What component (IC) to use for collecting A and !A signals into a single TTL signal?

Question

I am building a circuit board to listen to quadrature encoder signals from a motor controller and send these signals on to an FPGA that will be used to count the pulses and direction. The signals coming out of my motor controller are:

• Phase A: A and !A
• Phase B: B and !B
• Phase C: C and !C (origin indicator)

In other words, each phase sends out two signals that are the opposite of one another at all times. I have used Quad EIA-422/423 (MC3486) line receivers to convert differential voltage signals into TTL signals, but is this the correct component for converting each phase from two signals to a single TTL signal? If not, is there a similar IC that will do the trick?

Thanks!

EDIT: It looks like I was treating these signals correctly. I was a little confused on the definition of "differential voltage" as it applies to the line receivers I am using (MC3486). Basically when A is high and !A is low, my line receiver outputs a high on the TTL channel. When A is low and !A is high, it outputs a low on the TTL channel. This is what I wanted it to do; I was concerned that I had misinterpreted the meaning of differential voltage.

Answer 1

It sounds like a point-to-point system you are trying to "sniff" the data from and because of this you'd be well-advised to offer a high-impedance connection which can suffer a reasonable bit of common-mode voltage on the two lines relative to your "sniffer".

I don't know what data rate it's using but it'd be hard-pressed to task a MAX999 comparator.

I'd use 10k resistors from each line to the MAX999 inputs and I'd have 2 series 470R resistors across the MAX999 inputs to: -

1. Attenuate the signal
2. Act as a mid rail connection from your local sniffer power

The MAX999 will work down to very low input levels and attenuating most data transmission levels by ten is not going to be a big issue. The MAX999 also has a littel bit of inbuilt hysteresis too and works at 5V or 3V3 with a push-pull output that can directly feed micros and PLDs etc..

Answer 2

A dedicated voltage comparator chip such as LM339 is designed to interface with a digital logic interface (to a TTL or a CMOS).

source

^{simulate this circuit – Schematic created using CircuitLab}