Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90051)

182 Chapter9
Describing Asynchronous Devices
How Connections Set Characteristics
How Connections Set Characteristics
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
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
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.