NOVELL TECHNICAL DISCUSSION

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          TITLE:  Printing and the Serial Interface
   DOCUMENT ID#:  TD.P.2
           DATE:  April 6, 1992
        PRODUCT:  NetWare
PRODUCT VERSION:  All
     SUPERSEDES:  N/A

        SYMPTOM:  None

Serial printing on the Novell network has on occasion been a source
of difficulty.  Much of this difficulty is overcome with an
understanding of the serial handshaking scheme.  The purpose of
this document is to improve that understanding.

Serial printers in the DOS world take advantage of the RS-232-C
interface standard.  This interface was developed specifically for
terminal to modem communications or DTE to DCE (Data Terminal
Equipment to Data Communication Equipment).  As PC's and printers
both consider themselves to be DTE, each have to be "tricked" into
thinking that they are communicating with DCE on the other side.
Now, if all printers did this in the same way, a standard cable
could be used to interface all printers with all PC's and the world
would be a happy place.  However, since the world is not perfect,
and printer vendors have decided to implement handshaking in
various ways, the following is the optimal cable pin-out. 
Explanatory notes referring to the reference numbers in this
diagram are listed below.


                PC                                Printer

          9 pin   25 pin                          25 pin

          --      1 (1)           FG              1

          3       2 (2) >DDDDDD?  TXD     ZDDDDD< 2
                               @DDDDDDD?  3
                          ZDDDDDDDDDDDDEDDY                
          2       3 (3) <DY       RXD  @DDDDDDDD> 3

          7       4 (4) ^         RTS           ^ 4 (10) >?
                                                          3
          8       5 (5) <DDDD?    CTS             5 <DDDDD4
                             CDDDDDDDDDDDDD?              3
          6       6 (6) <DDDDY    DSR      3      6 <DDDDD4
                               ZDDDDDDDDDDDED?            3
          5       7 (7) DDDDDDDY  SG       3 @DDD 7 (7)   3
                                           3              3
          1       8 (8)  (NC)     DCD      3      8 <DDDDDY
                                           3
          4      20 (9) ^         DTR      @DDDD> 20 (11) 
                                                  (Handshaking)


(1) Frame Ground.  Not necessary and, in fact, may cause ground
loop problems if PC and printer are connected to different power
sources.

(2) Transmit Data (output).  The PC sends data to the printer via
this pin.

(3) Receive Data (input).  The PC can receive data from the printer
on this pin.  This is used for XON/XOFF (software) handshaking.  If
XON/XOFF is used, the PC needs only pins 2, 3, and 7 to function
properly.

(4) Request To Send (output).  This pin is brought high (^) and
kept in that state. Some printers use this pin to enable an input
on the printer.  The print server and rprinter, until version
1.22R, dropped this signal when no data was being sent to the
printer.  This process has been the cause of many problems.
Versions 1.22R and later were changed to hold this pin high at all
times.  Our recommendation is to avoid the use of this pin.

(5) Clear To Send (input).  This is the pin we use for hardware
handshaking.  If pin 5 is high, we send data to the printer.  If
pin 5 is low, we stop sending data.  If pin 5 stays low for about
10 seconds, we report the "OFF LINE" condition.  It is necessary to
connect the pin on the printer that the printer uses for signaling
a "don't send" condition to the PC to this pin.  This is pin 20 for
DTR handshaking but vendors commonly use other pins for their
handshaking.  If hardware handshaking is being used, pins 2, 5, and
7 are the only pins we need to function properly.

(6) Data Set Ready (input).  We don't use this pin.  However, DOS
needs both pins 5 and 6 to be high to send data to the printer, so
the recommendation is to tie pins 5 and 6 together to provide DOS
compatibility.

(7) Signal Ground.  This is needed to provide a reference for the
signal levels.

(8) Data Carrier Detect (input).  Not used.  Should be not
connected (NC).

(9) Data Terminal Ready (output).  See note four (4).

(10) When the printer is powered on, pin 4 on the printer is pulled
high and kept high (^).  It can be used, therefore, to enable input
pins that, on some printers, may be used to control the receive pin
(pin 3) on the printer.  The recommendation is to tie this pin to
pins 5, 6, and 8.  This may not work on all printers.

(11) Pin 20 on the printer is often used for the handshaking
signal.  If the printer supports DTR handshaking it should be using
pin 20.  Many printers use other pins; pins 11, and 19 being very
popular.

This pin-out is not the panacea for all serial printing handshaking
problems, but it should work as the interface for most printers.