MODEL 800. GDI Communications, LLC. Dual 1200 BAUD Modem. Verdi, Nevada. rzidnanga-arnnt= Pnnc1 rsf 17

Transcription

1 MODEL 800 Dual 1200 BAUD Modem GDI Communications, LLC Verdi, Nevada rzidnanga-arnnt= Pnnc1 rsf 17

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3 Model 800 User's Manual 800 USER MANUAL Rev. B GDI Communications, LLC 280 I-80 Exit 1 West Verdi, Nevada This manual applies to units with Serial Numbers D and above Drawing Number A00539 Rev B

4 TABLE OF CONTENTS General Characteristics 3 General Information 4 Installation 5 Switch Settings 5 Interface 4 Theory of Operation 5 System Description 5 Block Diagram Discussion 5 Carrier Detection 6 Data Transmission 7 Reception of Data 8 Modem Control 8 Soft Carrier Detect 9 Anti-Streaming 9 Maintenance - Alignments - Waveforms 10 Form Factor Standard Model 400 General Characteristics Connector: Printed circuit board edge connector; two rows of 22 bifurcated contacts on inch centers (44 pins). Keyed between Pins C & D and Pins H & J. Connector Pin-Out Standard Model 800 (dual modem) Power Requirements (Max.) 75 ma (+12V ) 75 ma (-12V) Maximum noise ripple, 500mV Environmental Operating Ranges temperature -37 to +74 C humidity 95% (non-condensing) Data Rate and Format baud Serial Asynchronous, by bit (Anisochronous) Modulation Type and Frequencies Phase Coherent FSK 1200Hz - Mark 2200Hz - Space 900 Hz - Soft carrier Line Interface 3002 Voice Grade Leased Line or Private Metallic Wire. Maximum Distance miles (depending on line facility and loading). Characteristic Impedance Receiver Characteristics Dynamic Range - +3dbm to -48dbm Carrier Detect Threshold -42±3dbm Carrier Detect Hysteresis - Demodulator Distortion <10% peak (1200 bps, -40 dbm, 1:1 data pattern) Receiver Frequency Tolerance - ±25Hz (for -40dbm) Receiver Bandpass Filter - provides greater than 20 db/octave active attenuation for frequencies outside operating band. Bit Error Rate - Less than 1 bit in 100,000 bits with a sig nal to noise ratio of 16 db. (flat weight Hz) Transmit Level -8dbm to Odbm adjustable (potentiometer) factory setting - Odbm (6000) Serial interface Meets EIA RS-232C and CCITT V.24 Indicators Transmit Data - XMT Receive Data - RCV Request to Send - RTS Clear to Send - CTS Receive Carrier - CAR Timing (Switch Selectable) RTS-CTS 6/12±2mS CAR Delay 4/8±2mS (Soft) Carrier Turnoff 5/10±2mS Receiver Squelch 3/6.5±2mS 7

5 Model 800 User's Manual Anti - Streaming (Switch Selectable) enable/disable (7 seconds) Duplex Mode Options (Switch Selectable) 2 wire Half Duplex or 4 wire Full Duplex GENERAL INFORMATION The Model 800 is a dual 1200 baud FSK Modem. The 800 Modem is made of two independent 400 type Modems. The only connection between the two Modem sections is the power supply. These units are shipped in the Full Duplex (4 wire), standard timing mode of operation. The 800 modem is designed for rack mount (card cage) or 170E operation (170s with dual modem slots). The Model 800 utilizes phase coherent FSK (Frequency Shift Keying) for speeds up to 1200 baud. Modem Fundamentals The term MODEM is a combination of two words, MOdulator and DEModulator. Modems are designed to convert digital data to an audio frequency (Modulation) so that it can be transmitted via a telephone line. The audio data is then converted back to digital information by a modem on the other end of the telephone line (Demodulation). By using this mechanism, two digital devices (such as a data terminal and a computer) can be connected via a telephone line. Modems can modulate digital information in several different ways. The Model 800 uses a technique known as Frequency Shift Keying (FSK). Digital information is binary in format; that is, data is represented by either a logic high or a logic low (1 or 0). FSK modems utilize this fact and generate a unique frequency for each logic level. The Model 800 generates two frequencies 1200 Hz = logic low and 2200 Hz = logic high. Communications terminology refers to a logic low as a MARK and a logic high as a SPACE. By utilizing frequency sensitive filters, modems are capable ofdifferentiating between MARK and SPACE frequencies in order to extract or demodulate received data. The rate at which a communication line changes state is known as its baud rate. The baud rate can be interpreted to be the equivalent of bits-per-second. The Model 800 is capable of communicating at baud rates up to The advantage of FSK techniques versus other techniques (that allow 1200 baud operation over normal telephone lines), lies in system timing requirements. Other methods rely on comparing the phase relationships of the two modems to extract data. This requires time to 'synchronize' or 'train'. The time required can be as much as 60 msec per "connection". The Model 800 is capable of operating in either half duplex (HDX) or full duplex (FDX) mode. HDX operation implies that the Model 800 can transmit or receive data, but that these actions cannot occur simultaneously. HDX is referred to as 2-wire mode. In FDX mode, the Model 800 can transmit and receive data simultaneously. FDX is known as 4-wire mode. INTERFACE The following table lists signal names, edge connector pin numbers, and signal descriptions that pertain to the Model 800. Model 800 Connections Signal Name Pin Function DR (Data Receiver) 2 Modulated data input to modem (4-wire only). DRR (Data Receiver Return) 3 Return circuit for DR DX (Data Transmit) X Modulated data input/output (2-wire) or modulated data output only (4-wire). DXR (Data Transmit Return) BA (Transmitted Data) Y Return circuit for DX. M Digital data input to modem.

6 Model 800 Connections BB (Received Data) P Demodulated digital data output to controller. CA (Request To Send) L Signals the modem that the 170 wants to transmit. CB (Clear To Send) N Flags the 170 that the modem is ready to transmit. CF (Carrier Detected) K Flags the controller that the sine wave, which carries data, is being received. +12VDC C&D Positive power source. -12VDC E&F Negative power source. AA, AB (Ground) A&B Modem power and digital signal common. INSTALLATION The GDI Model 800 Modem is designed to be used in conjunction with private, unterminated 2-wire HDX or 4-wire FDX metallic lines, or voice grade 3002 leased lines. Maximum line distance is restricted to 10 to 20 miles depending upon line characteristics and loading. The Model 800 is designed for rack (card cage) mounting. All of the electronics are contained on a single x 6.25 inch printed circuit board. Prior to using the Model 800 it is necessary to ensure that it is set for the proper "mode" of operation (i.e., HDX or FDX). The Model 800 is shipped from the factory set to operate in FDX (4-wire) mode. Should the application require HDX (2-wire) operation, it is necessary to set the switches in accordance with the "Switch Settings" table. Please note that to change timing parameters, switch positions 3,4,6, and 7 should be changed together. They should be all OFF or all ON. Also, if "Fast" timing is desired to reduce handshake time requirements, all "local" modems connected must support, and be configured for, "Fast" timing. POWER ON CHECKS After power up, the modem will reflect the status of the interface via five light emitting diodes (LED) which are located on the leading edge of the printed circuit board. The five LEDs indicate the presence of Transmitted Data (from the users terminal), Received Data (from incoming carrier), Carrier Detected, Request To Send, and Clear To Send. The Transmit LED will only be active when the users terminal is transmitting (sending) data. Any discrepancies in these LED outputs indicates a failure. Full/Half Duplex on/off - full duplex off/on - half duplex RTS - CTS Timing off - 8±2 msec on - 4±2 msec Switch Settings Carrier Turnoff Timing off - 10±2 msec on - 5±2 msec Local Echo enable/disable off - local echo enabled (required for full duplex) on - local echo disabled (normal for half duplex) Receiver Squelch Timing (Half duplex, local echo disabled) off - 6.5±1 msec on - 3±1 msec Receive Line Signal Detector (Carrier Detect) off - 8±2 msec on - 4±2 msec Anti-Streaming enable/disable off - disable on - enable I The Model 800 MDC Modem has jumpers in place of the switches so that it is permanently selected for full duplex operation with standard timing (only switch 1 is on). THEORY OF OPERATION System Description The 800 Modem is two independent 400 type Modems. This section will talk about the op-

7 Model 800 User's Manual eration of a single modem but it will pertain to both modems. The 800 modem consists of an "A" and a "B" Modem. Just add a "B" to the part reference designations to get the actual part number for the Modem "B" section. Modem "A" (modem on the top half of the board) is the primary modem and connects to ACIA 1 in the 170E. Modem "B" connects to ACIA 2 on the 170E if the 800 is plugged into modem slot 1 in the Controller. When plugged into the first slot in the R Card cage, Modem "A" is the first modem and Modem "B" is the second Modem. Page 14 in the Drawing Appendix is a Functional Block Diagram of one section of the Model 800 Modem. The following text describes the Block Diagram in detail and references each block to the appropriate logic depicted in the schematic (at the end of this manual). Block Diagram Discussion As shown in the Block Diagram, FSK data can enter the modem from two different sources; the Host Computer via DTR (4-wire only) or DTX (2-wire only). The digital data sent to the modem from the Users Terminal (BA) will be modulated and driven out on the DTX line (2-wire or 4-wire operation). If the modem is set up for HDX operation (Switches 1,2,5 - off,on,on), the transmitted data will not be echoed back to the users terminal via the Received Data line (BB). In HDX operation the Model 800 is capable of transmitting to the 2- wire line or receiving from the 2-wire line; however, these actions cannot occur simultaneously. When the Model 800 is configured for FDX 4-wire operation, (Switches 1,2,5 - on,off,off) it is capable of receiving and transmitting simultaneously. When in FDX, data can be transmitted to the line (via DTX) while simultaneously received data from the line (via DTR) is being forwarded to the users terminal via circuit BB, Received Data. In either FDX or HDX mode, data will always be sent to the line via the DTX line. Digital data from the users terminal enters on the Transmitted Data (BA) input. The data will be buffered from EIA RS-232-C levels and forwarded to an oscillator. The oscillator will output one of two frequencies, (a third frequency is generated under special circumstances discussed later). If the digital data received is a logic low (MARK), the oscillator will output a 1200 Hz frequency. Should the input be a logic high (SPACE), the oscillator will output a 2200 Hz frequency. This "frequency shift" is how the modulator identifies a digital "one" (SPACE) from a digital "zero" (MARK). The modulated data is fed to a transmitter squelch circuit which will inhibit the transmission of data should the Request To Send (CA) be false (low). If Request To Send is true (high), the modulated data will be forwarded to a line driver and then impedance matched (via a Transformer) and driven out through the transmission wire pair on a line known as Data Tip Transmitter (DTX). The Request To Send logic will squelch the receiver in HDX mode to insure that the transmitted data is not "echoed back" to the users terminal (dependent on the setting of Switch 5). This is the path for transmitted data in both HDX and FDX modes. When receiving FSK signals in the HDX mode, data enters the Data Tip Transmitter (DTX) input. The received data is transformer coupled to a line receiver, filtered, limited, demodulated, filtered and presented to the users data terminal equipment on the Received Data (BB) output. In FDX mode, received data audio signals enter the Data Tip Receiver (DTR) input. The data will then follow the same path as data received in HDX mode. The users data terminal equipment will not receive data from the BB output unless the Carrier Detected logic has sensed the presence of a carrier (refer to Figure 3). The Model 800 logic is designed such that a MARK condition (1200 Hz) on the telecommunications line will enable the Carrier Detected circuitry. This circuit will latch and will remain active until either a sustained fre-

8 quency outside of the receive filter passband or a line fault occurs. The Model 800 is capable of generating a sustained frequency out of the normal receiver passband. When Request To Send goes false, the Model 800 will alter the oscillator output from either 1200 Hz or 2200 Hz to approximately 900 Hz, or less. This 900 Hz signal, known as soft carrier disconnect, will flag the modem, on the receiving end of the data stream, that communication has been suspended. Only the presence of a MARK condition on the telecommunications line in conjunction with a Request To Send (CA) signal will be able to re-establish the communications link. Carrier Detection Refer to the schematic at the end of this manual. The presence of the carrier is detected in the same manner regardless of the mode of operation (i.e. HDX or FDX). Data received will be buffered by amplifier U1-d and then filtered by a two stage bandpass filter (U1-a,b). This filter network is designed to reject any frequencies which are outside of the carrier signal range. After the received signal has been filtered, it is boosted by U1-c and fed to a level detection circuit (U8-a,b,c). Amplifier U8-a is used at unity gain, driving D8 and D9 to create a "full wave" rectified voltage at the inverting input of U8-b. This DC voltage varies in relation to the strength of the receiver carrier signal. Low-pass amplifier U8-b filters this DC voltage and presents it to U8-c. Whenever the rectified signal is greater than 6 VDC, U8-c will go high allowing capacitor C31 to charge through Resistor R67. At the end of a four millisecond charging period, comparator U7-a will be biased "ON". This "ON" condition will enable the Carrier Detected (CF) output as well as the Carrier LED (L2). At the same time, the received data is also presented to a hard limiter (U3-d). The hard limiter circuitry establishes the input level to the demodulator circuitry. The demodulator consists of IC U2. The first amplifier stages U2-c and U2-d filter out the SPACE frequency (2200 Hz) and converts it to a negative DC voltage through D1and D2. The second amplifier stages of U2-a and U2-b filter out the MARK frequency (1200 Hz) and converts it to a positive DC voltage through D3 and D4. These DC outputs are summed at the input of U3-c. Amplifier U3-c acts as a low-pass filter to "clean-up" and amplify the output from the summing junction. This output is then fed through an additional four-stage, lowpass filter (IC U4) and presented to the EIA driver (U7-c) as well as comparator U7-d. Comparator U7-d is biased such that, when a MARK state is demodulated, the output of U4-d will force the inverting input of comparator U7-d to a level lower than the bias voltage on Pin 12 of U7-d. This forces the output high which reverse biases diode D10. As long as the carrier frequency is present, U7-d will keep Diode D10 reverse biased. As long as the carrier is of sufficient amplitude, U8-c will keep capacitor C31 charged through R67. Once the carrier frequency has been detected, comparator U7-a will forward bias diode D12, thus changing the bias level on comparator U7-d. This effectively "latches" the carrier active unless the carrier amplitude goes too low or the modem receives an input frequency which is outside of the receive filter passband. Should the modem receive a signal out of its passband, the receive filter will reject the frequency, the output of U8-c will go low, and capacitor C31 will discharge. This will cause the Carrier Detected (CF) line to go false (low).the absence of Carrier Detected will again change the bias to U7-d, which will forward bias diode D10 and inhibit Carrier Detected until a signal of the correct amplitude and frequency is again received. The absence of Carrier Detected, disables the Received Data (BB) output of the modem. Note that the Carrier Detected (CF) signal is affected by Switch 5. In HDX mode the presence of Request To Send is used to suppress the Carrier Detected (CF) signal (Switch 5 must be "ON"). As before, the suppression of Carrier Detected disables the Received Data (BB) circuit. This prevents transmitted data from being "echoedback" to the user. Referring back to the schematic, when the Request To Send line is true (high), Transistor Q1 will be "ON". This will bias

9 Model 800 User's Manual comparator U5-c Pin-9 low resulting in a high output on Pin-8. Comparator U7-b Pin-7 will also go low, thus inhibiting Carrier Detected at comparator U7-a Pin-3 (assuming Switch 5 is closed). Comparator U7-a Pin-1 will be driven low. This will inhibit the EIA output of comparator U7-c (BB). Data Transmission The reception of data is basically the same Data to be transmitted via the communications in either FDX or HDX modes, only the entry path line enters the modem on the Transmitted Data into the modem differs. In the HDX mode, data (BA) input. The data at this point is digital data enters on the DTX input and is transformer coupled (via T2) to amplifier U1-d. In FDX mode, and will be at an EIA level. Assuming a SPACE state on the input (high), Transistors Q5, Q7 and data enters on the DTR input and is transformer Q8 will be "OFF". This condition will, effectively coupled to U1-d via T1. Amplifier U1-d acts as a remove resistors R86 and R93 from the oscillator line receiver. Amplifiers U1-a and U1-b form a circuitry (amplifiers U6-a,c,d). These amplifiers bandpass filter designed to discriminate against form a free running oscillator, which will oscillate frequencies other than those in the 1200 to 2200 at 2200 Hz (assuming Q7 and Q8 are "OFF") or at 1200 Hz if Q7 and Q8 are "ON". Transistors Hz range. This filtering minimizes susceptibility Q7 and Q8 alter the current flow to U6-d and U6- to noise. a by inserting Resistors R86 and R93 in the circuit. If a logic low (MARK state) appears on the The transmission of FSK data by the GDI Modem Control BA input, the oscillator will output a 1200 Hz frequency; if a logic high (SPACE state) occurs, the logic. When the user connected to the modem Model 800 is controlled by the Request To Send oscillator will output a 2200 Hz frequency. wishes to send data over the telephone line, he Hence, a different frequency will be output for raises (asserts) the Request To Send input. The each logic level; this is known as Frequency presence of Request To Send (logic high) will Shift Keying. Amplifier U6-d has a Diode (D15) connected such that the output can never "saturate" (i.e., be a square wave). The sine wave turn Transistor Q1 "ON". This will turn Transistor Q2 "OFF" and at the same time present a logic low output from amplifier U6-a is fed to amplifier U6-b (0 volts) on the inverting input of comparator U5-c which acts as a line driver. The output of U6-b is (Pin-9). Pin-10 of U5-c is DC biased at +6 VDC. Transformer coupled (via T2) to the telephone The output of U5-c will then be a logic high, line (DTX). The output level of the transmitted which reverse biases Diode D18. Capacitor C35 signal is determined by Potentiometer R106. The will charge through Resistor R115. This charging frequency of the oscillator can be adjusted with time will be approximately 12 milliseconds. During Potentiometer R89. Refer to the Alignment Section in this manual for adjustment procedure. U5-b (Pin-7) will be low. This allows the receiving this 12 ms charge time, the output of comparator In HDX mode, the transmitted data is fed not modem 12 ms to detect the presence of a carrier, only to the outside world (via DTX) but also goes prior to data being transmitted. The presence of to the receiver circuit (because Switch 2 is "ON"). Request To Send, will reverse bias Diode D17 The primary impedance of T2 is approximately (through comparator U5-a) thus enabling the 600 ohms. This impedance in conjunction with R108, forms a voltage divider which will prevent output Line Driver U6-b. the receiver section from being overdriven. The data will follow the same path as described in the Carrier Detect section of this manual. The data n will be buffered, filtered, limited, demodulated, refiltered and presented to the users terminal (assuming that Switch 5 is "OFF"). This feature facilitates "loopback" testing. Under normal circumstances in HDX mode (Switch 5 "ON"), the transmitted data will disable the receiver and the carrier detect outputs from the modem. Reception of Data

10 Should Request To Send go false, Transistor Q1 will turn "OFF", forcing U5-c Pin-8 low. If Pin-8 is low, Diode D18 will be forward biased. This will cause Capacitor C35 to rapidly discharge, thus dropping Clear To Send (CB) immediately. If Pin- 8 is low, Diode D16 will be reverse biased, and Capacitor C34 will discharge through R112. This will keep the Line Driver U6-b enabled for the period of time it takes C34 to discharge (approximately 10 ms). At the same time, because Request To Send went false (low), Transistor Q3 turned "ON". This will alter the frequency of the oscillator to 900 Hz. The 900 Hz signal will be transmitted to the telecommunication line, where it will be received by the modem at the other end of the line. The receiving modem will reject this frequency because it is outside of the receive filter passband range (1200 to 2200 Hz) and, therefore, will drop its Carrier Detected signal, signifying that the data transfer between modems has ceased. Note that Request To Send could go false (low) when the data on the line is in SPACE condition. This could cause problems as the oscillator is designed to output a 900 Hz tone only if transistors Q2, Q7 and Q8 are "ON". Remember that Q7 and Q8 are only "ON" during a MARK state. For this reason comparator U5-d Pin- 14 will go positive whenever Request To Send goes false (low). This will forward bias Diode D20 which will turn on transistor Q6, thus assuring a MARK state will exist independent of the BA input. For 2-wire mode only (Switch 5 "ON"), the Carrier Detected Disable line will be held active (low) while Request To Send is true (high), (i.e., while data is being sent). When Request To Send goes false (low), the Carrier Detected Disable line will be held active (low) for the period of time it takes to discharge C36 through R120 (approximately 7.6 ms). If an adequate incoming carrier is being received, the Carrier Detected condition will be achieved approximately 3.1 ms later (due to the charging of C31 through R67 at the input of comparator U7-a). During HDX operation, data may be received by the GDI Model800 modem approximately 11 ms after Request To Send goes false (low). Soft Carrier Detect During the time that Request to Send went false, (low) and the 900 Hz tone is being sent for the period of 10 ms, the receiving modem is detecting the 900 Hz tone as an out of band frequency. The 900 Hz tone is a requirement for switched communications or polling and addressing systems. The 900 Hz tone is present on the transmission line for a period longer than the receiving modem takes to recognize a loss of carrier and squelch the receiver. This protects the receiving modem from detecting any line ringing or energy present caused by the removal of the transmitting tone. In order to protect the receiver from staying on line too long or not recognizing a loss of carrier, due to noisy lines or in-band noise, the soft carrier detector plays an important part in squelching the receiving modem which will insure a loss of carrier and eliminate any data transmissions errors. After the soft carrier tone has been filtered and is boosted by U1-c and fed to the hard limiter U3-d it is presented to the filters U3- a,b. Bandpass filter U3-a is designed to pass a frequency of 900 Hz which is rectified by D7. This rectified voltage presents a positive DC at the inverting input of U8-d this causes the output (pin 14) to go negative. Amplifier U8-d acts as a lowpass filter, to clean up and amplify the output from the rectifier, this negative voltage will forward bias D19. This will inhibit carrier detect by discharging C31 and forcing the output of U7- a (low) squelching the receiver. U3-b is a band-pass filter designed to pass a frequency of 1700 Hz which is the center frequency of the operating band. As long as there is energy within the operating band U3-b will create a negative voltage through rectifier diode D6 which will bias amplifier U8-d input pin 13 negative. This forces the output high to prevent the amplifier U8-d from squelching the carrier detect circuit during valid carrier and only allows the soft carrier detector to squelch when the soft carrier frequency is present.

11 Model 800 Users Manual Anti-Streaming Anti-streaming is a feature that prevents a modem connected to a malfunctioning controller from monopolizing the communication path. In multi-drop, polled, environments a Master' sends out a logical address and expects a response from the addressed device. If another device is transmitting carrier (because of a stuck Request to Send), the addressed device will be unable to respond. The anti-streaming feature of the Model 800 (Switch 8 on) prevents this by limiting the amount of time that the modem can continuously transmit to around 7 seconds. If Request To Send remains continuously active for longer than this period, the modem will override the condition, and turn itself off until the next state-change of Request to Send. Request To Send (CA) enters the modem at P1-L. When CA goes high, (active) it turns on Q10 and the RTS LED. This turns off Q11, allowing C3 to charge through R14. When C3 has charged to around 6V, U5 pin 14 goes low. This turns off modem enable circuitry through SW1-8, D26, and R92. If SW1-8 is off, the modem behaves normally. The anti-streaming feature should only be enabled at 'slave' sites in polled schemes, where the response packet is limited to 5 seconds or less. The anti-streaming function in the 800 MDC is not selected. 2. Oscillator Frequency R89 3. Demodulated Data Symmetry R15 TOOLS REQUIRED: 1. Oscilloscope 2. AC RMS Voltmeter 3. Frequency Counter 4. Slot Head Screw Driver Each adjustment is described below. Transmit Level Adjustment 1. Apply Request To Send (CA High). 2. Transmit a MARK (BA Low). 3. Connect 6000 resistor between P1-X, P1-Y. 4. Connect the AC voltmeter to P1-X, P1-Y. 5. Adjust R106 for 775 mv RMS (0 dbm). Oscillator Frequency Adjustment MAINTENANCE Alignments There are three alignments for each modem section that can be made to the GDI Model 800 Modem. All are made by adjustment of individual Potentiometers as follows: 1. Apply Request To Send (CA High). 2. Transmit a MARK (BA Low). 3. Connect the frequency counter to P1-X, P1-Y 4. Adjust R89 for 2196 Hz indication 5. Transmit MARK (BA Low) 6. Verify a Hz indication. Demodulated Data Symmetry Adjustment 1. Set Modem to FDX mode. 2. Apply Request To Send (CA High). 3. Transmit an ASCII "U" (1:1 bit pattern). 4. Interconnect P1-2 to P1-X and P1-3 to P1-Y. 1. Transmit Level R106

12 5. Connect the Oscilloscope to P1-P, P1-A (-). 6. Adjust R15 for a symmetrical square wave as shown below. Carrier Detect Threshold Measurement 1. Apply Request To Send (CA High). 2. Transmit an ASCII "U" to the Modem. 3. Connect a variable 600 ohm attenuator with a 4. minimum range of 50 db between output P1- X/P1-Y and input P1-2 / P1-3 (4-wire configuration required). 5. Connect the AC Voltmeter to P1-2(+), P1-3(-). 6. Starting at minimum attenuation, increase attenuation until 6.2 mv RMS (-42 dbm) is indicated on the AC Voltmeter. The modem should still receive and demodulate the ASCII "U". This would be confirmed by observing Received Data at Pin P. The Model 800 Modem has six test points on the front edge of the card so that you have easy access to the audio input, the received data, and Ground for each modem section. The test points are labeled as follows: AUDIO IN...received FSK at the input transformer GND... Ground RXD... The received data (RS-232) The AUDIO IN signal may be very small is some systems, on the order of 10 millivolts if the Modem is at the end of a long line.