Subject: Re: CANON off camera flash cord. Operation with 630. Any info?
X-Organization: Concordia University, Montreal, Quebec
I have been using Leung's one-piece cord with the EOS 630 for
quite some time now (about 1 year?), with very good results. The
device is well-built, from Canon parts, and has withstood
considerable knocking around in my camera bag. A strain relief is
included and appears to work well. When connected to the EOS 630
by Leung's cord, the 430EZ flash behaves as if it were mounted
directly on the hot shoe. The cord is expensive, but in my
shooting, it has been worth the cost.
The following is my original posting about the cord:
From: email@example.com (PETER SHIZGAL)
Subject: Canon EOS off-camera shoe cord
There have been several recent postings concerning the use of
Canon's off-camera shoe cord with EOS-series cameras.
Introduction of this cord lowers the cost of attaching a single
Speedlite to an EOS camera and provides better functionality than
the older multi-piece setup when only a single flash is used. The
older setup can accommodate multiple flashes, but at the cost of
sacrificing the preflash, A-TTL mode, flash head autozoom,
infrared AF auxiliary light, and TTL mode distance readout. These
functions share a data line that must be disconnected because of
the possibility of contradictory preflash distance readings from
I have contacted a senior technical rep at Canon USA regarding
the use of the one-piece off-camera shoe cord and have confirmed
that it will NOT work with the EOS 630 and RT. The cord works
fine with the EOS-1, 620, and 650. The cord will also work with
the EOS 10, Elan, and Rebel, but exceeds the FCC noise
I own an EOS 630 and a 430EZ Speedlite, so this wasn't good news
for me. Fortunately, a fellow netter, Paul Sandy, recommended
that I contact David S.H. Leung, a Canon (and Contax-Yashica)
specialist in Ilford, England, regarding a custom-built cord.
Using Canon parts, Leung manufactures a cord that allows the 430
EZ Speedlite to be connected off-camera to the EOS 630 while
maintaining full functionality. (Leung's rep won't comment on
whether on not the FCC noise spec is exceeded.) I have tested
this one-piece cord, and as far as I can determine, it behaves as
advertised. Simply put, the Speedlite appears to function as if
it were mounted on-camera.
I was pleased by the service provided by Leung's shop. I
contacted them by FAX and received a prompt and informative
reply. They somehow managed to ship the off-camera cord from
England to Canada in less than a week.
Now for the not-so-good news: The price is 68.00 sterling (about
$120 US) including tax and shipping. I would imagine that a low
volume of production and the use of Canon parts contribute to the
high price, which is more than twice the cost of the Canon
off-camera shoe cord (~ $50 US via mail order). However, the
Canon off-camera shoe cord won't work with the EOS 630 and RT, so
this comparison is moot for users of these cameras. The Canon
setup that will work consists of the TTL Hot Shoe Adapter II, an
off-camera shoe adapter, and a connecting cord. These items total
about $95 US via mail order, not including shipping. Moreover,
Leung's cord offers full functionality whereas the three-piece
Canon setup does not. Finally, it is possible that Leung would
agree to work with user-supplied components and would lower his
The person who answered my inquiries concerning Leung's cord is
Alan Burch. He can be reached care of
David S.H. Leung
33 Goodmayes Road
Ilford, Essex IG3 9UN
Tel. 81 590 3268
81 599 6657
FAX: 81 590 0293
The shop is open every day except Thursday and Sunday.
Exchange rates have changed since I posted this, and the price
may have changed as well.
Peter Shizgal CSBN, Concordia University Montreal, Quebec, Canada
Mike Coren managed to get some updated information:
From: firstname.lastname@example.org (Mike Coren)
Subject: Re: EOS 630 w/off-camera 430EZ (repost)
I called David Leung, the firm in Essex which Peter Shizgal
recommended as a source for an off-camera shoe cord for the
EOS-630 (see Canon EOS FAQ list version 1.65 Alpha, question 2.7,
"Can I get an off-camera flash cord for my 630/RT?"). The cord
is still available, and costs GBP 74.99. At the present exchange
rate (Jan 1993), that's about US $115. They can ship overseas,
and when doing so will subtract the 17.5% Value Added Tax (VAT)
but will also charge for shipping (the two usually cancel each
other out). They claim they've never had one go bad, but will
provide a one year warrantee. They take VISA and Mastercard but
not American Express. Their address is:
33 Goodmayes Road
Ilford, Essex IG3 9UN
011-44-81-590-3268 from the US (dunno about Canada).
When they first began selling the cables about five years ago,
they made them themselves using Canon parts. Now they take
Canon's off-camera shoe cord and use the same endpoints but
rewire it with a different cable in between. That's all they
would tell me without giving away any trade secrets! The person
I spoke to said they did tell Canon about this modification and
their success with it, but given that Canon is no longer making
the 600-series I wouldn't hold my breath waiting for it from
Q: Would it be better to replace the 630 with a 10s (or A2!) &
use Canon's cord?
That'll work, but it's a rather more expensive solution.
Michael D. Coren (email@example.com)
Canon has disabled the A-TTL preflash out-of-range warning on
all bodies after the EOS 1, 630, and RT.
Originally, Canon claimed this was done because it wasn't
accurate beyond 25 ft. For objects of low reflectance, the
flash would signal a false alarm. However, those who have used
this feature have found it very useful.
Canon now claims that the preflash exposure warning is disabled
due to a patent held by another company.
I am told that the patent which precludes the incorporation of
preflash exposure warnings also precludes after flash exposure
warnings. Thus, it is unlikely that this feature will be made
available through Canon. However, some 3rd-party flashes do still
offer after-flash exposure warnings.
Except for the EOS A2/5, all bodies in the EOS line require the
430EZ in order to support variable fill flash ratio for
shoe-mounted flashes. This includes the Elan which can vary the
fill-flash ratio, but only for the built-in flash.
For flashes without such controls, quite a lot of fiddling is
required. In programme modes the exposure compensation control
affects both flash exposure and ambient exposure so it is not
possible to use exposure compensation to control the fill-flash
ratio. To vary the fill flash ratio you can first use the manual
exposure mode to set the correct exposure level for the ambient
light. In this way you control the shutter speed (must be equal
to or slower than the maximum sync speed) and aperture manually.
You can then control the flash output by changing the film speed
setting, thus "fooling" the flash into giving more output (if the
film speed is lowered) or less output (if the film speed is
rasied). Don't forget to reset the film speed after you sre
This subject is covered in more detail in section 4A of the
Speedlite Reference Guide (for availability see section 1.1.)
Distance info is only useful so that the camera can apply the
"guide number" formula: f-stop = GN/distance. This is only a rough
approximation. Off-The-Film (OTF) metering measures the true
exposure of the film, which is much more accurate than the guide
In A-TTL, Canon uses a preflash to actively judge the effective
flash-to-subject distance rather than passively. This factors in
light loss from bouncing off different kinds of surfaces (for
example, flash modifiers), and light gains from bouncing off
different kinds of surfaces (like ceilings and walls). You simply
cannot make the remotest use the GN calculations for anything
but direct flash.
Personally, I avoid direct flash, and most light modifiers are
bound to void the GN calculations, either because of light loss,
or because they require the head to be up (like the Lumiquest
pocket bouncer). So for most shots, the distance information
would have to be thrown away.
It should be fine. TTL metering stands for "Through The Lens"
metering. So, no metering takes place from the flash sensor. The
sensor on the flash is only used for A-TTL metering. If you're
still worried, just shoot some test exposures to make sure [I'm
not sure if TTL exposure compensation works.]
The sensor on the flash is required for A-TTL metering. That's
when the 430 EZ emits a preflash, checks the return signal with
its own sensor, and then computes an aperture that will work.
It's best to think of the A-TTL preflash/sensor combination as a
kind of reflected light flash meter built into the flash itself
(and it's supposedly quite accurate too).
By blocking the sensor, the A-TTL program will definitely get
confused. Similarly, if the A-TTL supporting flash is on a flash
bracket it may give slightly different apertures if the sensor
isn't pointed in the right direction.
Some have tried fooling the A-TTL programme by blocking the
sensor and found that it stops down more instead of less! This
is because the IR preflash head is right beside it, and whatever
you're using to block the sensor is probably in front of the IR
emitter as well.
So basically, you've put a reflector in front of the
emitter/detector pair. If you carefully cover just the sensor,
you will get the anticipated result; the flash will try to select
the maximum aperture. The easiest way to avoid this is to tilt up
the flash head so that the preflash exits from the main flash
When you are using the TTL Hot Shoe Adaptor 2 with the Off-Camera
Shoe Adaptor, you lose A-TTL. The A-TTL assumes the flash is in
the camerashoe, or at least near it. It can be wildly inaccurate
when far off camera, so it is disabled when using the off-camera
TTL accessories. There is also the problem of multiple flashes
returning conflicting information on the data bus. [Again, I
don't know how flash exposure compensation is affected by this.
Canon confirm it doesn't work unless you are using an EOS A2/5.]
If you're using a modifier that goes over the main flash head
(filters, Sto-Fen Omnibounce, etc.), note that the preflash in
direct mode comes from the IR sensor below the main flash head.
This can confuse the A-TTL programme since the IR preflash will
not be filtered, giving inaccurate results.
By tilting the head up slightly, or using something to release
the bouncelatch, the 420EZ/430EZ will assume you are bouncing the
flash and will fire the preflash through the main head. This
should yield more accurate results. If the head is being tilted
up, perform tests to confirm evenness of flash coverage. The
flash may require manual zooming to a wider angle to give better
The A-TTL sensor, being in the flash and not in the camera may
yield bad apertures if filters are being used over the lens. In
this case, you'll have to find some way to put a similar filter
over the sensor.
In fiddling with my 430EZ, it seems as though it doesn't figure
the exposure compensation into the selected aperture. If you say
you want flash overexposure by 3 stops, it gives the same
aperture as selecting flash underexposure by 3 stops.
Note that in a TTL (or A-TTL) flash system auto flash exposure is
changed by changing the flash duration at a fixed apertures. Thus
you would not expect an aperture change. In a non TTL flash
exposure system, flash exposure is changed by changing the
aperture for a fixed degree of flash illumination. For more info
see the Speedlite reference guide, p57 and 71.
If we assume that A-TTL requires a separate sensor, then there
are a few reasons that Canon may choose to avoid putting the
sensor in the body itself:
Including an extra sensor would incur an extra cost on every
body manufactured. This could make Canon's bodies marginally less
competitive. This is especially true if the majority of Canon
owners do not wind up using A-TTL.
Cramming a 3rd sensor into the body would make the body more
complicated in order to split the optical path yet again in order
to feed the A-TTL sensor.
3) Dim viewfinder:
Bleeding off more light to another sensor would make the
viewfinder dimmer than it already is.
A-TTL was developed for the T-90 which did not have a
multi-segment metering sensor. Thus, it was unable to distinguish
foreground exposure from background exposure during the preflash.
Personally, I'm inclined to believe #4, and that Canon has simply
neglected to update its flash system. It is technically quite
feasible to use the existing evaluative metering sensor to
perform preflash exposure computations. [Rumour has it that
Canon is currently throwing around ideas for a new generation of
flash system. Cross your fingers, folks!]
The locking pin on the 430EZ is spring loaded (to be compatible
with bodies that didn't support locking pins). If you leave the
locking pin down at the same level as the hot shoe contacts when
the flash is not mounted, you can quickly and securely mount the
flash by just slipping it into the shoe. The pin will rise like
the contacts when being inserted into the shoe, and then pop down
into the little hole when the flash is properly seated. This
trick is especially useful for bodies without a built-in flash.
Canon publishes the "Speedlite Reference Guide". Stephen Page
posted the following summary:
From: firstname.lastname@example.org (Stephen Page)
Many thanks to someone (sorry, I've lost the posting!) who passed
on a tip about the Canon USA booklet, "Speedlite Reference Guide"
(Chuck Westfall, 1991). This is an excellent little book and it
contains some essential information which Canon have not bothered
to tell the world in their instruction manuals.
The booklet can be obtained from Canon USA, One Canon Plaza, Lake
Success, NY 11042, USA; telephone +1-516-488 6700. They sent mine
immediately and there was no charge.
I've summarized some key points below which may help others. Do
get the book, though; it's got lots more of the same. Also write
to Canon and blast them for a pathetic user manual... I cannot
see how anyone could work out how to use the flash correctly
without this book. My summary is valid for EOS-1/430EZ; other
models have slight differences. Early models of Speedlites work
[Anyone know if Canon have any other hidden reference guides
from national offices in the US or elsewhere? ]
1) Types of flash exposure.
In "normal" flash exposure, the exposure is calculated for the
foreground subject and the background is underexposed.
In "fill-in" flash exposure, the shutter speed and aperture are
set to expose the BACKGROUND correctly. The flash duration is
adjusted to expose the subject correctly. The Speedlites operate
in fill-in mode most of the time (see below).
2) Flash exposure and the camera modes.
In Program mode, the camera will use fill-in exposure provided
that the light level is EV10 or above. If the ambient light is
below EV10, the shutter speed will be locked at 1/60 sec and the
flash will work in "normal" mode, i.e. the background will be
In Aperture-priority (Av) or Shutter-priority (Tv) modes, the
camera will ALWAYS use fill-in exposure mode. The exposure will
be set to match the background (ambient light level), so if the
ambient light is low then a slow speed and/or large aperture will
3) Tinkering with the light levels.
When the ambient light is above EV10, the Speedlite's flash level
is reduced on a linear scale to a maximum of the equivalent of
1.5 EV steps less than the standard exposure level (at EV13 and
above). This is to preserve the natural lighting a bit, by not
blasting it away with flash.
4) Exposure compensation.
The exposure compensation controls on the camera affect BOTH
background and foreground exposure.
To adjust the foreground independently of the background, use the
control on the flash (430EZ) itself.
5) A-TTL vs TTL.
The Speedlites can be switched to use "TTL" rather than the
default "A-TTL" mode. Some flashes, e.g. ring flash ML-3, only
use TTL mode.
TTL mode disables the preflash (near-infrared in normal flash
head position, white in bounce position). This has the advantage
that it doesn't make the subject think that the picture has been
taken, and it doesn't set off any slave units. However, in TTL
mode you get no pre-exposure distance measurement, so the camera
can't scream if you are out of range. You can work it out by
using the distance scale, or by looking at the indicator light
AFTER the flash.
I hope this helps some of you to work out the mysterious
behaviour of your equipment!!
Stephen Page, email@example.com or uknet!aclondon!sdpage
From Chuck Westfall:
In a direct flash situation, the near-infra-red preflash located
below the main flash head is used with all current A-TTL
Speedlites (300TL, 300EZ, 420EZ and 430EZ). Since the intensity
of the preflash illumination is a known quantity, calculating
subject distance becomes a simple matter of comparing the level
of illumination returned to the external sensor of the A-TTL
Speedlite with the original amount. The bounce flash situation is
slightly different in that the preflash is emitted through the
main flash head, but the principle remains essentially similar.
The difference is that in a bounce flash situation, the external
flash sensor factors in the light loss caused by the increased
flash-to-subject distance as well as the amount of light absorbed
by the bounce surface. The net result is equally accurate. Please
keep in mind that the objective of the A-TTL preflash is to
measure flash-to-subject distance, which is not necessarily the
same as camera-to-subject distance.
From Chuck Westfall:
With the EOS 5, you can select the ambient light metering pattern
of your choice in all Creative Zone exposure modes except Program
AE, which cancels your metering selection and replaces it with
peripheral metering as with previous EOS models. This ability to
select ambient light metering patterns is a definite improvement
for the EOS 5 over earlier EOS cameras as well as the T90.
From Chuck Westfall:
The EOS 5 uses the same 3-segment sensor as the EOS 10s for its
TTL flashmetering. The EOS 5 designers elected to use the
3-segment TTL flash sensor from the EOS 10 mainly because it cost
less than developing a newer sensor, but also because they felt
it was quite adequate for its purpose given the layout of the
camera's 5 focusing points from left to right.
From Chuck Westfall:
During A-TTL or TTL flash photography with the EOS-1, ambient
light is measured by the peripheral metering pattern regardless
of the user-selected metering pattern, including center-weighted
average metering selected by CF8.
From Chuck Westfall:
A comparison of differences in communication between the flash
and the camera when multiple flash off-camera accessories are
used is covered on page 62 in the SRG in Section 2, fourth
From Chuck Westfall:
Unlike NiCd Pack TP, Battery Magazine TP is not equipped with a
fuse to prevent damage to Transistor Pack E which can be caused
by defective "C"-size NiCd cells.
From Chuck Westfall:
As a cost-reduction measure, the EF-M camera body does not
contain a TTL flashmetering sensor (or its associated circuitry).
Therefore, it depends on the external exposure control system
built into the Speedlite 200M for automatic flash photography.
Because of its lack of TTL flashmetering, the EF-M is not
recommended for use with the 200E, 300EZ or 430EZ EOS system
When the EF-M is used in either Program AE, Aperture-priority AE
or Shutter-priority AE with the 200M (with the 200M set for
automatic operation), it automatically locks its shutter speed at
1/90 and sets the lens aperture according to film speed (f/5.6
for ISO 100, f/8 for ISO 200, f/11 for ISO 400, etc.). These
settings are adequate for flash snapshots in low-light or indoor
shooting conditions, but they cannot be recommended for fill-in
flash photography in bright conditions if the correct exposure
for the existing light exceeds the camera's shutter
When the EF-M is set for manual mode and the 200M is set for
automatic, you can override the shutter speed setting in the
range from 30 seconds to 1/90 and you can select any aperture you
want. This gives you a limited capability for fill-in flash
photography. In a typical outdoor situation, your best bet is to
set the shutter speed to 1/90, and adjust the aperture until the
viewfinder's metering scale indicates correct exposure for the
existing light. However, please keep in mind that your effective
flash-to-subject distance range is controlled by the combination
of guide number, film speed and aperture setting. Here's a chart
showing the usable range (in feet) for a variety of film
speed/aperture combinations with the 200M:
Film Speed (ISO) f/2.8 f/4 f/5.6 f/8 f/11 f/16
100 23.5 16.5 11.8 8.3 5.9 4.2
200 33.0 23.5 16.5 11.8 8.3 5.9
400 47.0 33.0 23.5 16.5 11.8 8.3
Realistically, on a sunny day your aperture setting is likely to
be in the range between f/11 and f/16 at 1/90 second with ISO 100
film, thus producing an effective range of approximately 6 ft.
with the Speedlite 200M. This range can increase only if the
existing light level is reduced, such as by shooting in a shaded
area or perhaps on a cloudy day.
There is what might be described as a "bug" in the Elan. It seems
that above about ISO 2500, TTL flash may not work properly.
Things seem to be fine with all other EOS bodies (even The
Rebels) all the way up to ISO 6400. The behavior of the Elan is
not a malfunction in the sense that it cannot be "repaired"
either under warranty or in any other way. Not all Elans show
this behavior--it seems to be a problem in the design which must
be related to some component tollerance. Before you complain too
much however, note that many Nikon cameras (e.g. the 8008) have a
recommended ISO maximum of ISO 1000 for TTL flash and all Elans
meet or beat that!
[From Chuck Westfall]
USE OF NON-DEDICATED FLASH UNITS WITH EOS CAMERAS
Before using any unfamiliar studio strobe or other non-dedicated
flash unit with an EOS camera, we STRONGLY recommend that you
determine its trigger circuit voltage. Excessive trigger circuit
voltage can damage the shutter units and X-contacts of most
modern 35mm SLR cameras. Please be aware of the following
Trigger Circuit Voltage: The "trigger circuit" is the part of any
electronic flash that detects the X-sync signal from the camera
and then fires the flash. In order to do this, it supplies a
certain amount of voltage through the sync cord or hot shoe to
the camera's X-sync contact. The camera's X-sync contact, whether
in the form of a PC socket or a dedicated hot shoe, is connected
to the camera's shutter by a very thin connecting wire. When the
first shutter curtain reaches the end of its travel, a small part
of the shutter mechanism contacts the connecting wire, thus
completing the circuit. At that moment, a change in resistance is
detected by the flash's trigger circuit, causing the flash to
Canon dedicated Speedlites pass no more than 6 volts DC through
their trigger circuits. This is the level for which all EOS
cameras, including the EOS-1 as well as the A2/A2E, 10s, 630, RT,
Elan, Rebel etc. are designed.
GENERALLY SPEAKING, WE CANNOT RECOMMEND THE USE OF ANY STROBE
WHO'S TRIGGER CIRCUIT VOLTAGE LEVEL EXCEEDS 6 VOLTS.
Most independently-made strobes that are "dedicated for Canon,"
whether they are shoe-mount or handle-mount, have safe trigger
circuit voltage levels, but there are no guarantees. Trigger
circuit voltages up to around 20 or 30 volts DC are not
immediately dangerous, but they may damage the camera's X-sync
contact, connecting wire or shutter unit over a period of time
However, many non-dedicated flash units, especially older studio
strobe designs, use trigger circuits that may exceed 50 to 60
volts DC. This level is excessive for almost every currently made
SLR, not just Canon. Certain independent flash equipment
manufacturers (Lumedyne & Wein), in recognition of the problem,
now sell a "sync filter" (connected between the power pack and
the sync cord) that lowers the trigger circuit voltage to a safer
High voltage conducted through the camera's connecting wire can
also generate electro-magnetic interference or "noise" which can
have various unwanted effects on camera operation. These can
include metering errors, shutter speed errors, aperture size
errors, shutter release failure, etc. The effect varies according
to the camera model, because the interference can affect
different parts of IC chips according to the circuit layout.
It's easy to test any electronic flash for trigger circuit
voltage level using a standard voltmeter. Digital voltmeters are
relatively inexpensive and easy to find at stores like Radio
Shack. The plus (+) lead is attached to the positive contact in
the PC sync cord, while the minus (-) lead is attached to the
ground contact of the PC contact on the sync cord. With hot shoe
flash units, the plus lead should be placed on the + contact in
the middle of the hot shoe, while the minus lead should be
grounded on the hot shoe's ground terminal, usually located on
the side above the lock nut. Usually the voltmeter or multimeter
has several different settings for DC voltage depending on the
range you're trying to measure. With an unknown strobe, it's best
to start in the high range just in case.
Here is a list of known trigger circuit voltage levels for
independently manufactured flash units and studio strobes. (Most
of this data provided courtesy of Charles Tucek, Midwest Region
Pro Market Representative):
Date Brand Name Model Number Trigger Circuit
9/4/92 Calumet (Bowens) PS Series 30.0
9/4/92 Calumet (Bowens) Traveller Series 15.0
9/4/92 Comet CX244 11.0
11/6/92 Dynalite All Current Models 10.0
9/4/92 Metz 60CT4 5.0
9/4/92 Speedotron D604 Brown Line 64.8
9/4/92 Speedotron 2403CX Black Line 66.3
9/4/92 Vivitar 283* 5.0
*This is accurate only for 283s manufactured during or after
SAFE WAYS TO FIRE STUDIO STROBES WITH EOS CAMERAS:
There are several ways of firing a studio flash without using
a sync cord at all. One way that works well is to use a 420EZ or
430EZ on 1/32 power manual flash, while the optical slave trigger
of the flash is activated. By using the bounce feature of the 420
or 430, it's possible to direct the light away from the subject
and into the optical slave trigger.
If that's not convenient, various companies including Quantum
Instruments and Lindahl sell radio slave triggers consisting of a
transmitter that connects to the camera's PC contact (or PC
socket adapter), and a receiver that attaches to the studio
strobe's power pack. In addition to radio slaves, there are also
some IR slave devices that work basically the same way, but
require a direct line of sight between transmitter and receiver.
(Don't forget to test the slave transmitter for trigger circuit
voltage as described above.) Be aware that the use of radio or IR
slaves usually causes a small delay in flash firing that can
cause problems when attempting 1/250 sync. Studio photographers
generally don't care about this, but sports photographers usually
do. In this case we recommend conducting a series of test
exposures at various shutter speeds to find the usable maximum
sync speed for any given equipment combination.
There are three things to look for if an EOS camera is not firing
a studio strobe:
Some of the independently manufactured PC socket adapters have
metal shoes. This may cause a short circuit with the dedicated
contacts in the camera's hot shoe, and can sometimes prevent the
flash from firing. The remedy for this is to use the Canon PC
Socket Adapter, which has a plastic shoe. Alternatively, covering
the 4 small contacts in the camera's hot shoe with electrical
tape should solve the problem.
Due to the polarity of the X-sync contact used with the EOS
A2E, A2, 10s, Elan and Rebel series cameras, electronic flash
units with reverse polarity compared to the camera's X-sync
contact will not fire. This can be a problem with some studio
strobes. The polarity is caused by a diode attached to the
camera's X-contact to protect the camera's built-in flash. When a
strobe with reverse polarity is attached to the terminal, the
strobe constantly sees a short circuit which prevents it from
(This is generally not a problem with the EOS-1, 620, 630, 650
or RT cameras because their X-sync contacts have no such diode.
They don't need the diode because they don't have a built-in
If you're sure that the PC socket adapter isn't the problem, and
if you are using an EOS A2E, A2, 10s, Elan or Rebel series camera
body, try reversing the polarity of the sync cord. On some studio
strobes, the end of the sync cord that plugs into the power pack
is a regular 2-bladed household-style plug. For this type, it's a
simple matter of turning the plug around 180 degrees. Other sync
cords may have headphone-jack type plugs; this type has to be
rewired in order to work. Some photographers make a reverse-wired
extension cord that plugs into their existing sync cord, while
others just rewire a whole cord and make that one their "EOS"
Last but not least, if the first two solutions don't work,
excessive trigger circuit voltage as discussed earlier in this
bulletin may prevent the EOS camera from firing the studio
strobe. In this case, it's best to use the solutions recommended
in "Safe Ways to Fire Studio Strobes with EOS Cameras."
[From Chuck Westfall via ACS]
Well...sort of. If the trigger circuit voltage conducted through
the sync cord is high enough, it melts the solder between the PC
terminal and its connecting wire, thus breaking the circuit. But
there's no guarantee that the camera will be protected from
damage caused by high trigger circuit voltage. Therefore, it's
the customer's responsibility to check that out before using the
[Sounds to me like it would be most unwise to depend on any
protection being afforded by the PC adapter! RMA].