circuit does not inhibit the operation of
the Ring Indicator [RI](CE) circuit on pin 22. This circuit must be turned ON
in order for a local DCE to automatically answer an incoming call.
22 RING INDICATOR, RI, CE, 125
The ON condition of this circuit normally indicates that a ringing signal
is being received by a local DCE. The ON condition is approximately
coincident with the ON segment of the ringing cycle. DCEs with answer mode
indication options installed will also maintain the ON condition of this
circuit after the incoming call has been answered and will remain ON as long
as there is an off-hook condition of the telephone line; when the call is
terminated and the telephone line is on-hook, this circuit will turn OFF.
23 SPEED SELECT, SS, CH, 111
This circuit is used on an optional basis to establish the speed mode of a
data call at the originating DCE. When the circuit is implemented, an ON
voltage from DTE will cause the data call to be established in the high speed
mode (e.g. 1200 baud for a 212A compatible modem). An OFF condition will
cause the connection to be established in the low speed mode.
-------------------- end included material --------------------friedl@mtndew.Tustin.CA.US (Stephen Friedl) (10/27/90)
In article <35147@cup.portal.com>, thad@cup.portal.com (Thad P Floryan) writes: > A few weeks ago a discussion concerning UPS and SPS (Uninterruptible and > Standby, respectively, Power Systems) appeared in this newsgroup. > > I commented that one should NOT pull the wall plug (of the UPS) to test it, > and was asked "why?". I commented "because you lose the protective ground." Thad's comments on safety are well taken, but there are other reasons why the pull-the-plug test is not so hot: it's not a fair test and it may lie to you because pulling the plug is not the typical way that power goes out. If you just pull the plug, the system just sees an open circuit on the input, and it has the normal notification of the power loss. If the power goes out for real, however, the entire building is connected to the power cord of the supply, effectively providing a dead short. For some makes of UPS (Safe, Datashield, Tripplite, and Topaz, for instance), the backup supply is connected to this dead short for a brief moment before the disconnect switch kicks in to break the connection with the outside world. This can introduce an additional delay of what is claimed to be from 20 to 50%. Other brands use different switching mechanisms such that the pull-plug test will give an accurate representation of how the whole thing will work. Disclaimer: I got this information from a technical manual provided by American Power Conversion, a vendor of UPS products, so they are not unbiased. I like their products, their technical documentation is superb, but they could be faking these photographcs of scope traces. For what it's worth, though, my experience has been that Datashield has a BIG difference between the two modes of power loss -- they're pretty miserable. Steve -- Stephen J. Friedl, KA8CMY / I speak for me only / Tustin, CA / 3B2-kind-of-guy +1 714 544 6561 / friedl@mtndew.Tustin.CA.US / {uunet,attmail}!mtndew!friedl "Read my lips: KNOW New Taxes" - George Bush (OK, so I took it from Playboy)
gil@limbic.ssdl.com (Gil Kloepfer Jr.) (10/29/90)
In article <554@mtndew.Tustin.CA.US> friedl@mtndew.Tustin.CA.US (Stephen Friedl) writes: >If the power goes out for real, however, the entire >building is connected to the power cord of the supply, >effectively providing a dead short. For some makes of UPS (Safe, >Datashield, Tripplite, and Topaz, for instance), the backup >supply is connected to this dead short for a brief moment before >the disconnect switch kicks in to break the connection with the >outside world. Actually, this is seems to be correct. When I pulled the plug on my Tripp-Lite 'UPS' a few weeks to 'test' it, I accidentally touched the prongs on the AC plug, and got a small shock from it. It didn't seem like enough to do any bodily harm (but who am I to tell?). It would seem to me that there is a small amount of leakage back through the AC plug when the supply is on battery power. I know that this isn't good -- in fact, I think it is illegal. Does anyone have any specific measurements of just how much juice the Tripp-Lite supply sends over the AC line when it is on battery power? I'm not about to attempt testing it with my fingers again ;-) -- Gil Kloepfer, Jr. gil@limbic.ssdl.com ...!ames!limbic!gil Southwest Systems Development Labs (Div of ICUS) Houston, Texas
dave@westmark.WESTMARK.COM (Dave Levenson) (10/29/90)
In article <727@limbic.ssdl.com>, gil@limbic.ssdl.com (Gil Kloepfer Jr.) writes: > In article <554@mtndew.Tustin.CA.US> friedl@mtndew.Tustin.CA.US (Stephen Friedl) writes: [ regarding the Tripp-Lite UPS, with the power cord disconnected... ] > Actually, this is seems to be correct. When I pulled the plug on my > Tripp-Lite 'UPS' a few weeks to 'test' it, I accidentally touched the > prongs on the AC plug, and got a small shock from it. It didn't seem > like enough to do any bodily harm (but who am I to tell?). It would > seem to me that there is a small amount of leakage back through the > AC plug when the supply is on battery power. Did you get this shock immediately as the plug came out of the wall socket, or some time later? > I know that this isn't good -- in fact, I think it is illegal. Does > anyone have any specific measurements of just how much juice the > Tripp-Lite supply sends over the AC line when it is on battery power? > I'm not about to attempt testing it with my fingers again ;-) I just unplugged my Tripp-Lite BC450 from the wall. (I'm posting this article on backup-power!) I find that I can touch the plug and feel nothing. A volt-meter across the power pins on this plug reads zero. Could your Tripp-Lite be a different, or older, or defective model? I can imagine a failure-mode in which the relay fails to disconnect the primary power from the load when it connects the secondary power to the load, but I would expect this to cause the whole thing to oscillate...as the secondary power would trick the sensor into thinking the primary power had been restored. Anybody at Tripp-Lite on the NET want to comment here? -- Dave Levenson Internet: dave@westmark.com Westmark, Inc. UUCP: {uunet | rutgers | att}!westmark!dave Warren, NJ, USA AT&T Mail: !westmark!dave [The Man in the Mooney] Voice: 908 647 0900 Fax: 908 647 6857
murphyn@motcid.UUCP (Neal P. Murphy) (10/29/90)
dave@westmark.WESTMARK.COM (Dave Levenson) writes: >In article <727@limbic.ssdl.com>, gil@limbic.ssdl.com (Gil Kloepfer Jr.) writes: >... >model? I can imagine a failure-mode in which the relay fails to >disconnect the primary power from the load when it connects the >secondary power to the load, but I would expect this to cause the >whole thing to oscillate...as the secondary power would trick the >sensor into thinking the primary power had been restored. No, it wouldn't oscillate. The load on the rest of the grid would suck the output voltage down to almost zero (the previously mentioned short). The sensor would not detect a power restoration, and the backup power would last a few seconds and probably get extremely hot. It might oscillate if one yanked the plug from the wall, though. NPN
gil@limbic.ssdl.com (Gil Kloepfer Jr.) (10/30/90)
In article <1349@westmark.WESTMARK.COM> dave@westmark.WESTMARK.COM (Dave Levenson) writes (in response to my article): >Did you get this shock immediately as the plug came out of the wall >socket, or some time later? I don't remember... >I just unplugged my Tripp-Lite BC450 from the wall. (I'm posting >this article on backup-power!) I find that I can touch the plug and >feel nothing. A volt-meter across the power pins on this plug reads >zero. Just to be a little more specific about the posting, after reading this one, I put a meter on the plug of the Tripp-Lite with my system on backup power. The (AC) voltage across the hot and neutral are about 1.9 volts (nothing really terrible) *BUT* the voltage from the hot to ground and neutral to ground are around 60 volts (each!). That would definitely be enough to give me the tingle I felt when I grabbed the plug! >Could your Tripp-Lite be a different, or older, or defective >model? Let's compare notes, okay-- I have a Tripp-Lite BC-450-b rev 790. I just bought it about 3 weeks ago, so my feeling is that it should be pretty well along in the testing stage from Tripp-Lite. I can think of one additional failure mode not mentioned -- I'm running the following devices from this supply: AT&T 3B1 computer Telebit Trailblazer-Plus AT&T UNIX-pc expansion box Tape drive on a HD case/supply (turned-off) Could one of these devices be feeding garbage back through the ground? How does the Tripp-Lite UPS handle "ground"? I betcha that one of the devices could conceivably be creating an electrical path from neutral to ground, as I have been bitten by that once already. This is an interesting discussion - please take a look at your supplies with a meter and post your findings. I'd like to fix a problem before it fixes me! Gil. -- Gil Kloepfer, Jr. gil@limbic.ssdl.com ...!ames!limbic!gil Southwest Systems Development Labs (Div of ICUS) Houston, Texas
thad@cup.portal.com (Thad P Floryan) (10/31/90)
[apologies if this appears twice; PORTAL has a propensity to crash and/or hang precisely at the moment I post an article thus giving no "success" indication] adams@ucunix.san.uc.EDU (James Warner Adams) in <9010282317.AA08682@ucunix.san.uc.edu> writes in response to my comments about RS-232 pin 1 grounding: ``Pin 7 should NEVER be directly connected to pin 1. This is a hallmark of poor design. ... '' I've never felt "comfortable" about that practice either, but I took my cue from AT&T documents. Been selling from 10 to 50 of (my product) every month since early 1983, mostly to phone companies and US Gov't agencies. And the product easily qualified FCC and VDE certification because I used the techniques in "Digital Design for Interference Specification" (by The Keenan Corp.) The product now uses (among other chips) the MC68681 dual-UART, but is so clean electrically the plastic case (PacTec) did not require NiCr paint or other shielding. And pin 1 is internally connected to pin 7 on that product. You figure. Quien sabe? :-) ``[... other good material deleted here ...] Anecdotal evidence aside, good practice dictates that a single ground reference point be used when cabling.'' VERY TRUE! I learned that early in the game while designing low-noise microwave pre-amps for "government" applications. ``When installing shielded cables, the shield is generally connected BOTH to the hood AND to pin 1. There should not be an independent pin 1 wire running inside the shield. This defeats the whole purpose of the shield and invites interference.'' I'll take your word on that (above), but it was my impression a "shield" is an electrostatic barrier whereas the "pin 1 ground" is simply a protective drain. ``Thad, did it ever occur to you that perhaps your "site" is one REASON for the awful interference you mention?'' Yep, and the reason for my noisy site (8 computers, 6 modems, and misc. other stuff) is that I literally had to remove the RF shielding from one computer after an upgrade ... the shield would have electrically shorted out the new daughterboards I installed; this problem should soon be cured since yet another modification will permit the vertical clearance I need for re-installation of the shields. I've been harping on that manufacturer for over a year now and I should have new daughterboards "Real Soon Now." Fortunately, the site is located in a sparsely settled area and I've verified non-interference with neighbors' TVs, radios, pagers, cellular phones, and other equipment; beyond 30' laterally or vertically (yes, I climbed my TV tower as part of the test) the RFI is subdued. For what it's worth, the "offending" system is 68020-based. Encircling the "site" are StarLAN, Ethernet, and RS-232 "networks" stretching out 150', and the RS-232 operates just fine at 38,400 baud into two systems and 19,200 baud into all the others. ``Remember, data processing and communications equipment can radiate RFI as well as being affected by it.'' True. And it's NOT my intention to make light of anyone's studied comments regarding shielding and grounding. I hope these recent discussions have stimulated thought and triggered re-evaluation of everyone's site(s). Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]
floyd@hayes.ims.alaska.edu (Floyd Davidson) (10/31/90)
In article <35425@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes: > >adams@ucunix.san.uc.EDU (James Warner Adams) >in <9010282317.AA08682@ucunix.san.uc.edu> writes in response to my comments >about RS-232 pin 1 grounding: > > ``Pin 7 should NEVER be directly connected to pin 1. This is a hallmark > of poor design. ... '' > >I've never felt "comfortable" about that practice either, but I took my cue >from AT&T documents. Been selling from 10 to 50 of (my product) every month [text deleted] >other shielding. And pin 1 is internally connected to pin 7 on that product. >You figure. Quien sabe? :-) The point is don't connect them together at the cable connectors. Eventually they will come together in one way or another, but never at the cable. > > ``[... other good material deleted here ...] > > Anecdotal evidence aside, good practice dictates that > a single ground reference point be used when cabling.'' > >VERY TRUE! I learned that early in the game while designing low-noise >microwave pre-amps for "government" applications. That is where they come together. It might in fact be a site ground bus located across the room. > > ``When installing shielded cables, the shield is generally connected > BOTH to the hood AND to pin 1. There should not be an independent pin > 1 wire running inside the shield. This defeats the whole purpose of > the shield and invites interference.'' > >I'll take your word on that (above), but it was my impression a "shield" is an >electrostatic barrier whereas the "pin 1 ground" is simply a protective drain. I won't (above). The protective ground, pin one should be connected straight through on a single wire, inside the shield. The shield should be connected to the hood and pin 1 on one end only. The other end should be isolated. The protective ground is just that. And the shield should be just a shield too. Mix the two and the shield probably won't. > ``Thad, did it ever occur to you that perhaps your "site" is one REASON > for the awful interference you mention?'' > >Yep, and the reason for my noisy site (8 computers, 6 modems, and misc. other I bet that occured real fast! :-) :-) [ text deleted ] >Fortunately, the site is located in a sparsely settled area and I've verified >non-interference with neighbors' TVs, radios, pagers, cellular phones, and My site is in a fringe area, also "sparsely settled". I've had to pay *very* close attention to even where my TV antenna is located relative to my computer room. I don't aim the antenna for max signal, I aim for a sharp null toward all the EMI from the opposite corner of the house! I've never seen a published in depth technical discussion on the theory and practice of shielding RS232 cables, but I would suggest that reading any of the many that are available on audio. In particular the grounding of the shield at both ends with a relatively high impedance line is asking for a ground loop. That may or may not be bad enough to interfere with any given RS232 signal. But if you want to see if it does happen or not, try it with a high impedance input on your hifi set! Floyd -- Floyd L. Davidson floyd@hayes.ims.alaska.edu floydd@chinet.chi.il.us Salcha, AK 99714 connected by paycheck to Alascom, Inc. When *I* speak for them, one of us will be *out* of business in a hurry.