Specifications

Configuring the Modem
Note that DRS controls both the transmitter and receiver. If DRS changes states while the modem is receiving a
preamble, data, or a turn-off sequence, the state of the receiver is unknown until the carrier drops and a new
preamble is received. We recommend that DRS does not change states until both the local and remote modems’
transmitters are in the idle state. When they are, both the local and remote modems’ DRS signal should change
states.
SW3-2 − Data Rate (Fast-Poll Mode Only)
SW3-2 ON = 4800 bps
OFF = 9600 bps
SW3-2 configures the modem speed. When the modem is in high-speed fast-poll mode (SW3-1 OFF), setting SW3-
2 ON selects 4800 bps, while setting SW3-2 OFF selects 9600 bps.
SW3-3 − Async Character (Fast-Poll Mode Only)
SW3-3 ON = 11 bits
OFF = 10 bits
Switch SW3-3 selects whether the async character is 10 or 11 bits long. When the modem is in high-speed fast-poll
mode (SW3-1 OFF), setting SW3-3 ON selects an 11-bit async characters, while setting SW3-3 OFF selects a 10-
bit async character.
SW3-4 − Auto RTS (Fast-Poll Mode Only)
SW3-4 ON = Enable Auto RTS
OFF = Disable Auto RTS
For data terminals that do not support hardware RTS, set SW3-4 to ON to enable auto RTS mode. In this mode,
TXD is detected at the modem and an internal RTS signal is turned ON. After training completes, the TXD is
transmitted to the remote modem. The transmitter turns off if no TXD is detected after 1 character length of idle
time. Auto RTS is used in fast-poll mode only (SW3-1 set to OFF).
SW3-5 − Transmitter Termination
SW3-5 ON = Switched by RTS
OFF = 600 Ω
SW3-5 is used for multi-point configuration networks. When multiple modems are connected on the same metallic
circuit:
 The transmitter termination should be of high impedance if the modem is not transmitting.
 The transmitter is only terminated with 600 ohms when RTS is asserted.
This configuration should be used for all slave modems to prevent the transmitting modem from being unnecessarily
burdened. To select this configuration, set SW3-5 ON on the slave modems.
If you use the modem with transmission lines that are transformer-coupled or with an impedance-isolated network
(such as a transformer bridge), set SW3-5 OFF for proper operation.
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