Black Box USB Director RS-232 User manual

CUSTOMER
SUPPORT
INFORMATION
Order toll-free in the U.S.: Call 877-877-BBOX (outside U.S. call 724-746-5500)
FREE technical support 24 hours a day, 7 days a week: Call 724-746-5500 or fax [phone]
Mailing address: Black Box Corporation, 1000 Park Drive, Lawrence, PA 15055-1018
Web site: www.blackbox.com • E-mail: [email protected]
MARCH 2001
ME380A
19.2-kbps RS-232 Long-Haul Modem
(LHM-19.2)
LHM-19.2
PATT
REM
ANA
DIG
PWR RTS TD RD DCD TEST SQ

1
FCC/IC RFI STATEMENTS, SAFETY WARNING, TRADEMARKS
FEDERAL COMMUNICATIONS COMMISSION AND INDUSTRY CANADA
RADIO-FREQUENCY INTERFERENCE STATEMENTS
This equipment generates, uses, and can radiate radio-frequency energy, and if not
installed and used properly, that is, in strict accordance with the manufacturer’s
instructions, may cause interference to radio communication. It has been tested
and found to comply with the limits for a Class A computing device in accordance
with the specifications in Subpart J of Part 15 of FCC rules, which are designed to
provide reasonable protection against such interference when the equipment is
operated in a commercial environment. Operation of this equipment in a
residential area is likely to cause interference, in which case the user at his own
expense will be required to take whatever measures may be necessary to correct
the interference.
Changes or modifications not expressly approved by the party responsible
for compliance could void the user’s authority to operate the equipment.
This digital apparatus does not exceed the Class A limits for radio noise emission from
digital apparatus set out in the Radio Interference Regulation of Industry Canada.
Le présent appareil numérique n’émet pas de bruits radioélectriques dépassant les limites
applicables aux appareils numériques de la classe A prescrites dans le Règlement sur le
brouillage radioélectrique publié par Industrie Canada.
EN 55022 warning: This is a Class A product. In a domestic environment, it might
cause radio interference. If it does, the user might be required to take adequate
measures to correct the interference.
Safety warning: Always observe standard safety precautions when you install,
operate, and maintain this product. Only qualified and authorized service
personnel should attempt to adjust, maintain, or repair it; this should never be
done by untrained or unauthorized persons.
TRADEMARKS USED IN THIS MANUAL
BLACK BOX and the logo are registered trademarks of Black Box
Corporation.
Any other trademarks mentioned in this manual are acknowledged to be the property of the
trademark owners.

2
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
EUROPEAN UNION DECLARATION OF CONFORMITY
The manufacturer declares that the LHM-19.2 conforms to the following EMC and
safety standards:
• EN55022 (1994): “Limits and methods of measurement of radio disturbance
characteristics of information technology equipment.”
• EN50082-1 (1992): “Electromagnetic compatibility: Generic immunity
standard for residential, commercial, and light industry.”
• EN60950 (1992/3): “Safety of information technology equipment, including
electrical business equipment.”
The LHM-19.2 herewith complies with the requirements of the EMC Directive
89/336/EEC and the Low Voltage Directive 73/23/EEC. The product was tested
in a typical configuration.
Also, in accordance with EN41003, we declare the safety status of the ports on
the LHM-19.2: The port labeled “DTE” has SELV (Safety Extra Low Voltage)
status, and the ports (terminals) labeled “XMT” have TNV (Telecommunication
Network Voltage) status within SELV limits.

3
NOM STATEMENT
NORMAS OFICIALES MEXICANAS (NOM)
ELECTRICAL SAFETY STATEMENT
INSTRUCCIONES DE SEGURIDAD
1. Todas las instrucciones de seguridad y operación deberán ser leídas antes de
que el aparato eléctrico sea operado.
2. Las instrucciones de seguridad y operación deberán ser guardadas para
referencia futura.
3. Todas las advertencias en el aparato eléctrico y en sus instrucciones de
operación deben ser respetadas.
4. Todas las instrucciones de operación y uso deben ser seguidas.
5. El aparato eléctrico no deberá ser usado cerca del agua—por ejemplo, cerca
de la tina de baño, lavabo, sótano mojado o cerca de una alberca, etc..
6. El aparato eléctrico debe ser usado únicamente con carritos o pedestales que
sean recomendados por el fabricante.
7. El aparato eléctrico debe ser montado a la pared o al techo sólo como sea
recomendado por el fabricante.
8. Servicio—El usuario no debe intentar dar servicio al equipo eléctrico más allá
a lo descrito en las instrucciones de operación. Todo otro servicio deberá ser
referido a personal de servicio calificado.
9. El aparato eléctrico debe ser situado de tal manera que su posición no
interfiera su uso. La colocación del aparato eléctrico sobre una cama, sofá,
alfombra o superficie similar puede bloquea la ventilación, no se debe colocar
en libreros o gabinetes que impidan el flujo de aire por los orificios de
ventilación.
10. El equipo eléctrico deber ser situado fuera del alcance de fuentes de calor
como radiadores, registros de calor, estufas u otros aparatos (incluyendo
amplificadores) que producen calor.
11. El aparato eléctrico deberá ser connectado a una fuente de poder sólo del
tipo descrito en el instructivo de operación, o como se indique en el aparato.

4
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
12. Precaución debe ser tomada de tal manera que la tierra fisica y la polarización
del equipo no sea eliminada.
13. Los cables de la fuente de poder deben ser guiados de tal manera que no
sean pisados ni pellizcados por objetos colocados sobre o contra ellos,
poniendo particular atención a los contactos y receptáculos donde salen del
aparato.
14. El equipo eléctrico debe ser limpiado únicamente de acuerdo a las
recomendaciones del fabricante.
15. En caso de existir, una antena externa deberá ser localizada lejos de las lineas
de energia.
16. El cable de corriente deberá ser desconectado del cuando el equipo no sea
usado por un largo periodo de tiempo.
17. Cuidado debe ser tomado de tal manera que objectos liquidos no sean
derramados sobre la cubierta u orificios de ventilación.
18. Servicio por personal calificado deberá ser provisto cuando:
A: El cable de poder o el contacto ha sido dañado; u
B: Objectos han caído o líquido ha sido derramado dentro del aparato; o
C: El aparato ha sido expuesto a la lluvia; o
D: El aparato parece no operar normalmente o muestra un cambio en su
desempeño; o
E: El aparato ha sido tirado o su cubierta ha sido dañada.

5
TABLE OF CONTENTS
Contents
Chapter Page
1. Specifications ............................................................................................. 6
2. Introduction ............................................................................................... 9
2.1 Overview .............................................................................................. 9
2.2 Functions ........................................................................................... 11
3. Configuration ........................................................................................... 14
4. Installation ................................................................................................ 18
4.1 Rackmounting (Optional) ............................................................... 18
4.1.1 Rackmounting a Single LHM-19.2 ........................................ 19
4.1.2 Rackmounting Two LHM-19.2s Side by Side ........................ 20
4.2 Connecting Data Cables ................................................................... 22
4.3 Connecting Power ............................................................................. 24
5. Operation ................................................................................................. 25
5.1 Power-Up ........................................................................................... 25
5.2 The LHM-19.2’s Front-Panel Indicators and Controls ................... 25
5.3 Power-Down ....................................................................................... 27
5.4 Reconfiguration ................................................................................ 27
6. Troubleshooting ...................................................................................... 28
6.1 Tests and Diagnostics ........................................................................ 28
6.1.1 PATT Button: Bit Error Rate Test (BERT) ........................... 28
6.1.2 ANA Button: Local Analog Loopback ................................... 30
6.1.3 REM Button: Remote Digital Loopback ............................... 31
6.1.4 DIG Button: Local Digital Loopback .................................... 33
6.2 Calling Black Box .............................................................................. 34
6.3 Shipping and Packaging ................................................................... 34
Appendix: RS-232 Pinout ............................................................................... 35

6
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
1. Specifications
Compliance — EMI/RFI: CE (EN55022, EN50082-1), FCC Part 15
Subpart J Class A, IC Class/classe A;
Electrical safety: CE (EN60950, EN41003)
Cable Required — Between ME380A units: 4-wire twisted pair, 19 to
26 AWG;
Between ME380A and DTE: EIA/TIA RS-232 cable with
DB25 male connector on ME380A end
Interface — To line: 4-wire telco style (do not connect to PSTN);
To device: Serial EIA/TIA RS-232/CCITT V.24, DCE
Protocol — To line: Synchronous;
To device: Synchronous or asynchronous (user-
selectable); for line transmission, async data is
converted to sync using ITU V.14 compliant methods
Clock Source — Either internal, external from local device, or recovered
from remote LHM-19.2 (user-selectable)
Data Format — User-selectable for any word length from 8 to 11 bits,
including 5, 6, 7, or 8 data bits; even, odd, or no parity;
and 1, 1.5, or 2 stop bits; see Table 3-1 in Chapter 3
Data Rate — 1.2, 2.4, 4.8, 9.6, or 19.2 kbps (user-selectable)
Transmit Level — 0, –3, –6, –9, or –12 dBm (user-selectable)
Transmit
Impedance — 600 ohms or “LOW” (user-selectable)
Receive
Impedance — 600 ohms or “HIGH” (user-selectable)
Return Loss — Greater than 15 dB
Carrier — Controlled by RTS or constantly ON (user-selectable)
CTS Signal — ON only when modems are synchronized
Encoding — 16-state, 8-dimensional Trellis coding
Modulation — QAM with error correction

7
CHAPTER 1: Specifications
Equalization — Adaptive
Synchronization
Delay — Startup: Up to 8 seconds;
Resync: Up to 5 seconds
Line Type — 4-wire, at least voice-grade
Maximum
Transmission
Distance — Unloaded lines: Up to 37.2 miles (60 km)—see Table 1-1;
Conditioned lines: Up to 62 miles (100 km), depending
on line quality
Table 1-1. Maximum transmission distance.
Wire Gauge Up to 9600 bps At 19.2 kbps
19 AWG 37.2 mi. (60 km) 28 mi. (45 km)
22 AWG 21.1 mi. (34 km) 16.1 mi. (26 km)
24 AWG 15.5 mi. (25 km) 11.8 mi. (19 km)
26 AWG 12.1 mi. (19.5 km) 9 mi. (14.5 km)
User Controls — (4) Front-mounted pushbuttons:
DIG (local digital loopback),
ANA (local analog loopback),
REM (remote digital loopback), and
PATT (bit error rate test);
(10) Internal:
(1) Dial for data rate;
(1) Four-position DIP switch for protocol, async word
length, and stop-bit shortening;
(8) Jumpers:
Transmit clock source;
Carrier control;
Transmit level;
Transmit-line impedance;
Receive-line impedance;
Enable/disable analog-loopback command on
Pin 18;
Pin 21 signal (RL for remote digital loopback or
SQD for signal quality);
Connect/disconnect signal ground and chassis
ground

8
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
Diagnostics — Continuous line-quality testing (results indicated with
SQ LED and optionally with SQD signal);
V.52 compliant bit error rate testing with pseudo-random
bit patterns (results indicated with SQ LED and
optionally with SQD signal);
V.54-compliant loopback tests:
Local analog loopback, mechanically or electrically
user-controllable;
Local digital loopback, mechanically user-controllable;
Remote digital loopback, mechanically or electrically
user-controllable
Indicators — (7) Front-mounted LEDs: PWR (power), RTS, TD, RD,
DCD, TEST, and SQ; also indicates line quality and
BERT results with SQD signal on Pin 21 if that function
is enabled
Connectors — (2) Rear-mounted:
(1) DB25 for cable to DTE;
(1) 5-position terminal block for 4-wire modem-to-
modem line, including optional ground for cable
shield
Temperature
Tolerance — 32 to 158˚F (0 to 70˚C)
Humidity
Tolerance — Up to 90% noncondensing
Enclosure — High-impact plastic
Protection — AC/DC overvoltage-protection circuits connected
through transformers to transmit and receive lines
Fuse — 250 V, 250 mA slow-blow
Power — Directly from outlet through detachable 5-ft. (1.5-m) line
cord and rear-mounted IEC 320 male inlet:
Input: 115 VAC, 47 to 63 Hz, up to 250 mA;
Consumption: 5 watts typical
Size — 1.6"H x 9.6"W x 7.6"D (4.1 x 24.4 x 19.3 cm)
Weight — 3.1 lb. (1.4 kg)

9
CHAPTER 2: Introduction
2. Introduction
2.1 Overview
The 19.2-kbps RS-232 Long-Haul Modem (LHM-19.2) is a medium-range, 4-wire
voice-band modem operating at data rates up to 19,200 bps. It is optimized for
metropolitan applications where the distance is too long for short-range modems,
but insufficient to justify the expense of long-range modems.
The LHM-19.2 operates in full duplex. You can attach pairs of LHM-19.2s to
conditioned or unconditioned 4-wire leased or private lines. They will be able to
communicate at selectable data rates up to 19,200 bps, as far as 41 miles (65 km)
over unconditioned lines (depending on the data rate and line quality), or as far as
62 miles (100 km) over conditioned lines (depending on the line quality only).
The LHM-19.2 uses an adaptive equalizer and 16-state 8-dimensional Trellis-
coded modulation to achieve as close to error-free communication as possible. The
LHM-19.2’s transmit level is selectable in 3-dBm increments, over the range of 0 to
–12 dBm. The modems are coupled to the attached 4-wire line through isolation
transformers which, together with other circuitry, protect against AC or DC
overvoltages.
Communication between LHM-19.2 pairs over the 4-wire link is always
synchronous. Each LHM-19.2 can derive the transmit clock internally, externally
(from the attached DTE), or by recovering it from the signal received from the
other modem. (Receive clock is regenerated from the data.) Communication with
the attached DTE can be either synchronous or asynchronous; when it’s
asynchronous, the LHM-19.2 performs V.14-compliant async-to-sync conversion.
For diagnostics, the LHM-19.2 performs continuous line tests. It indicates line
quality with the SQ LED and (optionally) the RS-232 SQD signal: The LED stays ON
and the signal stays high if quality is good, but if the bit error rate ever rises above
1x10
-3, the LED and signal flicker for intermittent errors or dim or even go
dead
for continuous errors. The LHM-19.2 can also perform V.54 testing, including
local
analog loopback and local and remote digital loopback. These tests can be controlled
with the LHM-19.2’s front-panel pushbuttons or by toggling the RS-232 LL and RL
signals on Pins 18 and 21 of its DTE interface. Finally, the LHM-19.2 supports an
internal pseudo-random V.52 Bit Error Rate Test pattern; it shows results with the
SQ LED and SQD signal the same way it does for the line-quality tests.
On the next page, Figure 2-1 shows a standard dual-modem application, and
Figure 2-2 shows a tail-circuit application using four modems.

10
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
Figure 2-1. Two LHM-19.2s in a direct point-to-point application.
Figure 2-2. Four LHM-19.2s in a tail-circuit application.
DTE (PC, server, etc.)
DTE (PC,
server,
etc.)
DTE (PC,
server,
etc.)
LHM-19.2
LHM-19.2
LHM-19.2 LHM-19.2
LHM-19.2
4-wire line
4-wire
line
4-wire
line
Digital
Service
Network
LHM-19.2
DTE (PC, server, etc.)

11
CHAPTER 2: Introduction
2.2 Functions
This section describes some of the LHM-19.2’s electronic functions, especially
those you might need to configure (see Chapter 3). Refer to Figure 2-3.
Figure 2-3. Block diagram for the LHM-19.2.
Timing Generator (Sync Clock)
This circuit is a PLL circuit. The LHM-19.2 can transmit data to the line at one of
five selectable synchronous rates: 1.2, 2.4, 4.8, 9.6, or 19.2 kbps. You can use the
XMT CLK jumper to select where the LHM-19.2 gets its transmit timing (sync
clock) from:
• INT (internal clock from the LHM-19.2’s own crystal oscillator)
• EXT(external clock from the attached DTE device, through RS-232 Pin 24)
• LOOP (receive clock recovered from receive signal and looped back)
In the vast majority of cases, one modem should be strapped to INT or EXT clock,
while the other is set to LOOP clock.
RTS
RTS
CTS
ENCODER
CONTROL
INTERNAL
TIMING
POWER
SUPPLY
MODULATOR
TIMING
CNTL/ON
TEST
PATTERN
TXC
EXT
CLK
REM
COMMAND
GENERATOR
ASYNC TO SYNC
CONVERTER
REM
TXD
BERT
PATT
REM
DIG
RLB
COMMAND
DECODER
DIG
LPBK
ENCODER
TRELLIS MODULATOR TRANSMIT FILTER
SQ
VITERBI
DECODER
LPBK
TIMING TIMING
RECOVERY
CARRIER
DETECT
ANA
RXD
RXC
DCD
ADAPTIVE
EQUALIZER
DEMOD-
ULATOR
LINK
EQUALIZER
RECEIVE
FILTER
AGC
SELECTABLE
TRANSMIT LEVEL
ANA TX
RX

12
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
Async-to-Sync Converter
If you configure the LHM-19.2 to accept asynchronous data and attach it to a DTE
that sends async data, the LHM-19.2 uses its internal V.14-compliant async-to-sync
converter to add clock timing to the data for transmission to the remote unit.
The V.14 standard specifies that differences between the LHM-19.2’s sync line
frequency and the DTE’s async data rate should be compensated for by shortening
or lengthening the stop bit of each async character. If the local modem’s frequency
is higher than the local DTE’s, the modem’s converter circuitry extends the stop
bit. If the local modem’s frequency is lower than the local DTE’s, the modem’s
converter deletes one stop bit from every four or eight characters (settings
identified as “25%” and “12.5%” respectively). At the other end of the line, the
remote modem will add a stop bit shortened by 12.5% to the end of every
character whose stop bit has been stripped before relaying the data to the remote
DTE. The “12.5%” setting is suitable for overcoming frequency deviations up to
1.1%, and the “25%” setting is suitable for overcoming frequency deviations up to
2.3%.
Use position 4 of the LHM-19.2’s 4-position DIP switch to select a 25% or 12.5%
frequency-deviation setting. For proper operation of the async-to-sync converter,
you’ll also have to set the async character length by adjusting DIP-switch positions 2
and 3. See Chapter 3.
Encoder
The LHM-19.2’s 16-state, 8-dimensional Trellis encoder encodes the input data
from the DTE to a sequence of complex symbols ready for rendering into analog
form.
Modulator
The modulator turns the input symbols from the encoder into analog signals using
a QAM technique.
Transmit Filter
The transmit filter eliminates out-of-band replication resulting from conversion of
the sample output obtained from the DSP to a continuous transmit signal.
XMT Level
Use the OUT LVL jumper to set the LHM-19.2’s XMT level (signal level) to 0, –3,
–6, –9, or –12 dBm. See Chapter 3.

13
CHAPTER 2: Introduction
Receiver
The LHM-19.2’s receiver is comprised of several interlinked circuits, as Figure 2-3
shows:
• The receive filter removes out-of-band signals and prevents signal spectrum
aliasing during analog-to-digital conversion.
• The receive signal then passes into a fixed pre-emphasized line equalizer.
•Ademodulator samples the received analog signal into a baseband digital signal.
•Anadaptive variable equalizer compensates for line distortion.
•Anautomatic gain control (AGC) circuit compensates for line attenuation.
•AViterbi decoder turns the received symbols back into the original transmitted
data.
V.54 Diagnostics
The LHM-19.2 performs V.54-compliant remote digital loopback and local analog
loopback tests, as well as local digital loopback. Local digital loopback can be
activated manually by pressing the DIG button on the local LHM-19.2’s front
panel. Remote digital loopback and local analog loopback can be activated
manually from the local LHM-19.2’s front panel (by pressing REM or ANA
respectively) or through the DTE interface (by toggling the RL and LL signals on
Pins 21 and 18). See Sections 6.1.2 through 6.1.4.
These loopbacks can be enabled or disabled separately by setting the LHM-19.2’s
LOOP—REM and LOOP—ANA jumpers. See Chapter 3.
During both of the digital loopbacks (but not local analog loopback), the
modem that is looping data back to the other modem will lower the Data Set Ready
(DSR) signal to the attached device.
V.52 Test-Pattern Generator
This feature allows for quick and easy testing of the local LHM-19.2 as well as the
communication link between the modems. While an LHM-19.2 is in local analog
loopback or remote digital loopback, you can press PATT pushbutton on its front
panel to begin a bit error rate test. The test-pattern generator circuit transmits a
pseudo-random pattern of bits and examines what it receives back. If it detects any
errors, the LHM-19.2 flashes, dims, or turns OFF its SQ LED and (optionally) the
SQD signal on Pin 21. You can even have a pair of LHM-19.2s perform this test
during point-to-point operation by pressing the PATT buttons on both modems at
the same time. (This will interrupt normal data transmission.) See Section 6.1.1.

14
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
3. Configuration
To set the LHM-19.2’s internal jumpers and switches, take these steps, making sure
that the modem is unplugged:
1. Loosen the two screws at the lower corners of the LHM-19.2’s rear panel.
2. Slide the inside of the LHM-19.2 out of its housing, as if you were sliding a
drawer open.
3. Adjust the jumpers and switches for the configuration you want. Refer to
Figure 3-1 on the next page and Table 3-2 on the pages that follow.
4. Slide the unit back into its housing and retighten the retaining screw.
If you will be attaching a DTE to the LHM-19.2 that will communicate with it
asynchronously, you need to configure the LHM-19.2 to expect the correct total
async word length from the DTE. (This is done with positions 2 and 3 of DIP switch
SW2.) To get the total word length, add the start bit, data bits, parity bit (if any),
and stop bits, as shown in Table 3-1. (Round down 1.5 stop bits to 1.) The
LHM-19.2 doesn’t support words shorter than 8 bits or longer than 11 bits.
Table 3-1. Total async word length.
Start Data Parity Stop Total
Bit Bits Bit(s) Bits
Always 1 5 None (0 bits) 2 8
Even or odd (1 bit) 1 or 1.5 8
29
6 None (0 bits) 1 or 1.5 8
29
Even or odd (1 bit) 1 or 1.5 9
210
7 None (0 bits) 1 or 1.5 9
210
Even or odd (1 bit) 1 or 1.5 10
211
8 None (0 bits) 1 or 1.5 10
211
Even or odd (1 bit) 1 or 1.5 11

15
CHAPTER 3: Configuration
Figure 3-1. The LHM-19.2’s circuit board.

16
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
Table 3-2. The LHM-19.2’s configuration settings.
Number in Function Possible settings Factory-
Figure 3-1 default
and identity setting
1Select data rate. 0 = 1.2 kbps
Data-rate dial 1 = 2.4 kbps
2 = 4.8 kbps
3 = 9.6 kbps 9.6 kbps
4 = 19.2 kbps
(other positions reserved)
2
DIP switch SW2
Position 1 Select protocol. SYNC - ON = Sync Sync
SYNC - OFF = Async*
Positions Select async word 2 OFF, 3 OFF = 8 bits
2 & 3 length (see Table 3-1). 2 OFF, 3 ON = 9 bits
2 ON, 3 OFF = 10 bits 10 bits
2 ON, 3 ON = 11 bits
Position 4 Select async stop-bit ON = 25%
shortening. OFF = 12.5% 12.5%
3Select sync clock INT = Internal clock INT
XMT CLK source. EXT = External clock from
DTE on RS-232 Pin 21
LOOP = Receive clock
(recovered from remote
modem & looped back)
4Select RS-232 Pin 21 SQ = Signal Quality (SQD)
LOOP—REM signal.† REM = Remote Digital REM
Loopback (RL)
5Enable/disable Local EN = LL enabled
LOOP—ANA Analog Loopback (LL) DIS = LL disabled DIS
signal (RS-232 Pin 18).
* Due to a quirk of the board’s circuit architecture, when you select "SYNC - OFF”
with DIP-switch position 1, you must set the XMT CLK jumper to “INT”.
† It’s not necessary to make use of either of these signals if you don’t want to.

17
CHAPTER 3: Configuration
Table 3-2 (continued). The LHM-19.2’s configuration settings.
Number in Function Possible settings Factory-
Figure 3-1 default
and identity setting
6Select transmit-line 600 = 600 Ω600 Ω
XMT IMP impedance. LOW = Min. impedance*
7Select receive-line 600 = 600 Ω600 Ω
RCV IMP impedance. HIGH = Max. impedance*
8Select output level to 0 = 0 dBm 0 dBm
OUT LVL the transmit line. –3 = –3 dBm
–6 = –6 dBm
–9 = –9 dBm
–12 = –12 dBm
9Isolate/tie together CON = Signal ground CON
CHAS GND chassis and signal tied to chassis ground
grounds. DIS = Signal ground
disconnected from
chassis ground
10 Select carrier behavior. ON = Carrier always ON ON
CARRIER LOW = Carrier controlled
by RTS (ON only when
RTS is high)
* You might need to select these values if the impedance of the line cable you
attach to the LHM-19.2 differs so greatly from 600 Ωthat the modem can’t
compensate for it in the “600” setting. (This is one of the things that can be
wrong when local loopback works but no data gets to the other end of the line
cable.)

18
19.2-KBPS RS-232 LONG-HAUL MODEM (LHM-19.2)
4. Installation
The 19.2-kbps RS-232 Long-Haul Modem comes with a 5-ft. (1.5-m) power cord
and this manual. It’s designed to be placed either on a flat surface or in a rack.
Things to keep in mind about where you place it:
• It must be installed within 5 ft. (1.5 m) of an easily accessible grounded 3-wire
115-VAC outlet and within 50 feet (15.2 m) of the associated DTE. It isn’t
designed to be bolted down.
• Allow at least 36 inches (90 cm) of clearance in front of the LHM-19.2 so you
can operate and maintain it. Be sure to leave 4 inches (10 cm) of clearance
behind the unit for signal lines and interface cables.
If you’re going to rackmount the LHM-19.2, follow the directions in Section 4.1.
Otherwise, skip ahead to Section 4.2.
4.1 Rackmounting (Optional)
If you want to, you can install the LHM-19.2 in a l9" rack. It’s 1U high (1.75",
4.5 cm) and slightly less than half as wide as the rack’s available horizontal
mounting space. Two different rackmount kits are available: You can use the
RM516 kit to mount one LHM-19.2 by itself (see Section 4.1.1), or the RM523 kit
to mount two LHM-19.2s side by side (see Section 4.1.2).
Make sure to disconnect each LHM-19.2 from AC power and from all data cables
before rackmounting it. Only an experienced technician should attempt to install,
operate, or maintain the LHM-19.2.

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CHAPTER 4: Installation
4.1.1 R ACKMOUNTING A S INGLE LHM-19.2
The single-chassis rackmount kit RM516 includes one short bracket and one long
bracket. You’ll screw the brackets against the side walls of the LHM-19.2’s case, as
shown in Figure 4-1.
Figure 4-1. Preparing a single LHM-19.2 for mounting in a 19" rack.
Take these steps to install the LHM-19.2 in a 19" rack:
1. Use the four screws and four flat washers included with the RM516 kit to
attach the kit’s two brackets to the side walls of the LHM-19.2. Thread the
screws into the two front holes on each side wall (nuts are already in place on
the inside of the wall).
2. After attaching the brackets to the LHM-19.2, fasten the brackets to the side
rails of the rack with your own screws, bolts, cage nuts, etc. (these aren’t
included with the kit).