USER MANUAL, MSD-4, PT# 101012
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2
EAST COAST DATACOM, INC.
1.2
THEORY
OF
OPERATION
In a polled or contention environment the typical MSD operation is as follows:
Data arriving at the MSD's master port is continually broadcast to all of the MSD-4 sub-channel
ports. When one of the DTE devices answers the poll from the host site, that DTE device will
raise RTS (Request To Send). When RTS is raised the MSD-4 scanner circuitry will lock on to that
port and allow that attached DTE device to communicate with the modem link. The MSD will
remain locked onto that port until RTS is dropped and CTS (Clear To Send) is dropped from the
modem. After RTS and CTS have dropped, the MSD will automatically begin scanning the ports
until another port raises RTS.
OPTIONALLY: The “OR-MODE” of operation may be selected and the contention mode with any
DTE device that the DATA lead transitions from Mark to Hold will have access to the Master
Port.(the user system software must only allow one device at a time communicate)
CHAPTER 2 - BASIC OPERATION AND FUNCTIONS
2.1
FRONT
PANEL
INDICATORS
AND
SWITCHES
A
Green
LED illuminates when AC Power has been applied. Four adjacent
Green
LEDs illuminate
and identify CTS, DCD, RXD and TXD on the Master Ports RS-232 signal status. Sub-channel Ports
A thru D Active LED’s are also green and only one of the four LED’s will ever illuminate at a single
time, when a sub-channel is active. Yellow LEDs are provided as a visual indication of a
streaming sub-channel, the LEDs illuminate only if anti-streaming is enabled.
2.2
FRONT
PANEL
SWITCHES
Positive latching switches are provided for each sub-channel Port A thru D for isolating or
removing attached terminals. To disable a sub-channel simply push the switch, so it is in the
outer most position.
NOTE:
The switch disables the ability of the sub-channels Pin 2 TXD to
transmit back to the Master Port. Pin 3 RXD from the Master Port is always sent out to all sub-
channels. This feature is by design for data broadcast applications and data is always outbound.
2.2
REAR
PANEL
CONNECTORS
AND
FUSES
Located on the rear of the product you will find an IEC Power Receptacle. The supplied power
cord plugs into this receptacle. This receptacle also contains a fuse drawer.
2.3
CLOCKING
The MSD-4 derives clocking from the attached Modem or DCE.
2.4
ELECTRICAL
INTERFACE
The MSD-4 is EIA RS-232 (CCITT V.24) compliant using female DB-25 connectors.
