e00a068.pdf

COM PUTER

Centronics/I 2 C Interface

with logic analyser

by Ingo Gerlach DH1AAD

e-mail: IngoGerlach@welfen-netz.com

When this interface is connected to an extended parallel printer port

(EPP), your PC can communicate using the I 2 C protocol, and it can also

make direct read and write accesses to eight data bits.

bilities port), it became possible to

directly read in 8-bit data via the

data bus. Following the introduction

of these modes, a steadily increasing

number of peripheral devices (such

as scanners) utilise the parallel inter-

face for communications with the

PC, since a suitably high data trans-

fer rate can only be achieved if the

data are read directly.

Hardware

Most of the I 2 C interfaces that have been

described in Elektor Electronics up to now are

based on a Philips application note. This cir-

cuit is no exception. However, its special fea-

ture is the combination of an I 2 C interface

with an 8-bit parallel interface. The parallel

port of the PC can be used for control, since

8-bit output was deﬁned for this port by IBM

from the very beginning. However, since the

data lines were only intended to be used for

output, it used to be necessary to

resort to a trick in order to read in

data. This consisted of using the sta-

tus port of the interface to read data.

Since the status port is only ﬁve bits

wide, data had to be read in 4-bit

nibbles.

With the development of new

modes such as EPP (enhanced par-

allel port) and ECP (extended capa-

The hardware of the I 2 C/8-bit inter-

face, as shown in Figure 1 , consists

only of a 25-pin connector and a few

driver and inverter gates. I 2 C com-

munications take place via gates

IC3b and IC3d, IC3e and IC3f. SDA

and SCL are driven via the two data

lines D0 and D1, and they are read

via UserB2 (pin 15) and Intr (pin 10),

respectively, on the EPP interface

connector K1. A few pull-up resistors

provide the proper voltage levels.

The clock and data lines of the I 2 C

bus are located on pin 3 (SCL) and

pin 5 (SDA) of the combination pin

connector K2, and they are also

brought out to solder posts, along

with ground.

The PC can determine whether an

adapter is connected ( init iic ) via the

loopback connection formed by gate

IC2b between pin 17 (nAStrb) and

pin 13 (XFlagUser3). The bi-direc-

tional 8-bit bus driver IC1 is acti-

vated via pin 16 of the control port

Elektor Electronics

10/2000

Versatile

COM PUTER

EPP interface pin

assignments

IC3d

SDA

Function

nWrite

SCL

IC3b

I C

Data D0

IC3e

IC3f

Data D1

Data D2

Data D3

IC1

Data D4

Data D5

DataD6

+5V

GND

SDA

SCL

STROBE

Data D7

Intr

nWait

UserB1

XFlagUser3

nDStrb

UserB2

3EN1

nInit

3EN2

nAStrb

18...25

Masse

74LS245

IC2a

IC3a

Output connector K2

pin assignments

IC2f

IC2b

IC3c

R11

R12

Function

10n

Strobe

SCL

SDA

IC4

IC2, IC3 = 74LS06

Ground

7805

+5 V (Output)

R10

Data D7

IC2c

IC2d

IC2e

Data D6

DataD5

IC1

IC2

IC3

470µ

25V

100n

10µ

63V

100n

Data D4

B80C1500

Data D3

Data D2

000096 - 11

Data D1

Data D0

PE (Input)

Figure 1. The hardware of the interface consists of a few gates and an 8-bit bi-

directional bus driver.

(nInit), which determines whether

the interface circuit works in the I 2 C

mode or the parallel mode (DLL io

enable and io disable functions). The

data transfer direction (read or write)

is selected via nWrite and gate IC3a

(Low = read, High = write). Finally,

the RC network R1/C1 and gate IC2a

provide the required data input

acceptance pulse on pin 11 of K1

(nWait). The correct timing for a read

sequence is shown in Figure 2 .

All these components, along with

a simple voltage regulator formed by

B1, C2-C7 and a 5-V regulator (IC4),

plus a function lamp (D1), are

mounted on the printed circuit board

shown in Figure 3 . The ICs must be

pull-up resistors are

mounted vertically, and their free

ends are attached in common to a

single wire that connects to +5 V at

the end of the row of resistors. A

small 9-V transformer, or better yet a

simple 9-V mains adapter, provides

an adequate source of power.

Ω

000096-11 . Since these are based on the C

functions outp and inp , which directly access

the hardware, they work only under Windows

95/98. They cannot be used under Windows

NT, since NT prohibits direct hardware

access. The ﬁle I2CEPP.BAS provides a sort of

interface between the DLL and an application

program, such as Visual BASIC. The functions

and calling parameters are declared and

described in this ﬁle.

Test program

DLL and BAS

The diskette includes a simply structured 8-

channel logic analyser, which can be used

without any additional hardware for testing

the interface and displaying eight TTL-level

signals on the screen. A maximum of 64,000

All functions are located in the file

I2CDLL.DLL , which is available

along with the rest of the software

on a diskette with order code

10/2000

Elektor Electronics

mounted in suitable sockets, since

some of the eleven wire bridges run

underneath these components. Most

of the 10-k

COM PUTER

data

n Write

data byte

On disk set #000096-1/4

SETUP LST 8.474 04-03-00

ST4UNST EX_ 33.149 04-03-00

SETUP EXE 59.392 04-03-00

SETUP132 EX_ 68.950 04-03-00

STKIT432 DL_ 12.439 04-03-00

VB40032 DL_ 471.576 04-03-00

VEN2232 OL_ 23.683 04-03-00

OLEPRO32 DL_ 89.431 04-03-00

MSVCRT20 DL_ 154.560 04-03-00

MSVCRT40 DL_ 23.256 04-03-00

CTL3D32 DL_ 15.385 04-03-00

VB4DE32 DL_ 11.179 04-03-00

COMDLG32 OC_ 74.496 04-03-00

MFC40 DL1 408.187 04-03-00

MFC40 DL2 134.144 04-03-00

MFC40LOC DL_ 14.265 04-03-00

TABCTL32 OC_ 128.754 04-03-00

RICHTX32 OC_ 117.028 04-03-00

COMCTL32 OC_ 318.415 04-03-00

CCTLDE32 DL_ 12.091 04-03-00

DBLIST32 OC_ 117.805 04-03-00

DBGRID32 OC_ 185.161 04-03-00

GRDKRN32 DL_ 124.298 04-03-00

MSRDC32 OC_ 68.918 04-03-00

MSRDO32 DL_ 147.902 04-03-00

MSRDDE32 DL_ 5.906 04-03-00

MSOUTL32 OC_ 43.502 04-03-00

LA EX_ 15.469 04-03-00

CD4017 L_ 8.567 04-03-00

DREHGEB1 L_ 9.367 04-03-00

DREHGEB2 L1 3.069 04-03-00

DREHGEB2 L2 6.144 04-03-00

I2C_DLL DL_ 12.243 04-03-00

GRAPH01 IC_ 505 04-03-00

COPYRI~1 TXT 188 17-08-00

CONTEN~1 TXT 822 17-08-00

CONTEN~2 TXT 1.035 17-08-00

CONTEN~3 TXT 411 17-08-00

Te l

Te s

nD Strobe

n Wait

X Flag

n Init

nA Strobe

CONTEN~1 TXT 0 17-08-00

COPYRI~1 TXT 182 17-08-00

cd4017 la 64.002 19-02-00

diagramm Bas 11.170 18-02-00

drehgeb1 la 64.002 21-02-00

drehgeb2 la 64.002 28-02-00

epp_la bas 2.901 22-02-00

FILEDI~1 BAS 11.566 19-02-00

GRAPH01 ICO 1.078 15-08-95

I2CEPP bas 6.023 04-03-00

I2C_DLL dll 22.528 21-02-00

la exe 36.352 04-03-00

la LA 64.002 19-02-00

la vbp 1.378 26-06-00

la vbw 116 26-06-00

la_main frm 19.205 26-06-00

la_main frx 1.090 26-06-00

MSSCCPRJ SCC 189 26-06-00

000096 - 12

Figure 2. A read sequence using the EPP

interface.

measurements can be made (controlled via

the Counts slider), and the region to be dis-

played can be selected using the StartShow

and EndShow controls. The measured values

can be stored and read back in. The Strobe

signal (pin 1 of K2) is pulled low during a

measurement.

It is also possible to initiate measurements

externally. The check box PE Trigger must be

set for this. In this case, the measurement

starts only after pin 2 of K2 goes Low.

SCL

SDA

(000096-1)

Literature:

Parallel Port Complete, Jan Axelson;

Lakeview Research, ISBN 0-9650819-1-5

COMPONENTS LIST

R11

Resistors:

R1 = 22k

Ω

R2-R9,R11,R12 = 10k

000096-1

Ω

R10

IC4

R10 = 1k Ω

Capacitors:

C1 = 10nF

C2 = 10µF 63V radial

C3 = 470F 25V radial

C4-C7 = 100nF ceramic

Semiconductors:

D1 = LED, high efficiency

IC1 = 74LS245

IC2,IC3 = 74LS06

IC4 = 7805

Miscellaneous:

K1 = 25-way sub-D plug (male), PCB mount

K2 = 26-way boxheader or pinheader

B1 = B80C1500 in round case (80V piv,

1.5A peak)

PCB, order code 000096-1

Project software on disk, order code

000096-11

Figure 3. The hardware is mounted on this small printed circuit board.

Elektor Electronics

10/2000

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