Quantcast Loop Current

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MIL-HDBK-232A
5.4.3.4.1  Balanced voltage digital signaling.  The preferred signaling
method is balanced voltage digital signaling.  Balanced voltage digital
signaling relies on detection of a specific difference of potential between
two points, regardless of the relation of either line to a ground reference.
balanced voltage digital signaling's most significant characteristics are
reduced crosstalk and greater distance at higher speed.  This is achieved by
a constant difference of potential between conductors over the length of the
cable.
5.4.3.4.2  Unbalanced voltage digital signaling.  Unbalanced voltage digital
signaling differs from balanced in that the signal line difference of
potential is to a common signal reference point.  A poor installation, or
excessive distance between the source and sink, often results in a
significant difference of potential between the signal and the reference.
5.4.3.4.3  Loop current.  Loop current has been predominantly used in
teletypewriter or telegraph applications.  Loop current interfaces should be
treated as high level (see 5.2.2).  Currents of 20 or 60 milliamperes are
common.  A loop current circuit consists of a loop which is alternately
opened and closed to allow current to flow, or alters the polarity of
current.  Two types of current loops are prevalent -- dry contact and wet
contact.  In dry-contact signaling, the current source is in the receiver.
The transmitter closes and opens the circuit, providing a complete path for
the current.  In wet-contact signaling, the current source is in the
transmitter.  The most common loop-current signaling schemes are neutral
(single current) and polar (double current).  Polar is the more prevalent of
these schemes.
5.4.3.5  Commercial standards.  The designer and installer may be faced with
commercial interfaces which may or may not allow the use of individual
returns.  The more commonly used commercial interfaces include Electronic
Industries Association's (EIA) RS-449 and RS-232C.  Most commercial
interfaces share a common problem which jeopardizes shielding and could be a
potential TEMPEST problem.  Connector backshells typically are plastic, which
does not provide a means of closing the shield.  While some metallic
backshells may be available, most cannot be relied upon to be RFI tight.
Further, the mechanical mating schemes often do not provide sufficient
bonding to permit adequate electrical shield integrity.  In general, the
designer may find commercial equipment to be a source of RFI, unless
procurement specifications clearly dictate that adequate measures be taken by
the supplier to correct such deficiencies.
5.4.3.5.1  EIA RS-449.  This interface conforms to the basic assumption of
this handbook in that all data and clock signals have individual returns.
The interface is implemented using a 9-pin connector (simplex, send only) and
a 37-pin connector (full duplex).  The 9-pin connector may not be provided.
The interface, has a significant shortcoming.  Pin 1 is designated to
terminate the cable shield.  As typically implemented, pin 1 takes the shield
currents into the equipment in order to shunt those currents to ground via
the power FPSS.  In some equipment, this may be the chassis, which may also
be the neutral reference.  This may result in noise on the power or neutral
currents being introduced on the shield.  The shield, then, should not be
terminated on pin 1.  If the equipment is provided with a metallic
backshield, and the shield is circumferentially bonded, a shield grounding
wire should be attached to the backshell and run to the equipotential plane.
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