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Nikon TTL Hot-shoe Interface:
Gnd = Ground, Earth, Mass, Common.
X = X (trigger) contact.
Q = Quench or TTL stop.
Rdy = Ready to fire
SP = Speedlight present (Monitor).
The Nikonos interface (below) is electrically identical. |
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Nikonos TTL Interface:
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Nikonos bulkhead socket. Nikonos camera. |
Nikonos plug on sync.cable. |
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Interface
Connections and Signals: |
Rdy (strobe ready to fire): When the strobe asserts a
+3V active high signal to the Rdy contact, a Nikon or Nikonos
film camera switches to X-sync speed if a higher speed has been
selected. Rdy high also turns-on the viewfinder lightning-bolt
symbol. The Nikonos and some other cameras draw a few mA from
the Rdy line, the viewfinder LED being powered directly through
a resistor. The ready line can therefore be used as a power supply
for low-power circuits which can run on 3V (but the Sea &
Sea strobes with Duo connector and the YS60N are a special case,
see below).
X (Trigger): The camera shorts the X-contact to ground
when the shutter is fired. Electronic SLR cameras use a thyristor
for this function, and the X-terminal short-circuit output current
from the strobe(s) must be limited to no more than about 1.5mA
to allow the thyristor to de-latch after firing. The strobe input
is typically a logic-gate with a pull-up resistor to the logic
B+ rail, and the value of the pull-up resistor sets the X-terminal
current in the logic-low state. Up to 2 strobes can be connected
in parallel, so strobe designers should choose the pull-up resistor
so that the X-terminal short-circuit current is no more than
about 750mA (e.g., the pull-up resistor
for 5V logic is 6.8KW). Earlier strobes
which use 3.3KW pull-up resistors
(or less) must be modified for dual-strobe use with electronic
cameras.
Q (Quench / TTL stop): In a conventional TTL flash system,
the camera integrates the light falling on the film (using a
phototransistor and a capacitor), and when the integrated level
reaches a threshold determined by the film ISO (ASA) setting,
the Quench line is shorted to ground via a transistor. The Quench
signal tells the strobe to switch-off the flash-tube current.
The camera calibration specification is that for ISO100, an aperture
of f/5.6, and constant LV15 (K=1.16) from a light box, the time
between X going low and Q going low (txq)
is 820ms. This is slightly shorter
than the 'ideal' 977ms (1/1024s),
to allow the strobe some time to switch off.
Out of ISO range: When the camera ISO setting is too high
for accurate TTL flash operation, a Nikon film camera holds the
Q line low as a warning signal, and TTL control is unavailable.
The Nikonos V holds Q down for ISO 400 or higher. Since Q low
with X high is a warning signal, this situation should never
be allowed to occur during the normal firing sequence, i.e.,
after firing the strobe, Q should be returned high before X is
returned high. In Nikonos SB-series strobes, and original Ikelite
Substrobes, Q low with X high causes the Rdy output to pulse
on and off (i.e., it causes the viewfinder ready symbol to flash). |
The SP (Monitor)
function:
("speedlight present" is probably not
what SP stands for, but is a good enough way of remembering what
it does).
If the interface is implemented as Nikon intended, when a TTL
capable camera is connected to a TTL capable strobe, a current
of around100 - 250mA flows along the
SP line (camera positive). Since the SP input of a Nikon SB-series
strobe is the base of a bipolar transistor, a voltage of 0.6
- 0.7V will be seen at the SP terminal of a Nikon strobe when
the current is flowing.
If the Camera cannot detect a current flowing in the SP line,
it will not output a Quench signal. Why this feature was included
is anybody's guess. It seems to serve no useful purpose and makes
the system less reliable. Independent users of the Nikon interface
often disable the SP function by grounding the camera terminal
via a resistor. A resistor of value between about 3.3K - 11KW works well for Nikon film SLR cameras.
3.3K - 6.8K works well for the Nikonos V and RS-AF. Drawing a
small amount of current from the SP line fools the camera into
believing that a TTL strobe is connected.
Note: The SP connection serves a different function with
the later Nikon digital SLR cameras (The dTTL and iTTL systems).
The interface protocol has changed completely and, unless an
adapter module is used, such cameras will only operate in manual
flash mode and SP must be left disconnected when using non-Nikon
non-dTTL compatible strobes. Hence the camera information given
here applies mainly to the Nikonos, Nikon 35mm film SLRs, and
the Fuji S2 (which uses the ordinary analog Nikon protocol).
In the Inon Z220, the SP signal from the camera (activated
when the shutter is half-pressed) is used to operate the focusing
light. Note that the strobe must be in Nikon mode for this functionality,
i.e., the magnet must be in the 'N' hole.
Ikelite overrides the SP function by placing a 3.3K resistor
from SP to ground inside the Nikonos plug of its ICS5 to Nikonos
TTL sync cables.. Hence only 4 wires are needed in an Ikelite
TTL Nikonos cable.
The Sea & Sea YS50TTL/N overrides the SP function
by placing a 6.8K resistor from SP to ground inside the strobe.
Hence 5 wires are needed in the cable.
Sea & Sea YS-Duo series strobes
(including the YS90A) and the YS60N will not assert
Rdy until SP is asserted. SP in this case is used to determine
whether a Nikonos TTL or a Sea & Sea TTL camera is connected,
but this deviation from standard practice makes Duo strobes partially
incompatible with the Nikonos IVa (The IVa must be switched to
the X-sync speed manually). TTL quenching is still possible with
SP disconnected, but the ready line always stays low. If you
want to activate the Rdy function, connect a 100K resistor from
X to SP (but note that the Rdy line voltage obtained is only
about 2.1V). A more detailed description of the Duo interface
is given below.
Nikonos Speedlights and true clones feed the SP line to
the strobe, and the strobe will not function in TTL mode if SP
is left un-connected. In these cases the strobe can be fooled
into thinking a TTL camera is present by connecting the strobe
SP terminal to a positive supply via a resistor. A resistor in
the range 22K - 47K from SP to Rdy is usually sufficient. |
Suggested interface
circuit for TTL converter with Nikonos 5 connector:
A TTL protocol converter needs only two open-collector active
low outputs: X and Q. In order to ensure that the maximum variety
of strobes will work correctly however, some additional components
are needed at the connector interface. An optional ready light
can be implemented by connectiong an LED and series resistor
between Rdy and Ground. A 100K resistor between X and SP injects
sufficient current into the SP terminal to activate the Rdy line
of the Sea & Sea YS60N and YS strobes with a Duo connector,
and to turn on the focusing light of the Inon Z220. Nikonos SB-series
strobes cannot be made to work in TTL mode by this means however,
and so require a resistor (22K - 47K) from Rdy to SP. |
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Maximally compatible
Nikonos interface.
(view looking into socket)
This circuit has been tested in TTL mode with:
Sea & Sea YS50TTL/N (GTO),
YS60N, YS90A,
Inon Z220,
Nikonos SB105 (GTO),
Ikelite DS50 (IGBT) s/n<70000,
DS125 (IGBT) s/n>5000. |
Note that this interface does not guarantee that a particular
strobe will always work with a TTL converter. An additional mandatory
requirement is that the strobe should be able to recover and
be ready to fire within the preflash to main flash interval of
the flash control system. Strobes with a GTO series control element
can recover in about 100ms, and so will often work with long-interval
preflash systems such as Olympus (125ms), but not with short-interval
systems such as Canon eTTL (70ms). Strobes with an IGBT series
control element can recover in about 30ms, and should work with
all presently available full-intensity preflash systems. It follows;
that since the Nikonos SB series speedlights all use GTO circuitry,
there is no point in putting the 33K resistor from SP to Rdy
if the TTL control system requires a short pre-to-main flash
interval (i.e., if legacy SB speedlights and clones cannot be
used, there is no point in making provision for them).
If there is room in the housing, a normally closed (NC) switch
can be arranged to keep SP shorted to ground until the shutter
release is half-pressed. Alternatively, a normally open (NO)
switch can be used to keep SP disconnected until the shutter
is half-pressed. Either of these measures will cause the Inon
Z220 focusing light to be activated with the shutter control.
One disadvantage of such a provision however, is that it will
cause the Rdy line from a Nikonos SB strobe to pulse on and off
until the shutter is half pressed. Consequently, if a
ready LED is included at the interface, it is advisable to provide
a switch or control enabling the user to opt for SP inactivation
only if an Inon Z220 is used. Alternatively, a more complicated
circuit can be used to suppress the ready LED when the SP output
is inactive (as is done in the Nikonos V). |
Ikelite
TTL Interface:
The original Ikelite TTL flash interface is a subset of the Nikon
interface with the SP function omitted. |
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ICS5 bulkhead connector (strobe or
housing). |
ICS5 plug on sync cable. |
T- and B+6.5V:
On an early Ikelite TTL Substrobe (pre DS-series), the pin labeled
T- is provided for compatibility with non-TTL cables. Such cables
fire the strobe by shorting between X and T-, whereas a TTL cable
fires the strobe by shorting between X and ground. For historical
reasons (Ikelite solid-state triggering (SST) system), T- is
not grounded, but connects via resistor to the gate of the tube-trigger
thyristor. This means that the strobe can also be triggered by
applying a current-limited (~1mA) positive pulse between T- and
ground.
On an Ikelite film SLR housing, the T- pin is not connected.
The T- receptacles are likewise not connected (N/C) in a #4103
Ikelite to Ikelite TTL cable or in a #4104.6 Ikelite to Nikonos
TTL cable.
In the Ikelite DS-series interface, the T- function has
been removed and replaced with a power-output connection (B+6.5V)
for the strobe-powered 4100.5 slave unit, the 4100.6 EV controller,
and hot-shoe protocol conversion modules in digital camera housings.
Hence Ikelite digital housings with TTL converters (e.g., Olympus
TTL housings, Canon eTTL housings) require a special 5-core sync
cord #4103.51 (marked with a blue band), the extra wire being
for the power supply connection.
 Given that the
T- function has disappeared, old 2-core manual Nikonos leads
must not be used with DS-series Substrobes.
SP: In an Ikelite film-SLR housing, the SP function is
disabled by connecting SP to ground via an 11K resistor (it varies)
inside the hot-shoe connector.
In non-Ikelite SLR housings, the SP connection us usually brought
out to a Nikonos bulkhead connector for compatibility with Nikon
SB-series speedlights and clones.
Ikelite wire colour codes:
Black = Gnd. White = X-contact, Green
= Quench, Red = Ready, Grey = T- or B+6.5V.
Some years ago, the author designed a simple TTL strobe (see
circuit diagram).
The interface was made to conform to the standard Ikelite TTL
interface, so that the unit could be used with an Ikelite cable.
The circuit should therefore illustrate many of the principles
discussed above. Note that the circuit shown was designed for
the Nikonos camera. For thyristor-trigger cameras, the X and
Q input pull-up resistors need to be changed to 8.2KW.
To work with an Ikelite Olympus housing or 4100.5 TTL slave,
the T- terminal can be disconnected from the trigger thyristor
gate input and connected to the internal (switched) B+ rail via
a 33W 2W resistor (which gives short-circuit
protection). |
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Sea &
Sea Duo Strobe Interface: |
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This is a non-standard Nikonos interface, its behaviour being
dictated by the additional requirements of the Motormarine II
system:
 The Rdy line serves
as the S (start integration) signal for the MM2 interface if
SP is not asserted. Hence the strobe with a Nikonos V connector
cannot switch a Nikonos IVa camera to X-sync mode (Nik III and
IVa cameras do not connect SP).
In the MM2 protocol,
the Q line also serves to light the TTL OK LED in the camera
viewfinder. Hence, if SP is not asserted, the Q line is only
pulled-up when X is pulled low; or, after firing, when the green
TTL OK light on the strobe is lit.
In the MM2 protocol,
the X line is only pulled up when the red Rdy light on the strobe
is lit. Voltage on the X line is sensed to light the red Rdy
LED in the camera viewfinder.
 If a 100K resistor
is connected from X to SP, the Rdy line becomes active, the TTL
OK function via the Q line is disabled, and the Q line is pulled
up continuously rather than only when X is low. The Rdy line
output voltage is sub-standard however (~2.1V), and so protocol
conversion modules powered from the Rdy line must be of very
low voltage design if Sea & Sea compatibility is required..
See the Sea & Sea interface article
for more details of the MM2 protocol. |
Illustrations and text © Dr D. W. Knight,
Cameras Underwater, Jan - Sept 2004.
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