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, Bad output circuit, or an overloading at the input. 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 output and an input to help you in troubleshooting 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. =================================================================================== Date: 05-06-89 (23:57) Number: 552 Comp-U-Ease To: ALL Refer#: NONE From: PHIL MARCELLO Read: YES Subj: ADDENDUM CHAPTER 4 # 0 Conf: (6) TeleComm ------------------------------------------------------------------------ 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 output and an input to help you in troubleshooting 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.