Operations to be made in this case are:

• Preparing DTE starting up the communication software (Hyper terminal if we use Windows or any other software).
• Verifying in the interface visualizer (VIF) as the C 103 is the generator circuit. Conventionally, C 103 side DTE and C 104 side DCEare generator circuits.
• Verifying the state of work of C 108 and C 105 circuits.
• Giving the high level of voltage using 9 Volts of the VIF C 106 and C 107. Simulating a transmission by keyboard and observing the C 103.
• Giving the high level of voltage also the C 109 and connecting the pin 2 (C 103) to the pin 3 (C 104). Data going out from serial port of DTE, with this connection come back to the serial port itself. Simulating a transmission by keyboard, starting up and deactivating local echo. 

The test consists in verifying the efficiency of connections among the 25 female pins to those of the male connector.

1. Connect the two ends of the interface cable to the VIF
2. Put all the switches in Off position.
3. Feed with 9 Volt the one with " + " sign the first one of 25 pins, and verifying that the correspondent LED is on.

NULL-MODEM cable is made of two connectors DBF, (female) because both of them have to be connected to the terminals that have connectors DBM (male). As shown in the picture transmission circuits C 103 must be crossed by reception circuits C 104.

When the two terminals turn up C 105, it is connected to the C 106 , to simulate the answer of the MODEM to the terminal, and to turn up C 109 of the remote terminal, that says that carrier is in line.
The same crossing must be made between the C 108 of a terminal and the C 107 of the other one and vice versa.
This cable can’t be longer than 15 meters because of the voltage drop in the line.

The test has the purpose of monitoring of the error rate in 15 minutes of transmission, by using a DATA TESTER and putting the remote MODEM in a loop of interface (Loop 2). Programming the VIF to create the remote loop of interface, as in the picture.
At fist put the MODEM as Originate and the other as Answer.
DATA TESTER PROGRAMMING

• Give the high level voltage to the C 105 and C 108;
• MODEM will answer giving the high level voltage to the C 106 and C 107, as well as the C 109 to signal the carrier in the line of the remote MODEM.
• Put Tx and Rx internal clock;
• Put Tx and Rx clock 300 b/s. With MODEM V21/23 interface rate must be exactly the same to that one of the line.
• Choose pattern PR 511 bit.
• Synchronize with the key SYNC the DT on the arriving bit.
• Put a pair of wrong bits in with the DT.
• Detect isochronal distortion , during the measure: put the switch, first in early and after in late; reading on the display the two values.
• Detect on the display the number of wrong bits and of wrong blocks.

BER = 100 * 
[wrong bits (minus wrong injected bits) / received blocks * 511]
BLER = 100 * (wrong blocks / transmitted)

• Measure again putting, between two MODEMs, fictitious line of 16 Km and/or a noise generator to simulate a noisy line.

Monitor error rate, is the purpose of this test. In a gap of 15 minutes of transmission, using two data tester.

Two MODEMS programming.

Put MODEM 1 in originate; MODEM 2 in answer. Conventionally, originate calls, hence it will transmit with the first channel (980 – 1180 Hz). The answerer will transmit, instead, with the second channel (1650 – 1850).

DATA TESTERS’ programming.

• Give the high level voltage to C 105 and C 108 with the data testers.
• MODEM will answer turning up the C 106 and the C 107, as well as the C 109 to signal the carrier in line of the remote MODEM.
• Put Tx and Rx internal clock.
• Put Tx and Rx clock on 300 bit/s
• Choose for the pattern PR 511 bit.
• Synchronize the two DT on the arriving bits with the key SYNC.
• During the measuring, inject a pair of wrong bits by DATA TESTER 1.
• Detect, isochronal distortion, during the measuring: put the switch first in early and then in late, read, on the display, the two values.
• Detect, on the display, the number of wrong bits and wrong blocks.

BER = 100 * 
[wrong bits (minus injected wrong bits) / received blocks * 511]
BLER = 100 * (wrong blocks / transmitted blocks)
Measure for the second time, putting, between two MODEMs, a fictitious line of 16 Km and/or a noise generator to simulate a noisy line.

The test allows to observe the functioning of modulation and demodulation blocks of a V21/V23 MODEM.

It is possible to study and measure the carrier emitted by the MODEM turning on the C 105, its frequency and amplitude.

The ouput of the MODEM is closed on a 600 Ohm resistance, that is the telephone line impedance.

We use an oscilloscope to detect the shape and the amplitude of the signal.

We use a spectrum analyser, instead, to detect the frequencies and the levels of the signals in line.

1. Connect the DATA TESTER (or a VIF) to the MODEM and close the output of the latter on a 600 OHM load;
2. turn on, by the DATA TESTER, both the C 108 and C 105. The MODEM will answer turning C 106 and C 107.
3. visualize the carrier emitted by the MODEM, by the oscilloscope and the spectrum analyser.
4. Emit with the data tester some bits and observe by spectrum analyser the spectrum of the modulated carrier.
5. Inject a 511 BIT pattern and observe the spectrum of the carrier.

Using a signal generator with a 600 Ohm of internal resistance, put in line a 1180 Hz frequency, if the MODEM is set as answer, or a 1850 Hz frequency if it is set as originate. Observe the C 109, and determine the minimum amplitude of the signal, interpreted by the MODEM as the carrier of the remote MODEM, to turn it on.