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 MIL-HDBK-232A
5.4.3.5.2 EIA RS-232C. This unbalanced interface is the more prevalent
interface used in commercial equipment. The electrical characteristics are
similar to the unbalanced interfaces defined in MIL-STD-188-114. A single
signal ground/common return is provided on pin 7, which is identified as
circuit AB. This is contrary to the guidance of this handbook, which
advocates separate return paths (see 5.4.3.5.4). Cables should be
manufactured on site to provide a separate return for each signal using
twisted pair cable. At the connector, the returns are tied together, then
terminated on pin 7. An alternative is the use of a commercially available
RS-232-to-RS-449 adaptor plug. Cables are available to support RS-449, thus
negating the need to make new cables. The standard also presents a problem
with proper signal grounding. These items of equipment typically do not
provide a signal ground stud that connects the equipment to the signal ground
reference. The EIA standard states, "A protective ground is provided in the
interface cable, terminated on pin 1 identified as circuit AA. Pin 1 is
further bonded to the equipment frame. It may be further connected to
external grounds as required by applicable regulations. Most often this is
connected to the fault protection subsystem. The signal ground/common return
which establishes the signal reference is brought to ground only in the data
communications equipment. Provision is made to connect circuit AB to circuit
AA, which may be removed on site." (See figure 38.) This portion of RS-232C
presents very serious problems. First, a fault current on one chassis may
flow into the second chassis, causing damage to the equipment and cable.
Second, signal return, being coupled to the FPSS, presents the possibility of
RED data appearing in the BLACK area. The standard further states circuit AA
is optional, while circuit AB is mandatory. To overcome.this situation, the
following retrofit actions are suggested:
a.
If circuit AB is strapped to circuit AA, remove the strap.
b.
In the cable connector, terminate the circuit AA lead on the circuit AB
pin.
c.
Provide a method to run a low-impedance signal ground conductor to the
equipotential ground plane. (See MIL-HDBK-419.)
NOTE: Equipment designed to commercial standards and some Government
equipment is designed for environments in which the only available ground is
the FPSS. Typically, the equipment chassis may serve as the neutral power
reference and signal reference. Such conditions may violate codes and
standards. Further, the FPSS is bonded to the frame. In such situations, it
is impossible to properly separate the grounds. As a minimum, the protective
ground must not be taken between equipment via the interface cables.
5.4.3.5.3 Other interfaces. Other interface schemes exist which have not
become de facto standards. The designer should use the knowledge of the
problems in the EIA interfaces to examine other interfaces to determine and
correct any problems which may be encountered.
5.4.3.5.4 Mixed interfaces. An installation may include equipment with
RS-449, RS-232C, and MIL-STD-188-114 balanced and unbalanced interfaces that
must be interfaced together in various combinations. Figures 39 and 40
depict how these interfaces may be accommodated. Whether balanced or
unbalanced voltage digital signaling is used, a dedicated signal return
should be provided for each clock and data signal. While this required in
balanced voltage digital signaling, it is not typical in unbalanced voltage
digital signaling of RS-232C. The rationale for dedicated returns is
threefold. First, by using twisted pair wire, the potential for crosstalk is
reduced because the magnetic field created by the current in the return
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