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This section explains how Hewlett-Packard uses RS-232-C serial
communication 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 certain functionality, as explained in the following
sections. | | | | | 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. In the following sections, 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. | | | | |
Functions on the PC
Side of Serial Printer Cables | |
FG (Frame Ground reference pin) It is not necessary for you to use this pin, and using the
pin can cause ground loop problems if the PC and printer are connected
to different power sources. You have a Frame Ground pin only on
a 25-pin connector, and it uses Pin 1. TXD (Transmit Data output pin) The PC sends data (the print job) through this pin which connects
to Pin 3 on the printer. The PC uses Pin 3 on a 9-pin connector
and Pin 2 on a 25-pin connector. This attaches the PC's transmit
pin to the printer's receive pin. RXD (Receive Data input pin) The PC receives XON/XOFF characters through this pin, which
connects to Pin 2 on the printer. The PC uses Pin 2 on a 9-pin connector
and Pin 3 on a 25-pin connector. This attaches the printer's transmit
pin to the PC's receive pin. RTS (Request To Send output pin) NetWare print services sets the signal level high for this
pin (and the DTR pin) on the PC and keeps it in that state. The
PC uses Pin 7 on a 9-pin connector and Pin 4 on a 25-pin connector.
This signal can be used as a source for a high signal level to enable
certain functions on some printers. If your printer cable connects
this pin to a pin on the printer that requires a high signal level,
printing will function properly. You can achieve the same results
by following the steps in note 12. | | | | | NOTE: Early versions of NetWare print services set the signal
level high on this pin when sending data and set the signal level
low when not sending data. The timing of the signal changes caused
many problems with printers that require a high signal level for
data input. Beginning with version 1.22R, NetWare Print Services
constantly maintains a high signal level for this pin. | | | | |
CTS (Clear To Send input pin) NetWare print services uses this pin for its hardware handshaking.
The PC uses Pin 8 on a 9-pin connector and Pin 5 on a 25-pin connector.
This pin connects to the pin on the printer that controls the flow
of data. This is generally pin 20 on the printer, but some printer
vendors use other pins (see note 14). If the signal level is high
for this pin, data will be sent to the printer. Data is not sent
when the signal level is low. | | | | | NOTE: If this signal level stays low for about 10 seconds
while servicing a print job, NetWare print services reports an "off-line"
condition. | | | | |
DSR (Data Set Ready input pin) NetWare print services does not use this pin. Other print
services may require a high signal level for both Pins 5 and 6 on
the PC for proper data flow control. Hewlett-Packard recommends
that you tie (solder) Pins 5 and 6 together to provide printing
compatibility for other print services. SG (Signal Ground reference pin) NetWare print services uses this pin to provide a reference
for signal levels. The PC uses Pin 5 on a 9-pin connector and Pin
7 on a 25-pin connector. The printer uses Pin 7. DCD (Data Carrier Detect input pin) Some PCs may require a high signal level on this pin to enable
the PC to receive data from the printer. The XON/XOFF protocol will
not function correctly with a low signal level. If you do not use
the XON/XOFF protocol, the signal level on this pin can be high
or low. The PC uses Pin 1 on a 9-pin connector and Pin 8 on a 25-pin
connector. Hewlett-Packard recommends that you maintain a high signal
level on this pin by tying (soldering) it to another pin that has
a high signal level. Tie Pin 1 on a 9-pin connector to either Pin
7 or Pin 4. Tie Pin 8 on a 25-pin connector to Pin 4 or Pin 20. DTR (Data Terminal Ready output pin) NetWare print services sets the signal level high for this
pin (and the RTS pin) on the PC and keeps it in that state. The
PC uses Pin 4 on a 9-pin connector and Pin 20 on a 25-pin connector.
This signal can be used as a source for a high signal level to enable
certain functions on some printers. If your printer cable connects
this pin to a pin on the printer that requires a high signal level,
printing will function properly. You can achieve the same results
by following the steps in note 12. | | | | | NOTE: Early versions of NetWare print services set the signal
level high on this pin when sending data and set the signal level
low when not sending data. The timing of the signal changes caused
many problems with printers that require a high signal level for
data input. Beginning with version 1.22R, NetWare Print Services
constantly maintains a high signal level for this pin. | | | | |
Functions on the Printer Side of Serial Printer Cables | |
TXD (Transmit Data output pin) The printer sends XON/XOFF data through this pin which connects
to Pin 2 on a 9-pin connector and Pin 3 on a 25-pin connector on
the PC. The printer uses Pin 2. This attaches the printer's transmit
pin to the PC's receive pin. RXD (Receive Data input pin) The printer receives data (the print job) through this pin
which connects to Pin 3 on a 9-pin connector and Pin 2 on a 25-pin
connector on the PC. The printer uses Pin 3. This attaches the PC's
transmit pin to the printer's receive pin. RTS (Request To Send output pin) The signal level for Pin 4 is always high as long as the printer
is on. This signal can be used as a source of a high signal level
to enable certain functions on some printers. We recommend that
you tie (solder) Pin 4 to Pins 5, 6, and 8 on the printer; however,
this may not work if your printer uses these pins differently. CTS, DSR, and DCD (Clear To Send, Data Set Ready,
and Data Carrier Detect input pins, respectively) One or more of these pins may be used to enable certain functionality
on the printer. We recommend that you tie (solder) these pins to
Pin 4 on the printer to maintain a high signal level (see note 12). DTR (Data Terminal Ready output pin) The printer often uses this pin for the hardware handshaking
signal. If the printer supports DTR handshaking, it should use Pin 20.
However, some printers use Pins 11 or 19 (or other pins besides
Pin 20) to implement their handshaking. The handshaking pin should be connected to Pin 8 on a 9-pin
connector and Pin 5 on a 25-pin connector on the PC (and also Pin 6
on either connector if you followed the recommendation in note 5).
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