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Byte-Wide Bidirectional Personal Computer Parallel Port Communication Interface

IP.com Disclosure Number: IPCOM000040234D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Heath, CA: AUTHOR [+6]

Abstract

A technique is described whereby byte-wide bidirectional communications are provided to enable point-to-point communication between two IBM personal computers (PC) for message and file transfers. Modifications are made to the existing PC parallel communication circuit, and new software is developed so as to enable data migration to occur between PCs with different media sizes, such as a 5.25" PC drive being able to exchange data with a 3.5" PC drive. In addition, printer interface cables are modified so as to enable printer software to sense which printer interface is being used for communications. Prior-art PC parallel circuit cards (PN 1501987) do not support byte-wide bidirectional communications capability since the output latch circuit is wired to ground.

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Byte-Wide Bidirectional Personal Computer Parallel Port Communication Interface

A technique is described whereby byte-wide bidirectional communications are provided to enable point-to-point communication between two IBM personal computers (PC) for message and file transfers. Modifications are made to the existing PC parallel communication circuit, and new software is developed so as to enable data migration to occur between PCs with different media sizes, such as a 5.25" PC drive being able to exchange data with a 3.5" PC drive.

In addition, printer interface cables are modified so as to enable printer software to sense which printer interface is being used for communications. Prior-art PC parallel circuit cards (PN 1501987) do not support byte-wide bidirectional communications capability since the output latch circuit is wired to ground. To enable parallel byte-wide bidirectional communications between two PCs, the following hardware modifications are made to the existing PC parallel printer circuit card: 1)Cut the land from pin #1 of U4(LS374) to ground. 2) Connect pin 1 of U4(LS374) to pin 15 of U7(LS174).

3) Connect pin 2 of U10(LS125) to pin 15 of U7(LS174).

4) Connect pin 3 of U10(LS125) to pin 16 of U3(LS244).

5) Connect pin 1 of U10(LS125) to pin 10 of U10(LS125). Steps 1 and 2 connect the output circuits to unused latch circuits so as to allow the processor to control the enabling and disabling function of the direction bit. Steps 3 and 5 allow the processor to read the status of the direction bit. Connection between the two communicating PCs is then made through a modified printer cable to provide point-to-point connection for eight data lines and two control lines.

The modification is accomplished by connecting ACK pin 10 of one PC to Strobe pin 1 of the other PC, and vice versa. Data lines of both PCs are pin 2 to pin 9, D0 to D7, respectively. To enable the software to sense which printer interface is being used for communications and to provide two control lines, the following modifications are added to the cable: In both PCs pins 14 and 15 are jumpered as are pins 13 and 16 so as to provide wiring for sensing the base address. To provide additional control lines, pin 12 of each PC is connected to pin 17 of the other PC. Pins 18-25 are grounded. The following pseudo-code is provided to illustrate...