Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90057)

Describing Asynchronous Devices
How Connections Set Characteristics
Chapter 9
166
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 e3000 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.
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.