02836 [alt-beam] Re: 74xx139 (H-bridge)

Alt-BEAM Archive

Message #02836

To: NSX - evenflow88@hotmail.com
From: Sean Rigter rigter@cafe.net
Date: Fri, 30 Apr 1999 18:35:47 -0700
Subject: [alt-beam] Re: 74xx139 (H-bridge)

You asked for it: an explanation of the famous 74xx139 (Z bridge) chip
which should answer your questions. This is not much different from
other explanations out there but a bit more oriented to the current
discussion. Anyway I found it a useful exercise to try to keep the
explanation simple but complete.

Here is how the 74xx139 (Zoelen bridge) is drawn in layout format:

+---------- Reverse
Motor | +------- Forward
GND | | | | +---- Engage (connected to GND = YES)
| | | | | | | |
.-+--+--+--+--+--+--+--+-.
| 8 7 6 5 4 3 2 1 |
| [|
| 9 10 11 12 13 14 15 16 |
`-+--+--+--+--+--+--+--+-'
| | | | | | | |
| | | | | Vcc
Motor | | +------- Engage
| +---------- Forward
+------------- Reverse

74xx139 ASCII DRAWING (c) AA van Zoelen
http://www.xs4all.nl/~vsim/z-bridge.html

It consists of 2 identical independent sections called dual 1 of 4
decoders. In this example, each section controls one motor. Let's look
at section 1 (the top pins) in schematic format showing the pin numbers
and a truth table for reference.

Motor 74AC139 truth table
| |
+--+--+--+--+--+ Combination A B E Y0 Y1 Y2 Y3
| 4 5 6 7 | ----------- ----- -----------
| Y0 Y1 Y2 Y3 | 0 0 0 0 0 1 1 1
| Section 1 | 1 1 0 0 1 0 1 1
| A B E | 2 0 1 0 1 1 0 1
| 2 3 1 | 3 1 1 0 1 1 1 0
+----+--+--+---+ 4 x x 1 1 1 1 1
| | |
| | |
| | +------- Engage
| +---------- Forward
+------------- Reverse

The 3 lines marked E, A and B are inputs which can be connected to Vcc
(1) or GND (0). These 3 inputs control the 4 outputs Y0,Y1,Y2 and Y3.
The outputs are all normally 1. The A and B inputs can be connected to
Vcc (1) and GND (0) in 4 combinations (00,10,01,11). These input
combinations cause 4 combinations to happen on the Y outputs
(0111,1011,1101,1110). The E input is normally called the ENABLE input
and it must be connected to GND (0) for anything to happen to the 4
outputs. When the ENABLE pin is connected to Vcc (1), the output
combination is always 1111 regardless of the A and B input combination
(xx).

In some BEAM applications, the A, B, and E inputs are connected to
MicroCore outputs (ie A=Nv1 and B=Nv3 and E=PNC output). The motor is
connected to the Y1 and Y2 outputs and will turn only if either output
Y1 or Y2 is GND (0) while the other is Vcc (1). From the TRUTH TABLE it
is clear that when the PNC output is 1 (ie right after power is
connected), the motor cannot turn since both Y1 and Y2 are 1.

The MOTOR is STOPPED if the PNC is still Vcc (PNC=E=1) which is
combination 4 in the table

After the PNC output settles down (0) and the process is circulating
through the MicroCore then two active input and output combinations will
occur that cause the motor to turn.

REVERSE if the process is active in Nv1 (Nv1=A=0) which is combination 1
in the table.

FORWARD if the process is active in Nv3 (Nv3=B=0) which is combination 2
in the table.

>From the table it is clear that no other combination of A and B can generate active (01 or 10) outputs on Y1 and Y2 and that outputs Y1 and Y2 can never both be 00.

The 74AC139 may also be useful in other applications for example in
controlling a HEAD (instead of the usual BiCore circuit). In that case
the output of left and right photo sensor circuits are connected to the
A and B inputs. Then think of turning FORWARD and REVERSE as turning
LEFT and RIGHT or UP and DOWN. When a head photo sensor detects a light
source input A or B will turn low (0) and the motor will start turning
in that direction until it receives equal light on both sensors and both
A and B are low (0). This is combination 1 from the table and therefore
Y1 and Y2 are both 1 and the motor is turned off saving power! This is
an improvement over a simple BiCore HEAD circuit which consumes power
even if the head is pointed directly at the light source. It should be
pointed out that the BiCore circuit outputs could also be turned off by
decoding the balanced or "locked on" condition.

The 74AC139 or equal can be used by itself to drive small motors. Two or
more chips can be stacked vertically with corresponding pins soldered
together. The Y1 and Y2 outputs can also be connected to a current
amplifier such as an H-bridge. Since the illegal output combination 00
cannot be generated by the 1 of 4 decoder, the Y1 and Y2 can be safely
connected to an H(bomb)Bridge without a possibility of generating a
mushroom cloud.

This information holds equally for section 2 (pins 9 to 15) of the chip.

These few BEAM examples are just the tip of the iceberg of many other
applications for which the 74xx139 Dual 1 of 4 decoder can be used.

BEAM, MicroCore and BiCore are (c) Mark Tilden
Z bridge is (c) AA van Zoelen

wilf

NSX - wrote:
>
> Hie all....i am wondering out where should i connect the pin labeled forward
> and reverse in the chip 74xx139 in beam tek's motor driver website.....more
> over the are forward and reverse on each side of the chip on on the left and
> the other on the right...any hints?and why does it have two engage/enable
> pins....wilf i know u know this one...care to help...and about the
> chat///mail me ASAP
>

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