Union Switch & Signal GENISYS II, Hardware Installation

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UNION SWITCH & SIGNAL GENISYS II System Hardware Installation
3-4 August 1999 SM-6900B Rev. 0.0
simplest connection that allows data to be passed in both directions between master and slave
units.
Some protocols and some specific types of interconnection require message “framing,” signals
that tell the port at the receiving end of the communication circuit where a message starts and
ends. This information is passed by cross-connecting RTS and DCD. In other words, RTS on the
master unit is connected to DCD on the slave unit(s); and DCD on the master unit is connected to
RTS on the slave unit(s). This control signal connection, coupled with the data signal connection
described in the paragraph above, is often referred to as a NULL MODEM connection or a
connection between serial ports implemented without the use of modems. The RTS signal on the
sending unit signals the beginning of the message when it is asserted. This occurs shortly before
data transmission begins. The RTS signal is deasserted shortly after the end of data transmission.
The NULL MODEM connection may always be used to connect two units located in the same
equipment facility that are to communicate using GENISYS protocol. The GENISYS protocol,
however, requires only that TXD, RXD, and COM are connected. The DCD signals may be
strapped in the asserted state for any unit supporting the GENISYS protocol or left disconnected
for GENISYS II or GENISYS 2000 units. (These units are designed to ignore the DCD signal
UNLESS half-duplex or keyed carrier communication is selected.)
3.2.2 Connecting GENISYS Protocol Serial Ports Using Modems
Whenever it is necessary to connect GENISYS protocol ports that are not located in the same
equipment facility, some type of modem or communication device that provides serial common
isolation and secondary transient protection MUST be used. While the operating characteristics
of modems vary widely, there are a few requirements that apply to all serial interface applications
that employ modems.
GENISYS protocols can only be passed through modems capable of passing asynchronous data.
Most low-speed modems (19,200 BPS. or less) and carrier sets have this capability. GENISYS
protocol implementations do not support the use of transmit and receive data clocks that are
separate from the data signals. Therefore, modems that support only a synchronous serial
interface to connected serial equipment cannot be used, as a rule.
GENISYS protocol requires modems that can process all messages without the use of either
hardware or in-band flow control. This means either the modems must not buffer outbound data
or the modem’s outbound data buffer must be large enough to hold the longest message expected
to be transmitted on the communication link. This constraint applies to modems only. Simple
carrier sets generally are not capable of buffering data. To ensure all of the above requirements
are met, care must be exercised in configuring modems that are more sophisticated.
GENISYS protocol may be transported using either half-duplex or full-duplex modems. Half-
duplex modems do not support simultaneous transport of data, unlike full-duplex modems. Half-
duplex modems require that both the master and slave modem interfaces be wired to key the
outbound carrier on and off using the RTS signal. Full-duplex modems allow the master-end
carrier to be strapped permanently in the ON state. Full-duplex modems have an advantage, in