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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">  <! REQUEST: REVIEW TITLE: Use of a PC Printer Port for Control and Data Acquisition AUTHOR: Peter H. Anderson EMAIL ADDRESS: pha@eng.morgan.edu INDEX: ORGANIZATION: Morgan State University <HTML><HTML><HEAD><TITLE>Use of a PC Printer Port for Control and Data Acquisition</TITLE> <META content="text/html; charset=big5" http-equiv=Content-Type> <META content="MSHTML 5.00.2314.1000" name=GENERATOR></HEAD> <BODY bgColor=#ffffff> <CENTER>link to <a href="http://et.nmsu.edu/~etti" rel='nofollow' onclick='return false;'>the Technology Interface </A> the Electronic Journal for Engineering Technology </CENTER> <a href="http://et.nmsu.edu/~etti" rel='nofollow' onclick='return false;'>the Technology Interface</A> /Fall 96 <CENTER> <HR> <H1>Use of a PC Printer Port for Control and Data Acquisition</H1></CENTER> <HR> <CENTER> <H4>by</H4> Peter H. Anderson <a href="mailto:pha@eng.morgan.edu" rel='nofollow' onclick='return false;'>pha@eng.morgan.edu</A> Department of Electrical Engineering Morgan State University </CENTER> Abstract: A PC printer port is an inexpensive and yet powerful platform for implementing projects dealing with the control of real world peripherals. The printer port provides eight TTL outputs, five inputs and four bidirectional leads and it provides a very simple means to use the PC interrupt structure. This article discusses how to use program the printer port. A larger manual which deals with such topics as driver circuits, optoisolators, control of DC and stepping motors, infrared and radio remote control, digital and analog multiplexing, D/A and A/D is avaialable. See http://www.access.digex.net/~pha A special thanks to Morgan State University students Towanda Malone, Christine Samuels and H. Paul Roach for their technical contributions and to New Mexico State University student Kyle Quinnell for preparing the html file. <CENTER> <H2>I. Printer Port Basics</H2></CENTER> <CENTER> <H3>A. Port Assignments</H3></CENTER> Each printer port consists of three port addresses; data, status and control port. These addresses are in sequential order. That is, if the data port is at address 0x0378, the corresponding status port is at 0x0379 and the control port is at 0x037a. The following is typical. <PRE> Printer Data Port Status Control LPT1 0x03bc 0x03bd 0x03be LPT2 0x0378 0x0379 0x037a LPT3 0x0278 0x0279 0x027a </PRE> My experience has been that machines are assigned a base address for LPT1 of either 0x0378 or 0x03bc. To definitively identify the assignments for a particular machine, use the DOS debug program to display memory locations 0040:0008. For example: <PRE> >debug -d 0040:0008 L8 0040:0008 78 03 78 02 00 00 00 00 </PRE> Note in the example that LPT1 is at 0x0378, LPT2 at 0x0278 and LPT3 and LPT4 are not assigned. Thus, for this hypothetical machine; <PRE> Printer Data Port Status Control LPT1 0x0378 0x0379 0x037a LPT2 0x0278 0x0279 0x027a LPT3 NONE LPT4 NONE </PRE> An alternate technique is to run Microsoft Diagnostics (MSD.EXE) and review the LPT assignments. <CENTER> <H3>B. Outputs</H3></CENTER> Please refer to the figures titled Figure #1 - Pin Assignments and Figure #2 - Port Assignments. These two figures illustrate the pin assignments on the 25 pin connector and the bit assignments on the three ports. <HR> <CENTER> Fig 1. Pin Assignments</CENTER> <HR> <CENTER> Fig 2. Port Assignments</CENTER> <HR> Note that there are eight outputs on the Data Port (Data 7(msb) - Data 0) and four additional outputs on the low nibble of the Control Port. /SELECT_IN, INIT, /AUTO FEED and /STROBE. [Note that with /SELECT_IN, the "in" refers to the printer. For normal printer operation, the PC exerts a logic zero to indicate to the printer it is selected. The original function of INIT was to initialize the printer, AUTO FEED to advance the paper. In normal printing, STROBE is high. The character to be printed is output on the Data Port and STROBE is momentarily brought low.] All outputs on the Data Port are true logic. That is, writing a logic one to a bit causes the corresponding output to go high. However, the /SELECT_IN, /AUTOFEED and /STROBE outputs on the Control Port have inverted logic. That is, outputting a logic one to a bit causes a logic zero on the corresponding output. This adds some complexity in using the printer port, but the fix is to simply invert those bits using the exclusive OR function prior to outputting. [One might ask why the designers of the printer port designed the port in this manner. Assume you have a printer with no cable attached. An open usually is read as a logic one. Thus, if a logic one on the SELECT_IN, AUTOFEED and STROBE leads meant to take the appropriate action, an unconnected printer would assume it was selected, go into the autofeed mode and assume there was data on the outputs associated with the Data Port. The printer would be going crazy when in fact it wasn't even connected. Thus, the designers used inverted logic. A zero forces the appropriate action.] Returning to the discussion of the Control Port, assume you have a value val1 which is to be output on the Data port and a value val2 on the Control port: <PRE> #define DATA 0x03bc #define STATUS DATA+1 #define CONTROL DATA+2 ... int val1, val2; ... val1 = 0x81; /* 1000 0001 */ /* Data bits 7 and 0 at one */ outportb(DATA, val1); val2 = 0x08; /* 0000 1000 */ outportb(CONTROL, VAL2^0x0b); /* SELECT_IN = 1, INIT = 0, /AUTO_FEED = 0, /STROBE = 0 */ </PRE> Note that only the lower nibble of val2 is significant. Note that in the last line of code, /SELECT_IN, /AUTO_FEED and /STROBE are output in inverted form by using the exclusive-or function so as to compensate for the hardware inversion. For example; if I intended to output 1 0 0 0 on the lower nibble and did not do the inversion, the hardware would invert bit 3, leave bit 2 as true and invert bits 1 and 0. The result, appearing on the output would then be 0 0 1 1 which is about as far from what was desired as one could get. By using the exclusive-or function, 1 0 0 0 is actually sent to the port as 0 0 1 1. The hardware then inverts bits 3, 1 and 0 and the output is then the desired 1 0 0 0. <CENTER> <H3>C. Inputs</H3></CENTER> Note that in the diagram showing the Status Port there are five status leads from the printer. (BSY, /ACK, PE (paper empty), SELECT, /ERROR). [The original intent in the naming of most of these is intuitive. A high on SELECT indicates the printer is on line. A high on BSY or PE indicates to the PC that the printer is busy or out of paper. A low wink on /ACK indicates the printer received something. A low on ERROR indicates the printer is in an error condition.] These inputs are fetched by reading the five most significant bits of the status port. However, the original designers of the printer interface circuitry, inverted the bit associated with the BSY using hardware. That is,