Table of Contents
Advertisement
Interface Cables
21
Building Custom Cables
Occasional situations may require use of non-standard cables. If you need to modify an existing
cable or design a new one, several precautions are necessary to ensure optimum performance.
This section provides information for determining wiring needs and suggests several helpful
assembly techniques that can produce better noise immunity and better system performance.
Safety Considerations
Electrical equipment safety grounds must be adequately maintained when designing cables.
Generally, a shielded multi-conductor cable is used for datacomm applications. The outer
shield is wired to the earth (safety) ground pin on the interface connector. The shield should
also be continued with a connection through the cable terminating connector to the safety
(earth or frame) ground of the device being connected. Grounding the shield at both ends
provides a path for electrostatic discharge currents, and helps maintain both frames at the same
potential, thus improving noise immunity. Be sure, however, that there are no ground voltage
differentials between power receptacles for devices connected to the same cable shield. This is
best ensured by feeding power to both the computer and modem or other device from the same
circuit breaker panel, or by taking other equivalent precautions.
For other than short cable runs, to ensure safe ground currents through the shield, install the
cable so that it is connected to the datacomm interface after the interface is installed in the
computer and the computer power has been connected. If the modem and computer are
connected to a different building grounds: before you connect the cable to the modem, use an
AC/OC digital voltmeter to measure the voltage difference between the shield pin of the cable
connector and the shield pin of the corresponding modem (or other device) connector. If the
voltage is more than 100 to 300 millivolts, alternate grounding should be installed to reduce or
eliminate the difference. After grounding is complete and cables are connected, use a current-
sense probe to measure cable shield currents. Shield currents at power frequency or below
should never be allowed to exceed a few hundred milliamperes under normal operating condi-
tions.
Using Interface Power Connections
The datacomm interface provides pins for
+
5V,
+
12V, and -12V power at its rear panel
connector. These connections are normally used with various modems or adapters supplied by
HP, and are not intended for other purposes. Improper use of these power connections can
seriously compromise the reliability of the computer.
Balanced Versus Unbalanced Connections
Note that the interface is designed for both balanced (RS-449/422) and unbalanced (RS-449/
423 and RS-232C) operation. This requires a dual set of drivers for certain outputs as well as
receivers that support both balanced and unbalanced inputs. All connections to the interface
cable or adapter are made through the 50-pin connector on the interface rear panel cover.
Connections are shown on the schematic diagram, and a suffix on each line identifier indicates
whether the input or output is for balanced or unbalanced operation as follows:
• TT(u), SO(u), TR(u), and RS(u) are all unbalanced driver outputs whose return paths to
ground are to TT(C), SO(C), TR(C) and RS(C), respectively .
• Balanced drivers for TT, SO, TR, and RS have their complementary outputs labelled as:
(A) for inverted output; (B) for non-inverted output.
Advertisement
Table of Contents
