Asynchronous devices can be connected to the DTC in the
following ways:
Directly connected to the DTC.
Remotely connected through a modem, a mux, a switch,
a PBX, a PAD, or Telnet/iX.
Direct Connections |
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A direct connection is one in which a device's
data communications cable is plugged into a port without being connected
to any other communications equipment. Two standards are available
for direct connections to HP 3000 Series 900 systems, RS-232-C
and RS-422.
RS-232-C direct connections need only consist of the wire
the device uses to send data, the one it uses to receive data, and
a ground wire. If RS-232-C cables are used, each device can be up
to 50 feet (15m) away from the DTC.
Cables for RS-422 use two wires for each send and receive,
plus a ground wire. These cables are less susceptible to noise and
can be used at longer distances from the DTC than devices using
RS-232-C cables. If RS-422 cables are used, each device can be up
to 4000 feet (1200m) away from the DTC.
The system console is normally located next to the computer.
The cable that attaches the system console to the access port uses
the RS-232-C specification.
The speeds supported for direct connections are 300, 1200,
2400, 4800, 9600, and 19200 bps. (Direct connected devices on DTC
16iX/16MX and DTC 72MX also support 38400 bps.) Note that the system
console does not support 19200 or 38400 bps.
Modem Connections |
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Modem connections are more complex because more equipment
is used. The RS-232-C standard is used for modem connections, with
more of the circuits defined by the standard implemented to support
the requirements of the modem connection. A device's RS-232-C
data communications cable is connected to a modem (or the device
has an integral modem); the modem
sends data through telephone lines to another modem. The second modem is
connected through a RS-232-C cable to a port on the DTC. Both of
the RS-232-C cables must adhere to a 50 foot (15m) length
restriction. However, no restrictions are placed on the distance
between modems.
Only full-duplex asynchronous modems
are supported for use with HP 3000 Series 900 computers.
Twenty-five pin RS-232-C cables must be used on each end of a modem
connection because, in addition to send, receive, and ground signals,
control signals are also required to control the modems.
Both terminals
and serial printers are supported over modems. However,
because data is traveling for long distances through telephone lines
of varying quality, parity
must be used to detect transmission errors.
This means that only printers that support printer-type 21 should
be used over modems, since parity cannot be used with other printer
type files.
Theoretically, remote devices are supported at the same speeds
as devices connected directly. However, most full duplex asynchronous
modems support only a limited set of speeds. Common
modem speeds are 300, 1200, and 2400 bps. Note that the device,
the modems, and the data communications configuration
must be set to the same transmission speed
for the connection to work.
