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Flip-Flop Line Direction Switchers

IP.com Disclosure Number: IPCOM000039325D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 70K

Publishing Venue

IBM

Related People

Edwards, DJ: AUTHOR

Abstract

A redrive circuit for a two-way open-collector-driven signal line requires constant monitoring to determine the direction of redrive. If a microcontroller is used for this, it will have very limited time if any for other tasks. It may also be too slow in changing the direction of redrive. These disadvantages can be overcome by using a dedicated line-switcher circuit. The line-switcher circuit described here, consisting of a J-K flip-flop and two inverters, has fast direction detection and switch times and also has inputs to allow a microcontroller to over-ride the line-switcher and also to switch the redrive off altogether. (Image Omitted) The line switcher monitors both sides of the line-driver, and if they are different, it switches the direction of the driver.

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Flip-Flop Line Direction Switchers

A redrive circuit for a two-way open-collector-driven signal line requires constant monitoring to determine the direction of redrive. If a microcontroller is used for this, it will have very limited time if any for other tasks. It may also be too slow in changing the direction of redrive. These disadvantages can be overcome by using a dedicated line-switcher circuit. The line-switcher circuit described here, consisting of a J-K flip-flop and two inverters, has fast direction detection and switch times and also has inputs to allow a microcontroller to over-ride the line- switcher and also to switch the redrive off altogether.

(Image Omitted)

The line switcher monitors both sides of the line-driver, and if they are different, it switches the direction of the driver. This is done by a modified J-K flip-flop circuit shown in Fig. 1. The inputs to the flip-flop are the states of the lines on either side of the drivers. The J-K flip-flop is clocked, for example, by an 8 MHz clock. When one side is high and the other is low, the low input is returned to the gate which will allow that low to propagate, by the output of the J-K flip-flop. The J-K flip-flop, by design, cannot switch the drivers on in both directions at the same time. When both inputs are low, it remains unchanged when clocked, as is required because this is the condition that arises when the line is being driven correctly. When both sides are high, a J-K flip-flop normally changes state when clocked. As both ends of the line are at the same level in this instance, it may appear that this toggle is all-right, but it is undesirable in this application, so there is a further addition to the circuit. The 74110 J-K flip-flop has three inputs ANDed together for the J input and the same for the K input. By taking the two line switcher inputs, inverting them, and then feeding each to an input on the other side of the flip-flop from its source, the flip-flop is also inhibited from changing state when both inputs are high because there will also be a low input to each side of the flip-flop. Fig. 2 shows function tables for the J-K flip-flop alone and as modified in Fig. 1. The microcontroller can read which direction the line-switcher has selected, by reading its output. It can also use the flip-f...