LPTCAP

Parallel print capture system

By Kris Heidenstrom (k@heidenstrom.gen.nz)

Release 1c, 02 April 2000

Introduction

It is mainly intended to allow PC-based capturing of data from "closed" systems such as obsolete word processors which use incompatible disks or disk formats, so that the old equipment can be thrown out, or microcontroller-based embedded systems, where the only way to access the data externally is via the unit's printer port, to eliminate paper printouts and/or add processing capabilities.

The LPTCAP system is not a product you can buy. The software is provided in this package, and may be used in any personal or commercial application. The hardware (the LPTCAP adapter) can be constructed by anyone with basic practical electronic construction skills, and is documented in full in this package. Parts cost is about US $20 excluding the case and cables. The author does not sell the adapter as a kit or product.

If you are browsing this document online, you may have noticed that it has no links to other parts of the package. This is because I would prefer you to get the full LPTCAP archive, which is available as http://home.clear.net.nz/pages/kheidens/lptcap/lptcap01.zip.

The LPTCAP system consists of a hardware adapter box called the LPTCAP adapter, and associated support software. The LPTCAP adapter box has two connectors:

• CN1 - a female 36-pin Centronics connector, and
• CN2 - a male 25-pin D-sub connector.

CN1 is physically and electrically similar to the Centronics connector on a parallel printer, and connects to the device which is sending printer data (known as the Sender) via the cable that would normally be used to connect the Sender to a Centronics parallel printer.

Data from the Sender is latched in the adapter, and can be captured by a PC (called the Capture PC) which is connected to CN2 (male 25-pin D-sub). The connection between CN2 and the Capture PC is through a short pin-to-pin cable which connects CN2 to a parallel printer port on the Capture PC. The Capture PC controls and interrogates the LPTCAP adapter through this cable.

The basic functions performed by the LPTCAP adapter are:

• Provide electrical compatibility with the Sender and the Capture PC,
• Capture parallel data from Sender and allow Capture PC to read captured data via a standard parallel port in either polled or interrupt mode,
• Provide timed Acknowledgement pulse generation,
• Allow the Capture PC to monitor commands from the sender,
• Allow the Capture PC to provide statuses to the sender.

The support software runs on the Capture PC. It consists of the LPTCAP software module which is included in the LPTCAP package in source (assembly language) and object form, to be linked into other programs. This module provides LPTCAP auto-detection, testing and capturing functions which are easy to use.

A small exerciser program called LPTEST (which uses the LPTCAP software module) is also included. This program performs tests and simple capturing functions. This program is included in source (Borland C) and executable forms, and can be used as a starting point for other programs, or for basic capturing functions.

Capturing can be done in polled mode, or interrupt-driven mode if the Capture PC's parallel port has interrupt capability. Interrupt-driven mode may be more convenient and/or efficient than polled mode in some applications, but is not required to prevent data overrun (as with serial communication, for example), because the Centronics electrical protocol requires an explicit handshake on each character. The LPTCAP adapter provides the handshaking signals and thus prevents data overrun.

Boring Legal Stuff

My philosophy is that this package should be freely available to anyone who wants to use it, and that no-one should deny this to anyone else. By "freely available" I mean easily available and with no specific charge apart from any normal data or disk charges.

You can use the hardware design and the support software freely in any personal or commercial application. You can distribute the software, in its complete and unmodified form only, as long as the author is identified, the package is identified as free and no specific charge (other than data charges or disk duplication) is made for it.

You are welcome to build LPTCAP adapters (the hardware part of the package) and sell them, but you must clearly identify the designer, and state that the hardware design and support software are freely available and that the potential buyer could get the design details for free and build the device him/herself.

You may not distribute the LPTCAP package in an altered or incomplete form. Please distribute only the original archive file.

If you generate a freely distributable derived version of the design or code such as a DOS protected-mode version or a version for a different operating system, please include a note that your code was derived from mine, and please consider sending me a copy of your package.

I provide no warranty that the LPTCAP code and the LPTCAP design will perform to any standard or at all. This is because I cannot control the environment in which you use the software and the hardware design. Any use of the LPTCAP system is entirely at your own risk. If this is not acceptable to you, then do not use the LPTCAP package.

You should also check the Caveats section for notes on the limitations of the design.

I have tested the design thoroughly and I believe that it is sound. If you have a problem with the LPTCAP design, please let me know and I will try to help. You can contact me at:

	Internet:	k@heidenstrom.gen.nz	(preferred)
	Snail:	Kris Heidenstrom
		c/- P.O Box 27-103
		Wellington
		New Zealand
	Phone:	Work: +64 4 385-6611
		Home: +64 4 475-7437
		(We are 12 hours ahead of GMT.)

Please feel free to contact me via email, or to send a postcard to the snail mail address above if you find the LPTCAP system useful.

Versions

Original release

Version 1a - released 28 March 1998

Correction to setup procedure - in the second stage when measuring across R1 and adjusting for half resistance, I had said to measure between U2 pin 8 and U1 pin 11 - the correct pins are U2 pin 8 and U1 pin 5. Thanks to Jay Pennell (jpennell@seanet.com) of Penntronics for finding this error.

Version 1b - released 28 July 1998

Correction to layout illustration to add connection points to CN1 pin 10 and CN2 pin 11, and change to the construction procedure to state that there are thirteen wires from the board to each connector including grounds. Thanks to Peter Scales (peter@sleepy.demon.co.uk) for reporting these errors.

Version 1c - released 02 April 2000

Changed email address from kheidens@clear.net.nz to k@heidenstrom.gen.nz.

LPTCAP Adapter Hardware

The schematic diagram (above) is contained in the file LPTCAP-S.GIF. As you can see, it is quite large, and should be printed if possible. Note that on the schematic, U1 is shown as six discrete components, named U1A to U1F.

The stripboard layout is given in the file LPTCAP-L.GIF. This view shows the adapter board looking down from above. See the Construction section for more information. Important note: There were two errors in the layout drawing which are fixed in release 1b - connections to CN1 pin 10 and CN2 pin 11 were not marked on previous versions.

Parts List

	Quantity	Reference(s)	Description
	3	C1-3	0.1µF 50V ceramic or monolithic capacitor
	1	C4	10µF 10V tantalum capacitor
	2	C5,6	470pF 50V ceramic capacitor
	3	C7-9	100pF 50V ceramic capacitor
	1	CN1	Female 36-pin Centronics connector, chassis mount
	1	CN2	Male 25-pin D-sub connector, chassis mount
	2	R1,2	50 kilohm preset potentiometer (trimpot)
	2	RSIL1,2	Single-in-line resistor network, 9-pin, 8-element, 100 kilohm
	1	JP1	3-pin jumper block or single-pole changeover switch (see below)
	1	U1	74HC14 integrated circuit - hex Schmitt trigger inverter
	1	U2	74HC74 integrated circuit - dual D-type flip-flop
	1	U3	74HC165 integrated circuit - serial-output shift register
	1		Jumper shunting block for JP1
	2		14-pin DIP IC socket, dual-wipe (for U1 and U2)
	1		16-pin DIP IC socket, dual-wipe (for U3)
	1		Piece of stripboard - 25 tracks by 20 holes
	1		Plastic or metal case
	2		Screws, nuts and lockwashers for mounting CN1
	2		Hexagonal tapped-head screws with nuts and lockwashers for CN2
	5m		Hookup wire (stranded, insulated)
	2m		Solder

Notes:

All of the above parts should be readily available from electronic hobbyist shops such as Radio Shack or the local equivalent. R1 and R2 are 50 kilohm preset potentiometers (trimpots). These are screwdriver-settable devices which are adjusted in the adjustment procedure. Single-turn or ten-turn types are both suitable. JP1 is listed as a three-pin jumper strip with a jumper shunt. This jumper selects the interrupt polarity, which may be different for different Capture PCs. If the LPTCAP adapter will regularly be used with several different Capture PCs, it may be worthwhile connecting a single-pole changeover (SPCO) switch such as a small slide switch or toggle switch, instead of the jumper, so that the interrupt polarity can be selected without opening the adapter case. U1-3 must be 74HCnn types. 74nn (TTL), 74LSnn, 74ACTnn and 74HCTnn types are NOT suitable. Typical manufacturers are Motorola, Philips, National Semiconductor, Harris and Texas Instruments. Use DIP (dual in-line package) devices, not surface-mount (SOIC) devices (surface mount devices do not go well with stripboard). U1-3 are static sensitive devices. Keep them in their anti-static tubes until you are ready to use them.

Construction

COPY /B PRNFILE.OUT PRN

You can measure the time taken to transfer the file and calculate the throughput if you wish. When the file has been transferred, use a file-compare program such as DOS's COMP to compare the captured file with PRNFILE.OUT on the Capture PC. If the files do not match, see the Caveats section for suggestions.

Signal Naming Conventions

The four control signals (-INIT, -SELECT, -AUTOFEED and -STROBE) are connected directly from the Sender's port to the same signals on the Capture PC, so they are named identically on both CN1 and CN2.

LPTCAP Adapter Technical Description

The LPTCAP adapter contains one 74HC14 hex Schmitt inverter chip (U1) which is used to invert various signals and clean up slow signal transitions from the parallel ports, one 74HC74 dual flip-flop (U2) which detects and latches data bytes from the Sender and generates Acknowledge pulses, and one 74HC165 parallel/serial-input, serial-output shift register (U3) which loads and holds each captured data byte and passes it serially to the Capture PC.

These devices must be 74HC types. Other types with compatible pinouts, such as 74LS and 74HCT, should not be used.

The default state of the adapter is with U2A in the reset state, i.e. U2 pin 6 high and the +-DRDY signal line in the inactive state (whether this is high or low depends on the setting of the IRQ Polarity switch or jumper). In this state, assuming that -RESETB is inactive (high), the adapter is ready to accept a data byte from the Sender machine.

When the Sender wishes to send a byte, it asserts the data value on D0-7 and then briefly pulses -STROBE active (low). This causes the BUSY latch (U2A) to flip to the set state, giving a BUSY indication back to the Sender, and switches the shift register (U3) from transparent load mode to shift mode, latching the data byte into the shift register in the process. The +-DRDY line goes active, indicating to the Capture PC that a data byte has been captured and generating a parallel port interrupt on the Capture PC if the interrupt is enabled and the port is interrupt capable.

The LPTCAP adapter will remain in the "busy" state until the Capture PC detects and acknowledges the new data. In this state, the Sender cannot send a new data byte, because BUSY is active and no -ACK pulse has been sent by the adapter.

When the Capture PC detects that new data is ready, via the +-DRDY line, either by polling the line or through the parallel port interrupt, it can read the data serially via the SDI and -SDI signals (-SDI is always the complement of SDI), using the -SCL signal to clock the data through U3. Initially when a data byte is captured, bit 0 is present on SDI. On every falling edge of -SCL from the Capture PC, the data shifts through the shift register by one bit, shifting new data in from the SDO line (which is used in the diagnostic test but not during normal operation) and presenting the next data bit of the captured byte on SDI and -SDI.

Once the Capture PC has read the data byte, it brings -DACK low, clocking a "1" into U2B and starting a pulse on the -ACK line back to the Sender (and to the Capture PC on the -ACKP line). Approximately 5 microseconds (set by R1 and C5) after the start of this pulse, U2A's clock goes high and U2A is reset (assuming that -STROBE from the sender is high, which it should be). Approximately 10 microseconds (set by R2 and C6) after the start of the -ACK pulse, U2B resets, ending the -ACK pulse.

The -RESETB signal is provided to allow the Capture PC to reset U2A without generating an -ACK pulse, and is used only during initialisation of the LPTCAP adapter. The 74HC74 device responds in a defined way when its set and reset inputs are both active (low) - both the Q and Q-bar outputs are forced high. With U2A, only the Q-bar output is used, so the reset input (from -RESETB) has priority over the set input (from -STROBE). Therefore, if -RESETB is active (low), U2A will be in a known state regardless of the state of -STROBE. This is important because the Capture PC cannot guarantee that -STROBE will be high (inactive) - it may be being driven active by the Sender. By forcing -STROBE low itself, the Capture PC can control U2A's Q-bar output via -RESETB.

The DDATA signal feeds the Data input of the acknowledge pulse flip-flop and should be held at logic "1" at all times. It has no practical purpose except to provide another source of supply current for the adapter.

The following diagram shows a typical data byte capture.

Parallel Port Registers On The Capture PC

In these tables, the "!" symbol indicates that the PC's parallel port circuitry inverts the data between the register and the hardware signal, i.e. a bit value of "1" in the register corresponds to an electrically low signal and vice versa. The "-" symbol indicates that the signal is electrically active-low, i.e. it is active or "true" when it is electrically low.

Therefore a signal name which starts with "-" or "!" (but not both) is in its active state when the register bit is 0, and a signal name with both "!" and "-", or neither, is in its active state when the register bit is 1. The polarity of +-DRDY depends on the setting of the IRQ Polarity jumper (JP1) or switch on the LPTCAP adapter. See the Status register section for details.

Data Register

Assembly Time Settings

	TINY	Selects tiny memory model
	SMALL	Selects small memory model
	COMPACT	Selects compact memory model
	MEDIUM	Selects medium memory model
	LARGE	Selects large memory model
	HUGE	Selects huge memory model
	SLOW	Selects slow timing
	VERYSLOW	Selects very slow timing

These constants are defined on the assembler command-line with the "/dname" option. For example, to specify large model and slow timing, the command line to the assembler (TASM or MASM) should include:

/dLARGE /dSLOW

A memory model should be specified. If no memory model is specified, during assembly the module issues a warning message and the module defaults to SMALL model.

The SLOW and VERYSLOW options may be useful during development. The VERYSLOW setting should not be used when generating production code. If neither SLOW nor VERYSLOW are specified, the module will be assembled for standard timing.

Refer to the comments in LPTCAP.ASM for more detailed information on the memory model support and the slow timing settings.

Functions Provided

	lptcap_version()	Returns version numbers and assembly settings
	lptcap_port() **	Detects LPTCAP adapter(s) on parallel ports
	lptcap_test() **	Diagnostics - logic and data loopthrough tests
	lptcap_test_fail_data()	Returns details of a diagnostic check failure
	lptcap_adjust()	Generates timed signals for adapter adjustment
	lptcap_get_cont()	Returns states of control signals from Sender
	lptcap_set_stat()	Sets states of printer status signals to Sender
	lptcap_send_ack()	Sends an acknowledgement to the Sender
	lptcap_poll_new()	Tests whether a new character has been captured
	lptcap_next_char()	Returns the next captured character
	lptcap_wait_char()	Waits (with or without timeout) for a character
	lptcap_autodetect_irq()	Auto-detects the IRQ number of the LPTCAP port
	lptcap_intmode_install()	Installs interrupt-driven capture mode
	lptcap_intmode_uninstall()	Uninstalls interrupt-driven capture mode

** The functions lptcap_port() and lptcap_test() set up global variables which are used by other functions, and must be called and return a successful result before other functions are used.

The code in the LPTCAP module manipulates the interrupt flag extensively. All functions return with the interrupt flag unmodified. Functions do not lock interrupts out for more than about 50 microseconds at one time (any operations which take longer than this are split into blocks, with the interrupt flag restored to its initial between the blocks) except for lptcap_adjust() which locks interrupts out for about 5 milliseconds at a time. Some functions explicitly enable interrupts temporarily during their execution.

Individual Function Descriptions

lptcap_port()

lptcap_test()

If this function is called while interrupt-driven receive mode is enabled (via lptcap_intmode_install()) it will return LPTCAP_ADAPTER_INTMODE and will not perform any logic or data tests.

This function should not be called while LPTCAP is capturing data as it will disturb the capturing process. It should not be called while the Sender is trying to send data. This means that any capture program should perform its initial setup and tests BEFORE the Sender is instructed to send data.

Return values are:

	0	LPTCAP_ADAPTER_PRESENT	Adapter present and working normally
	1	LPTCAP_ADAPTER_NO_PORT	No parallel port has been selected
	2	LPTCAP_ADAPTER_MISSING	Adapter is not attached
	3	LPTCAP_ADAPTER_INTMODE	System is running in interrupt-driven mode
	4	LPTCAP_ADAPTER_NO_DRDY	Adapter failed to activate +-DRDY signal
	5	LPTCAP_ADAPTER_ACKSTUK	Adapter -ACKP is stuck in active state
	6	LPTCAP_ADAPTER_NO_DACK	Adapter failed to activate -ACKP signal
	7	LPTCAP_ADAPTER_ACKLONG	Adapter -ACKP pulse too long
	8	LPTCAP_ADAPTER_BSYSTUK	Adapter indicated BUSY active after -ACK pulse
	9	LPTCAP_ADAPTER_STRSTUK	-STROBE line stuck low (see notes below)
	10	LPTCAP_ADAPTER_SDISERR	Adapter SDI and -SDI states inconsistent
	11	LPTCAP_ADAPTER_DATAERR	Data loopthrough error (see notes below)

All non-zero values indicate a problem with the LPTCAP adapter or the parallel port of the Capture PC, except return codes LPTCAP_ADAPTER_BSYSTUK and LPTCAP_ADAPTER_STRSTUK, which may indicate that the Sender is already trying to send data when lptcap_test() is called, or possibly that the Sender is faulty or is connected but powered off.

For a return value of LPTCAP_DATAERR, the byte values that were sent and received in the data loopthrough test that failed are available through the lptcap_test_fail_data() function and may be displayed in the error message to possibly help a user to diagnose the problem.

This function temporarily uses timer channel 2 for timeout checking. This may interfere with any background sound generation or timing that is being done by other parts of the program using timer channel 2.

lptcap_test_fail_data()

This function returns an unsigned integer which is comprised of the "sent" value (in the low byte) and the "received" value (in the high byte) from the data loopthrough test that failed, when lptcap_test() returns an error code of LPTCAP_ADAPTER_DATAERR. This may be displayed as part of the failure report message if desired.

lptcap_adjust()

This function generates a pattern of accurately timed signals which are used during adjustment of the LPTCAP adapter. See the Adjustment section for details.

The "ticks" parameter specifies the duration of the test. One timer tick is approximately 55 milliseconds, so one second is about 18 ticks and one minute is about 1092 ticks. If a zero "ticks" parameter is supplied, lptcap_adjust() will generate the test signal until the user presses the Ctrl key.

This function may be used even if lptcap_test() has not been called, or when lptcap_test() returned an error code. This is because it may be necessary to adjust the LPTCAP adapter before lptcap_test() will report success.

This function should not be called while LPTCAP is capturing data as it will disturb the capturing process. If it is called when the LPTCAP system is operating in interrupt-driven receive mode (via lptcap_intmode_install()) it will return without generating any adjustment signal.

This function temporarily uses timer channel 2 for timekeeping. This may interfere with any background sound generation or timing that is being done by other parts of the program using timer channel 2.

lptcap_get_cont()

lptcap_set_stat()

The default and initial values for these signals are the "0" values, which are "normal" (i.e. "no-error") values. This function need not be used in normal circumstances, since the outgoing signals are set to their default values when lptcap_port() is called.

lptcap_send_ack()

lptcap_poll_new()

lptcap_next_char()

In polled mode, this function examines the LPTCAP adapter directly, and in interrupt-driven mode, it examines the buffer pointers and checks for missed interrupts - see notes on lptcap_poll_new() for details.

lptcap_wait_char()

In polled mode, this function examines the LPTCAP adapter directly, and in interrupt-driven mode, it examines the buffer pointers and checks for missed interrupts - see notes on lptcap_poll_new() for details.

lptcap_autodetect_irq()

If lptcap_autodetect_irq() returns LPTCAP_IRQ_NONE or LPTCAP_IRQ_SEVERAL, interrupt-driven capture (if enabled) will probably fail to work properly. A return value of LPTCAP_IRQ_INVERTED means that the IRQ Polarity switch or jumper on the LPTCAP adapter must be changed to the other position. When this has been done, lptcap_test() must be called to detect the new state of this switch/jumper, then lptcap_autodetect_irq() may be called again.

Any other value indicates a valid and correct IRQ number that is associated with the LPTCAP port. On PC and PC/XT machines the IRQ number will be in the range 2-7. On the AT and later machines the IRQ will be in the range 3-15 excluding 8 and 13.

If interrupt-driven capturing is not required, there is no need to call this function at all.

This function does not enable the interrupt (except briefly during its testing), and does not install the LPTCAP interrupt-driven receiver.

The irqmask parameter is a bitstring which specifies which IRQs are to be tested. Most interrupt-capable parallel ports are wired to IRQ7 or IRQ5, and some may be jumper-selectable to other IRQs. Bits 0 to 15 of the irqmask parameter correspond to IRQs 0 to 15, and a 1-bit enables testing of that IRQ. Therefore a typical value for irqmask is 0x00A0, allowing IRQs 5 and 7 to be tested, though any value, including 0xFFFF, can also be used.

Some IRQs are not available on the slot bus, and these are not tested by this function even if enabled in irqmask. On PC and PC/XT machines, IRQs 2-7 are available on the slot bus (IRQ0 and IRQ1 are used on the motherboard). On AT and later machines, IRQs 3-7, 9-12, 14 and 15 are available on the slot bus (IRQ0, IRQ1, IRQ8 and IRQ13 are used on the motherboard and IRQ2 doesn't exist).

This function will probably fail on MicroChannel machines (IBM PS/2). It will also probably fail if used in a DOS compatibility box within a multi-process operating system such as Windows 3.x, Windows 95, Windows NT, OS/2 or Linux.

Many soundcards use IRQ7. If the sound card has not been initialised, its IRQ line driver may be in the high-impedance state and the parallel port may be able to use the IRQ line. However, software that uses the sound card (such as Windows or game software) may leave the buffer enabled, and this may prevent the parallel port IRQ from working after these programs have been run.

This function should not be called while LPTCAP is capturing data as it will disturb the capturing process.

lptcap_intmode_install()

The LPTCAP port IRQ number can be determined by calling lptcap_autodetect_irq() but some parallel ports do not have interrupt capability, in which case interrupt-driven receive mode will not work.

This function does not attempt to confirm that the specified interrupt request line is associated with the LPTCAP port, nor that the port is interrupt capable, nor that the interrupt line is not in use by some other peripheral.

The bdp parameter must point to an lptcap_bufdescriptor structure (see the typedef in LPTCAP.H), which is formatted as follows:

	volatile unsigned int tail_index; /* Tail (index into buffer) */
	unsigned int head_index;	/* Head (index into buffer) */
	unsigned int buf_size;		/* Size of buffer in bytes */
	unsigned char * buf_base;	/* Pointer to base of buffer */

Memory for the buffer descriptor and the buffer area must be allocated by the calling program. The caller must provide the buf_base and buf_size parameters in the descriptor structure; this function provides initial values for tail_index and head_index (both 0).

The maximum buffer size is limited to 65535 bytes, even in the "huge" memory model, because the buf_size is a word (16-bit). The maximum number of data bytes that can be held in the buffer is one less than the buffer size.

The calling program may remove data from the buffer manually, by manipulating the head_index value, or it may use the lptcap_poll_new(), lptcap_next_char() and lptcap_wait_char() functions.

Enabling interrupt-driven receive mode causes the behaviour of the character input functions (lptcap_poll_new(), lptcap_next_char() and lptcap_wait_char()) to change - they use the interrupt-driven receive buffer, instead of going direct to the LPTCAP adapter hardware.

The interrupt line associated with the LPTCAP parallel port can be found by lptcap_autodetect_irq(). If an incorrect interrupt number is chosen, data capture may appear to work (because software will capture data in polled mode if the interrupt handler is not activated properly), but throughput will be reduced.

Using lptcap_port() to change to a different port while interrupt-driven capturing is installed will cause strange behaviour, so don't do it!

If a program uses interrupt-driven capture mode, steps must be taken to ensure that lptcap_intmode_uninstall() is called before the program terminates. This may involve a critical error handler and/or a Ctrl-Break handler, since either of these conditions can usually terminate a DOS application without allowing it to clean up. If this occurs, the interrupt vector is left pointing to code in the LPTCAP module which could be overwritten when another program is run, which would cause a system crash.

lptcap_intmode_uninstall()

Do not use the LPTCAP software on a machine which is running in slow mode (i.e. a machine which has a turbo switch which is set to "slow"). This may cause lptcap_test() to occasionally report bogus LPTCAP_ADAPTER_NO_DACK failures.

The LPTCAP hardware adapter is not heavily protected against electrical interference or damage due to common-mode or differential-mode surges being induced into the cables. It is very important to follow these guidelines:

• Do not use the LPTCAP adapter in a harsh electrical environment.
• Make the cable from the LPTCAP adapter to the Capture PC as short as possible.
• The cable to the Sender should also be as short as practical.
• Both cables should be screened if possible.
• Both cables MUST have electrical continuity between the connector shields. This is essential to help prevent damage to the ICs in the LPTCAP adapter when cables are plugged in and unplugged.
• If electrical interference causes data corruption, ensure that both data cables are screened, try powering the Sender and the Capture PC from the same electrical outlet point, and try moving the suspected interfering appliance(s) to a different outlet point.

During testing, the prototype adapter performed well and was able to transfer one-megabyte pseudo-random data files without error. However, the Centronics interface is susceptible to electrical interference, so if data accuracy is important, you should perform some testing by sending a known data file through the adapter and trying to induce interference in the following ways:

• For all cables and connectors, grasp and tug or jiggle the cable near the connector,
• Test for poor connections in the LPTCAP adapter by repeatedly tapping it sharply (e.g. with a piece of wood),
• Test susceptibility to electrical interference by switching nearby appliances on and off repeatedly.

If capturing is unreliable with certain Senders or Capture PCs, or certain combinations, try adding a 10 kilohm resistor between pins 4 and 14 of U2.

Performance

The LPTCAP system is not optimised for high throughput. The hardware constraints of the standard parallel port severely limit the maximum data transfer rate, and software changes would make little difference to the throughput.

Using the SLOW or VERYSLOW options when assembling the LPTCAP module will affect throughput. SLOW will roughly halve throughput, and VERYSLOW will reduce throughput by a much greater amount (depending on the speed of the Capture PC).

Throughput is limited by both the Sender and the Capture PC. The Capture PC generally does more work than the Sender and needs to be fast if throughput is significant. Prototype throughput test results were:

	Sender	Capture PC	Throughput
	100MHz Pentium	100 MHz Pentium	16100 bytes/sec
	8 MHz XT	75 MHz Pentium	6000 bytes/sec
	75 MHz Pentium	8 MHz XT	1400 bytes/sec

These tests were done with standard timing (i.e. SLOW and VERYSLOW not used).

Only the first line, using fast machines for both Sender and Capture PC, has a data transfer rate higher than 115200 bps (the maximum rate available on a standard PC serial port).

Throughput in interrupt mode is generally slightly lower than polled mode, unless the software on the Capture PC is performing other tasks in addition to capturing data.

Summary

The hardware can be built by anyone with some practical electronic construction experience. The support software can be linked into user-written programs, or the test program can be used for one-off data capturing.

The design has some limitations (generally these are inherent in the Centronics protocol), which are discussed in the Caveats section.

Anyone is free to use the LPTCAP system for their own personal or business purposes, provided that they don't deny others the same opportunity. The design details and support software are freely available to any user.

I hope you find the LPTCAP system useful.

Kris Heidenstrom

02 April 2000