9600 bps, Dual Modem 170 Card Private Wire Only

Transcription

1 bps, Dual Modem 170 Card Private Wire Only Users Guide 280 Interstate 80 West Exit 1 / PO Box 1330, Verdi, NV Phone: Fax: traffic@sgdi.net

2 TABLE OF CONTENTS General Characteristics 3 General Information 4 Interface 4 Installation 5 Switch Settings 5 Theory of Operation 5 System Description 5 Block Diagram Discussion 5 2 and 4 Wire (1/2 & Full Duplex) Operation 6 Mark and Space FSK Description 6 Modem Handshake Control 6 Soft Carrier Disconnect 6 Anti-Streaming Topic Discussion 6 General Characteristics Form Factor Standard Model 800 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 Series. 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 Modulation Type and Frequencies Phase Coherent FSK 11,200 Hz- Mark 17,600 Hz - Space 7,800 Hz - Soft carrier Line Interface 3002 Voice Grade Leased Line or Private Metallic Wire. Maximum Distance 7-10 miles (depending on line facility and loading). Characteristic Impedance - 600ohm Receiver Characteristics Dynamic Range - +3dbm to -42dbm Carrier Detect Threshold -42±3dbm Carrier Detect Hysteresis - Demodulator Distortion <10% peak (9600 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 signal to noise ratio of 16 db. (flat weight Hz) Transmit Level -8dbm to Odbm adjustable (potentiometer) factory setting - Odbm (600ohm) Serial interface Meets EIA RS-232C and CCITT V.24 Indicators Transmit Data - TXD Receive Data - RXD Request to Send - RTS Clear to Send - CTS Receive Carrier - DCD 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 Anti - Streaming (Switch Selectable) Enable/Disable Duplex Mode Options (Switch Selectable) 2 wire Half Duplex or 4 wire Full Duplex

3 GENERAL INFORMATION This section is included for the technician who has little or no experience with modems. For those technicians familiar with modems, the Theory of Operation section contains detailed information of the Model 896 Modem. The Model 896 is a dual asynchronous modem designed for rack mount (card cage) operations. The Model 896 utilizes phase coherent FSK (Frequency Shift Keying) for speeds up to 9600 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 hardwire line. The audio data is then converted back to digital information by a modem on the other end of the line (Demodulation). Modems can modulate digital information in several different ways. The Model 896 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 896 generates two frequencies 9600 Hz = logic low and 17,600 Hz = logic high. Communications terminology refers to a logic low as a MARK and logic high as a SPACE. By utilizing frequency sensitive filters, modems are capable of differentiating 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-persecond. The Model 896 is capable of communicating at baud rates up to The advantage of FSK techniques versus other techniques (that allow 9600 baud operation over standard copper path), 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 896 is capable of operating in either half duplex (HDX) or full duplex (FDX) mode. HDX operation implies that the Model 896 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 896 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 896. Model 896 Connections EIA Signal Name Pin Pin Function CHA CHB DR (Data Receiver) 2 22 Modulated data input to modem (4-wire only). DRR (Data Receiver Return) 3 21 Return circuit for DR DX (Data Transmit) X 20 Modulated data input/output (2-wire) or modulated data output only (4-wire). DXR (Data Transmit Y 19 Return circuit for DX. Return) BA (Transmitted Data) M 11 Digital data input to modem. BB (Received Data) P 13 Demodulated digital data output to controller. CA (Request To Send) L 10 Signals the modem connected device wants to CB (Clear To Send) CF (Carrier Detected) +12VDC -12VDC AA, AB (Ground) transmit. N 12 Flags the connected device modem is ready to transmit. K 9 Flags the connected device that the sine wave, which carries data, is being received. C&D E&F A&B Positive power source. Negative power source. Modem power and digital signal common.

4 INSTALLATION The GDI Model 896 Modem is designed to be used in conjunction with private, 2-wire HDX or 4-wire FDX metallic lines, or voice grade 3002 leased lines. Maximum line distance is restricted to 7 to 10 miles depending upon line characteristics and loading. Transmit and Receive interface termination impedance is 600 Ohm balanced, using Transformer coupling. The Model 896 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 896 it is necessary to ensure that it is set for the proper "mode" of operation (i.e., HDX or FDX). The Model 896 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. To use Modem in HDX (2-wire) configuration, copper pair must connect to Modem Pins X & Y for CH-A, or Pins 20 & 19 for CH-B. 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. 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 user's terminal is transmitting (sending) data. Any discrepancy in these LED outputs indicates incorrect install or failure Full/Half Duplex on/off - full duplex off/on - half duplex RTS - CTS Timing off - 8±2 msec on msec Carrier Turnoff Timing off - 10±2 msec on msec Local Echo enable/disable off - local echo enabled (required for full duplex) on - local echo disabled (normal for half duplex) Switch Settings Receiver Squelch Timing (Half duplex, local echo disabled) off msec on msec Receive Line Signal Detector (Carrier Detect) off - 8±2 msec on msec Anti-Streaming enable/disable off - disable on enable THEORY OF OPERATION System Description FSK data can enter the modem from two different sources; the Host Computer via DTR/DRR (4-wire only) or DTX/DRX (2-wire only). The digital data sent to the modem from the Users Terminal (BA) will be modulated and driven out on the DTX/DRX line (2-wire or 4-wire). If the modem is set up for HDX operation (Switches 1,2,5 - off,on,on), the transmitted data

5 will not be echoed back to the users terminal via the Received Data line (BB). In HDX operation the Model 896 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 896 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/DRX) while simultaneously received data from the line (via DTR/DRR) 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 user's 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 11,200 Hz frequency. Should the input be a logic high (SPACE), the oscillator will output a 17,600 Hz frequency. This "frequency shift" is how the modulator identifies a digital "one" (SPACE) from a digital "zero" (MARK). The received 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 1). The Model 896 logic is designed such that a MARK condition (11,200 Hz) on the telecommunications line will enable the Carrier Detected circuitry. This circuit will latch and will remain active until either a sustained frequency outside of the receive filter passband or a line fault occurs. The Model 896 is capable of generating a sustained frequency out of the normal receiver passband. When Request To Send goes false, the Model 896 will alter the oscillator output from either 11,200 Hz or 17,600 Hz to approximately 7,800 Hz, or less. This 7,800 Hz signal, known as soft carrier disconnect, flagging 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. Anti-Streaming Anti-streaming is a feature that prevents a modem connected to a malfunctioning controller from monopolizing the communication path. In multidrop, 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 896 (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.