INTRODUCTION:
The LDM-MR19.2 is a short-range modem which operates at full- or half-duplex with synchronous or asynchronous transmission over unconditioned telephone lines. With an extended range of up to 34 miles (55 km), the LDM-MR19.2 operates at any of
eight selectable data rates, up to 19,200 bps. By using conditioned differential diphase modulation (Eurocom Std. D1), the LDM-MR19.2 provides immunity from background noise, eliminates normal line distortion, and enables efficient transmission and
reception of serial data over twisted-pair cable. The LDM-MR19.2 is coupled to the telephone line through isolation transformers. The transformers and electronic circuitry protect against AC or DC overvoltages. The protection circuitry allows operation
even when DC is connected to the line because of the following:
Transmit Level and Transmit Receive impedance's are independently selectable.
Transmit timing is provided internally, derived externally from the data terminal, or regenerated from the Receive signal.
Receive timing is regenerated from the Receive signal.
Line communication is always synchronous. when you set the modem to asynchronous mode, it performs as async-to-sync conversion in compliance with the CCITT V.22 bis standard. The LDM-MR19.2 also features V.54 diagnostic capabilities for
performing local analog loopback and local and remote digital loopback. In the digital loopback mode, the operator at either end of the line may test both of the modems and the line. The operator can control the loopback with either the front-panel
pushbuttons or Pins 18 and 21 of the V.24/RS-232 interface.
FUNCTIONAL DESCRIPTION:
ENCODER:
The encoder modulates the input data from the DTE using the "conditional diphase modulation" technique. You can configure the encoder to operate in one of five modes:
1. 4-wire full-duplex
2. 4-wire half-duplex
3. 2-wire half-duplex
4. 4-wire multipoint
5. 2-wire multipoint
MODULATION TIMING:
The modulation-timing circuit supplies the Transmit clock to the encoder. Four clock sources are available:
1. INT. CLK.= internal clock--from the modem's internal crystal oscillator.
2. EXT. CLK. = external clock--from DTE, Pin 24.
3. RCV. CLK = receive clock recovered from Receive signal.
4. ASYNC = asynchronous timing for working with the async-to-sync converter in an async application.
Setting the XMT TIMING jumpers determines the timing option.
ASYNC-TO-SYNC CONVERTER
:
The LDM-MR19.2 provides internal conversion from async to sync in compliance with CCITT V.2 bis. According to this standard, frequency deviation between the modem and the DTE is compensated for by shortening or lengthening the stop bit of
the async character. When the modem's frequency is higher than the DTE's, the local converter extends the stop bit. When the modem's frequency is lower than the DTE's the local converter may delete one stop bit every four or eight characters. The
remote converter adds a shorter stop bit before sending the data to the remote DTE. For frequency deviations of up to 1.1%, the stop bits should be shortened every eight characters (12.5%). For frequency deviations of 1.2 % to 2.3%, the stop bits should
be shortened every four characters (25%). User Switch position 2 (S2) of the "ASYNC LENGTH" DIP switch to select 25% or 12.5% frequency deviation. For proper operation of the async-to-sync converter, set the async character length by adjusting
switch positions 3 and 4 (S3 and S4) of the "ASYNC LENGTH" DIP switch.
XMT LEVEL:
Four options are available for the XMT level (signal level): 0, -3, -6, or -9 dBm. XMT level is controlled by the XMT LEVEL jumper.
RECEIVER:
The receiver contains several circuits.
1. The RECEIVE FILTER removes all the out-of-band frequencies.
2. The AUTOMATIC EQUALIZER is made up of several equalizers activated according to data rate.
3. The digital AGC (automatic gain control) automatically compensates for the attenuation of the line. You may choose between the two modes of AGC operations by setting the AGC jumper:
ON: The AGC operates continuously.
CNTR: The AGC is active only when DCD is ON. When DCD is OFF, the AGC will remain in the state to which it was most recently set.
In full-duplex point-to-point applications, there is no difference between the two AGC modes of operation. However, there is a difference in multipoint applications, where the master modem communicates with modems at different distances. When the
master modem is set to AGC-ON, the master can communicate across long distances (up to its maximum range) to the slave modems. However, this setting increases the RTS-to-CTS delay. For example, under AGC-ON, the CTS delay of all slaves must
be set to 64 ms when the data rate is less than 9.6 Kbps.
When the master modem is set to AGC-CNTR, it can't transmit or receive across as much distance. However, the polling goes faster. For example, under AGC-CNTR, the CTS delay of all units can be set to 8 ms when the data rate is greater than or
equal to 4.8 Kbps.
V.54 DIAGNOSTICS:
You can activate V.54 loops with either the front-panel pushbuttons or Pins 18 and 21 can be enabled or disabled separately by the SWITCH, PIN 18 and PIN 21 jumpers, respectively. When you use the LDM-MR19.2 as a tail end to a DDS
network, set the V.54 DELAY jumpers of the modems located close to the DDS network to ON to prevent multiple loopbacks. The DELAY switch prevents the last modem from receiving the complete V.54 data sequence and being induced into a
loop. V.54 is required only for operation in synchronous mode.
NOTE:
The V.54 DELAY jumper must be set to OFF when
you operate in asynchronous mode.
