Date Code Code 20010518 Installation 3-7
SEL-2020 Instruction Manual
For sites where the main issue is cable length, you can use short-haul modems connected by wire.
This alternative is a compromise between the low cost and short cable for direct EIA-232
connections and the isolation and noise immunity of higher-cost fiber-optic links.
You must provide power to any modem that you install between the SEL-2020 and another
device. You can use the SEL-2020 to power some types of modems connected to its rear-panel
ports. With the proper jumper connections, the EIA-232 outputs of the SEL-2020 will support
modems which accept +5 Vdc power. The total current drawn by all of the external modems
powered by one SEL-2020 should not exceed 0.5 amp. See Table 3.4 and Figure 3.2 for the +5
Vdc power jumper settings for each port. None of these jumpers are installed at the factory.
Some modems power themselves from the control and data lines. These modems do not require
connection to the +5 Vdc power.
Telephone Line Communications
A telephone dial-up link is one option for off-site communications with an SEL-2020. Use a
modem to convert from the audio telephone line to an EIA-232 interface on the SEL-2020. You
can use the optional SEL internal modem to connect a telephone line to Port 8 of the SEL-2020.
Or you can use an external modem connected to any of the rear-panel ports. You can set the
SEL-2020 to answer the phone and to initiate calls based on conditions that you select.
You should use telephone line protection equipment where the line enters the building, to ensure
personnel safety and reduce damage to equipment from ground-potential rise and other hazardous
conditions. Connect the line protection equipment to the modem following standard commercial
telephone wiring standards.
If you use one telephone line for both voice and SEL-2020 communications, set the SEL-2020
modem port to ignore a specified number of rings before answering, so that personnel at the site
can answer the phone before the modem answers. You can also provide a hook-switch in the
phone line, so on-site personnel can disconnect the telephone line from the modem; however, you
may want to use some type of timer instead to disconnect the line to prevent them from leaving
the modem disconnected.
If you have one telephone line to communicate with a mix of telephones and modems in a site,
you typically use a telephone port switch. Connect the protection equipment to the telephone
port switch, and the telephone port switch to the SEL-2020 modem and other devices with
standard telephone wiring.
Data Flow Control
All SEL devices, including the SEL-2020, support XON/XOFF software data flow control. You
should select this option, or accept the XON/XOFF = Y default for any communication setting
where the SEL-2020 is connected to another SEL device. Set RTS/CTS = N to connect an
SEL-2020 to any SEL device.
The SEL-2020 also supports RTS/CTS hardware data flow control. You should select the
RTS/CTS option only if the connected device uses RTS/CTS, and does not use XON/XOFF flow
control. Consult the instruction manual or contact the device vendor to determine the proper
flow control technique for each non-SEL device. If you select RTS/CTS hardware data flow
control, make sure that the cable you are using to connect the device to the SEL-2020 has
conductors for RTS/CTS.
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