Output of file : CHAPTER.04 contained in archive : CLASS.ZIP

Chapter IV. Electrical Characteristics

Having a literal understanding of the interface is important.
Now a look at the electrical characteristics of the interface.

As was stipulated earlier, for our purpose, a high voltage was
said to be an "on" condition while a ground voltage or negative
voltage was to be considered an off condition.

Most troubleshooters use a "breakout box" or "Interface monitor"
that sits between the modem connector and the cable, actually
being put in the circuit. "LED"'s are used to tell you the state
of the signal, low-high; on-off.

If you took a voltmeter and measured the voltage of a control
signal in the "on" state, you would measure something in the order
of 5 volts to 12 volts.

Depending on the manufactuer and logic circuits used, and of three
conventions are used.

Convention 1 - Convention 2 - Convention 3

ON Condition + 5 volts + 5 volts + 12 volts

OFF Condition 0 volts - 5 volts - 12 volts

-----------------------------------------------------------------
Sample of 0 volt to + 5 volt Convention

5 volts -------- ÚÄÄÄÄÄÄ -
³ 
³ Good Level - "ON" Condition
³ 
2.5 volts --------- ³ -----------------
³ 
³ Improper Voltage - Bad circuit
³ 
.5 volts --------- ³ -----------------
0.0 volts ÄÄÄÄÄÄÄÄÄÙ Good Level - "OFF condition"

This convention is not used very much anymore in the
world of interfacing interfacing.

The more preferred and now more commonly used is the convention
using +12 volts as an "ON" condition and -12 volts as the off
convention.

12.0 volts ³ -----
³ 
³ Good Voltage Levels for
³ an "ON" Condition
³ 
2.5 volts ³ -----
³ 
0.0 volts ³ Bad Voltage Levels something is wrong
³ 
2.5 volts ³ -----
³ 
³ Good Voltage Levels for
³ an "OFF" Condition
³ 
12.0 volts ³ -----

The variance in levels has to do with variations in the power
supply powering the circuits, resistance in the cables,
and the load requirements of input circuits our output is
connected too. To accomodate these variations, the wider
the swing in voltage is seen as a safer bet that a true
voltage level will make it to the other end of the cable.

The bad voltage area may indicate the cable is to long,

12.0 volts ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
³ ³ Control Signal On
³ ³
³ ³
³ ³
2.5 volts ³ ³
³ ³
0.0 volts ³ ³
³ ³
2.5 volts ³ ³
³ ³
³ ³
³ ³
³ ³
12.0 volts ³ ÄÄÄÄÄÄÄÄÄÄÄÄÙ Control signal off

While some control circuits are always on or always off
others are turning on and off as required.

Our clocks (in synchronous interfacing) and our data
will also be continuously going from low to high and
high to low.

They must enter the safe voltage zone to be exceptable
levels.

12.0 volts ³ ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ
³ ³ ³ ³
³ ³ ³ ³
³ ³ ³ ³ ON area
³ ³ ³ ³
2.5 volts ³ ³ ³ ³ ----------
³ ³ ³ ³
0.0 volts ³ -------³ ------³ -----³ Twighlight Zone
³ ³ ³ ³
2.5 volts ³ ³ ³ ³ ----------
³ ³ ³ ³
³ ³ ³ ³
³ ³ ³ ³ OFF area
³ ³ ³ ³
12.0 volts ³ ÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÙ

Of course, you will need a Data Scope to see these oscillating
signals.

While signals in the 3 volt range may make it, I would have
serious doubts about long term operation. The closer you
operate to the unacceptable voltage levels, the more likely
you are to experience problems down the road.

Striving for clean healthy full voltage signals should also
be a desired goal.

As a result, based on voltage levels and impedance, it is
recommended that the cable between computer and modem, when
an RS232 interface is used, should not exceed 50 feet.
It is possible to operate beyond that limit, but it is not
recommended.

I tried drawing what I needed here, but without 45 degree
angle lines, I just could not do it.

Anyways, in dealing with Voltage levels on the Digital
Interface, it is important to know what constitutes an
problems.

The Output of an interface device is a Logic Driver.
A Logic Driver is an amplifier that allows for greater
current flow or more power to the output.

Logic circuits normally have very low power requirements,
however when those logic signals leave the circuit board
they need a boost.

If you were to look at an output of a Logic Driver that is
not connected to anything, you should see a clean healthy
voltage. A Logic driver has a pull up resistor on the
output to insure this.

If then, you connect the interface, and you see the signal
being drawn down to a much lower voltage, you would suspect
a short circuit of some type on the INPUT device.

If you look at the input on an interface. You are looking at the
input of a logic driver. This input is tied to ground via a
resistor to keep large voltage spikes from blowing it apart.
This resistor allows the input to float. Mean that the input
may be sitting at any "extraneous" voltage between 0v and 2.5v
THIS is perfectly normal. It is only when an output device is
connected to the input that you will see a real Logic Level
Voltage.

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