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Buffered 4-Port RS-232 Modem Sharing Device

Expand leased line, polled environments without adding computer ports or communications links.


Please call 877-877-2269 for a replacement product or part.
Extended Double Diamond™ Warranty Available (3 Additional Year)

Product Features

  • Increases network efficiency.
  • Transparent to data.
  • Fully cascadable.


  • DCE/DTE selectable.
  • Internal or external clocking.

  • The Buffered Modem Sharing Device is a network expansion device for modem sharing or port sharing applications in polled or contention environments. It allows up to four devices to share a modem/DSU or computer port. Any combination of terminals and modems may be used in a network environment.

    Each port may be selected as a DCE or DTE interface. Once installed, system and network efficiency are increased through higher host processor utilization coupled with the significant decrease in idle time between host/terminal traffic sessions.

    Ideal for either synchronous or asynchronous network environments, the Sharing Device is protocol transparent at data rates up to 128 Kbps. It may be configured to provide clocking for the entire network. To prevent data transmission errors caused by clock differentials throughout the network, an 8-bit buffer is provided. In applications where the master port and the selected port provide their own clocks, data is clocked into the buffer at the receive clock rate of the active port and clocked out using the master port transmit clock.

    In applications where the Clear To Send control signal is not provided by the master port, an artificial RTS/CTS is available. Additionally, several RTS/CTS delay settings are available.

    The Buffered Modem Sharing Device provides optional Anti-Streaming circuitry. Once enabled, Anti-Streaming will automatically remove a defective terminal or modem from service if the Data / Control criteria is present for the user predefined selection period.

    Housed in a sturdy metal enclosure and equipped with an internal 110/220VAC switch selectable linear power supply, the DMM-4 will provide in excess of 100,000 hours of reliable service.

    Tech Specs

    Dimensions 1.75”H x 17”W x 9”D (4.4 x 43.2 x 22.9 cm)
    Indicators Power, Transmit Data, Receive Data, Channel Active, Channel Stream
    Interface EIA RS-232, CCITT V.24 using DB-25 female connectors, ±15KV ESD protection
    Power 100–120 to 200–220VAC @ 10%, 50/60Hz, 0.16/0.08A, External 110/220 volt select switch, IEC Power Inlet, (2) 5-mm fuses
    Terminal Service Modes Sequential scanning for RTS, DCD, or data
    Timing Internal or external
    Weight 4.5 lb. (2.1 kg)

    This product includes:
  • (1) power cord
  • Manual

  • * Technical specification weight is the unit weight. It is not the packaged shipping weight. For shipping weight, please contact Black Box customer service at 877-877-2269.



    Double Diamond Warranty

    Extended 1 Year Warranty Available

    Protect your purchase with an additional 1 Year Double Diamond™ warranty!
    Cost: $69.60

    Extended 3 Year Warranty Available

    Protect your purchase with an additional 3 Year Double Diamond™ warranty!
    Cost: $139.19


    Double Diamond™ Warranty (Standard)

    Take the risk out of equipment purchases with the Double Diamond™ Warranty from Black Box Network Services.

    The Double Diamond Warranty is our warranty that protects your Black Box® brand products* from accidental damage, including drops, water, and power surges.

    The Buffered 4-Port RS-232 Modem Sharing Device comes with a standard 3 Year Double Diamond™ Warranty (Standard) already included.

    During the warranty period, Black Box Network Services will repair or replace your damaged equipment within one week of receipt at NO CHARGE!*

    Plus, get an Extended Double Diamond Warranty.

    You can also extend your warranty or protect a previously purchased Black Box® brand product** for an additional year—for only 10% of the purchase price! Or, extend your coverage for a full three years for only 20% of the purchase price!

    Ordering an extended warranty is easy: When viewing a product in your shopping cart, just select the one or three years of additional coverage in the dropdown menu under “Warranty Options.” For details, contact Customer Service at 724-746-5500.

    * Warranty subject to certain exclusions and limitations ** Certain products may not be eligible.

    What’s more, we also have a 45-day return policy. Simply call Customer Service for a Return Authorization (RA) number, return your product in new condition and in the original package, and receive a full refund.