Three-level, self-clocked data transmission system

• US 4,271,526 A
• Filed: 06/05/1979
• Issued: 06/02/1981
• Est. Priority Date: 03/02/1979
• Status: Expired due to Term

• Current Assignee: Unisys Corporation
• Original Assignee: Burroughs Incorporated (Terreno Realty Corp.)
• Inventors: Flora, Laurence P.
• Primary Examiner(s): Pitts, Harold I.
• Application Number:
  US06/045,805
• Time in Patent Office: 728 Days
• Field of Search: 375/17, 375/20, 178/68
• US Class Current: 375/288
• CPC Class Codes: H04L 25/4925 using balanced bipolar ternary codes

Abstract

A serial digital data transmission system wherein pairs of binary input signals are encoded into a self-clocked three-level signal having high, low and intermediate levels for driving over a transmission line with level transitions occurring in response to an applied clock signal, and with a valid signal being employed to indicate data presence. Selectable encoding is provided in response to detecting any accumulated excess of high or low prior transmission levels so as to continually seek maintenance of an intermediate average level for the transmitted signal. The clock and valid signals are regenerated at the receiver for use in deriving the binary signal data from the received three-level signal.

Claims

1. A self-clocked three-level digital data transmission system comprising:

encoding means responsive to input binary signals and an applied clock signal for encoding said input binary signals into a three-level signal having first and second levels and an intermediate level and having level transitions occurring at times determined by said clock signal;

said encoding means being operable to encode pairs of input binary signals into corresponding encoded pairs of sequential levels in a manner such that at least one encoded input pair is selectably encodable into either a first pair of sequential levels having a positive average value relative to said intermediate level or into a second pair of sequential levels having a negative average value relative to said intermediate level, said encoding means being operable to select said first or second pair in a manner so as to seek a predetermined average level for the encoded three-level signal relative to said intermediate level;

a transmission line;

driving means for coupling the encoded three-level signal to one end of said transmission line for transmission thereby; and

receiving and decoding means coupled to the other end of said transmission line for receiving the transmitted three-level signal, deriving said clock signal therefrom, and employing the derived clock signal for decoding the received three-level signal into binary form.

2. The invention in accordance with claim 1, wherein said first and second levels are equal and of opposite polarity relative to said intermediate level.

3. The invention in accordance with claim 1, wherein said intermediate average level is substantially zero.

4. The invention in accordance with claim 1, 2 or 3, wherein a valid signal is applied to said encoding means along with said input binary signals for indicating the presence thereof, wherein said encoding means operates to provide encoding only when said valid signal is applied, and wherein said receiving and decoding means includes means for regenerating said valid signal from the received three-level signal.

5. The invention in accordance with claim 4, wherein said input binary signals applied to said encoding means include a plurality of start bits, and wherein said receiving and decoding means include means for detecting said start bits for use in determining the polarity of the received three-level signal.

6. The invention in accordance with claim 4, wherein said three-level signal is comprised of a high level H, a low level L and an intermediate level I, and wherein said encoding means is operable to encode an input pair into a pair of encoded levels as follows:

space="preserve" listing-type="tabular">______________________________________ INPUT PAIR ENCODED PAIR ______________________________________ 00 IH or IL 01 LH 10 HL 11 HI or LI ______________________________________

7. The invention in accordance with claim 6, wherein said encoding means selects between the encoded pair IH and IL for a 00 input pair and between the encoded pair HI and LI for a 11 input pair in response to detecting the accumulated average level of the encoded three-level signal.

8. The invention in accordance with claim 4, wherein said encoding means operates in response to an input pair designatable as AB to produce an encoded pair designatable as FS in accordance with the following Boolean equations:

##EQU3## wherein M is true when the encoded signal has an average level which is between L and I;

wherein the F_H and F_S equations set forth the relationships which determine whether the first encoded level F of an encoded pair will have an H or L level, and wherein the S_H and S_L equations set forth the relationships which determine whether the second encoded level S of a transmitted pair will have an H or L level, an I encoded level being produced when the respective equations above are inapplicable.