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Features / Benefits

  • Interface - X.21, ITU V.11, (3) DB-15 Female Connectors
  • Data Rates - Synchronous up to 1.024Mbps
  • Anti-Streaming - Automatic or Manual for unattended Circuit Restoral
  • Individual Subchannel Enable/ Disable Switches
  • Integral Elastic Tail Circuit Buffer - FIFO capable of 512 bits per channel
  • Up to Two DTE or DCE's per Link
  • Transparent to Data
  • Supports Fall Back Clocking
  • Sturdy Metal Enclosure
  • 110/220VAC Switch Selectable


The DMM-X21 is a X.21 Port Sharing Device that provides the network manager with a cost effective means of expanding existing, leased line polled networks without adding computer ports or communications links. With the DMM-X21/2, up to 2 DTE or DCE devices can share the same port and communications link using the contention and control protocols normally resident in the host hardware and software.

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 synchronous network environments, the DMM-X21/2 is protocol transparent at data rates up to 1.024Mbps. Data arriving at the Master Port is continually broadcast to all Subchannels. The attached terminal device that raises the Control (C) or Indicate (I) signal is automatically given control of the DMM-X21/2 until data transmission is complete. Clocking is accomplished from the attached Digital Modem.

The DMM-X21/2 incorporates optional Anti-Streaming circuitry. If enabled, Anti-Streaming will automatically remove a defective terminal from service if the Control (C) or Indicate (I) signal is present longer than the user predefined selection period.

Elastic Tail Circuit Buffers compensate for systems having different clock sources. Large 512 bit buffers enable the DMM-X21/2 as an ideal choice for VSAT or land line applications.

The DMM-X21 is equipped with an internal 110/220VAC switch selectable linear power supply and has a three year warranty.

x.21 port sharing device application diagram


Application Multiple X.21 Synchronous DCE or DTE devices contending in a polled environment, to share one Digital DCE or DTE interface
Capacity Up to Two X.21/2 devices
Interface X.21, V.11 Compliant using DB-15 female connectors
Data Format Data transparent at all data rates
Data Rates Up to 1.024Mbps
Timing From Composite (Master) Port,Sub-channel Port 1
or Port 4
Buffers Optional Unidirectional 8 bit buffer 512 bits per channel
 Anti-Streaming Automatic.....Selectable time out intervals

Disable.........Selectable via dip switch

 Front Panel Indicators....Power, Send/Receive Data, Channel Active, Channel Stream

Switches.....Enable/Disable of each Subchannel

Power Source 100-120 to 200-220VAC @10%, 50/60Hz, 0.32/0.16A, external 110/220 volt select switch, IEC Power Inlet, (2) 5mm Fuses
Environmental Operating Temperature . .32º to 122º F (0º to 50º C)

Relative Humidity.. . . . . . .5 to 95% Non-Condensing

Altitude. . . . . . . . . . . . . ... .0 to 10,000 feet

Dimensions Height ....... 1.75 inches (4.44 cm)

Width ........ 13.35 inches (33.90 cm)

Length ....... 9.00 inches (22.86 cm)

Weight 4.5 pounds (2.1Kg)
Warranty Three Years, Return To Factory
Regulatory Approvals Tested According to UL 60950-1:2003, CAN/CSA-C22.2 No. 60950-1:2003CE Safety EN60950 and CE EMC EN300-386-2
Ordering Information Model: DMM-X21/2
Description: 2 Port X.21 Digital Sharing Device
Included with each unit: 1) Operations Manual

2) U.S.A. Grounded Power Cord, Part # 713015

3) Optional Power Cords

....A) United Kingdom, Part # 713016

....B) Continental Europe, Part # 713017

....C) Other: Specify Country on Purchase Order


1) Spare Data Center Fuses

....A) 160ma Fuse, Qty (2) Part # 714000

....B) 80ma Fuse, Qty (2) Part # 714001