Cabling printers

Optimal serial cabling

This topic explains how RS-232-C serial communications can be used for printing.

NOTE: Anyone making changes to their cabling pin configurations should be familiar with soldering techniques and with tracing wires using a multimeter or color-coding.

Serial printers cabled to PCs generally use the RS-232-C interface, which was developed specifically for terminal-to-modem communications, or communication between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). Proper ``handshaking'', the exchange of signals to control the flow of data between hardware devices, allows data to flow correctly between DTE and DCE devices.

Since PCs and printer are both DTE, they must send signals as DTE. However, they must be cabled so that they receive signals as if from DCE. Because PC and printer vendors implement this serial handshaking in different ways, many problems occur when the signals are not correctly routed between them.

The following chart shows optimal cabling between 9 and 25-pin PC connectors and a 25-pin printer connector for NetWare print services. Each pin handles a separate functionality.

Optimal serial pin connections

NOTE: Superscript numbers in the graphic correspond to notes on the next couple of pages. In parallel communication, a ``high'' signal level ranges around 5 volts. A ``low'' signal level ranges around 0 volts. In serial communication, the ``low'' range is -3 to 3 volts, the ``high'' ranges are -15 to -3 and 3 to 15. Within this documentation, the conventions ``input'' and ``output'' determine what controls the signal. ``Input'' means the signal is controlling the device; ``output'' means the device is controlling the signal.