An extract on #womensecret
Eleven 78-rpm records by Johnson were released by Vocalion Records during his lifetime. A twelfth was issued posthumously. Johnson's estate holds the copyrights to his songs.
The Complete Recordings, a two-disc set, released on August 28, 1990, contains almost everything Johnson recorded, with all 29 recordings, and 12 alternate takes. (Another alternate take of "Traveling Riverside Blues" which was released by Sony on the CD King of the Delta Blues Singers and was included in early printings of the paperback edition of Elijah Wald's Escaping the Delta.)
To celebrate Johnson's 100th birthday, May 8, 2011, Sony Legacy released Robert Johnson: The Centennial Collection, a re-mastered 2-CD set of all 42 of his recordings and two brief fragments, one of Johnson practicing a guitar figure and the other of Johnson saying, presumably to engineer Don Law, "I wanna go on with our next one myself." Reviewers commented that the sound quality of the 2011 release was a substantial improvement on the 1990 release.
The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are either in the range of +3 to +15 volts or the range 3 to 15 volts with respect to the "Common Ground" (GND) pin; consequently, the range between 3 to +3 volts is not a valid RS-232 level. For data transmission lines (TxD, RxD, and their secondary channel equivalents), logic one is defined as a negative voltage, the signal condition is called "mark". Logic zero is positive and the signal condition is termed "space". Control signals have the opposite polarity: the asserted or active state is positive voltage and the deasserted or inactive state is negative voltage. Examples of control lines include request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR).
The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of 5 V, 10 V, 12 V, and 15 V are all commonly seen depending on the voltages available to the line driver circuit. Some RS-232 driver chips have inbuilt circuitry to produce the required voltages from a 3 or 5 volt supply. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to 25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.
Because the voltage levels are higher than logic levels typically used by integrated circuits, special intervening driver circuits are required to translate logic levels. These also protect the device's internal circuitry from short circuits or transients that may appear on the RS-232 interface, and provide sufficient current to comply with the slew rate requirements for data transmission.
Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop. Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential serial connections such as RS-422, RS-485, and USB can tolerate larger ground voltage differences because of the differential signaling.
Unused interface signals terminated to ground will have an undefined logic state. Where it is necessary to permanently set a control signal to a defined state, it must be connected to a voltage source that asserts the logic 1 or logic 0 level, for example with a pullup resistor. Some devices provide test voltages on their interface connectors for this purpose.