MAXSMBUS Datasheet MAXSMBUS | P1-P3

___MAXSMBus Functionality Check

Follow these steps to verify that the MAXSMBus inter-

face board is functioning properly. All necessary soft-

ware is supplied on a disk with the companion EV kit.

Instructions for operating the software are included in

the EV kit manual.

1) Connect a +9VDC supply (+7V minimum, +30V

maximum) to the MAXSMBus interface board at the

terminals labeled POS9 and GND in the lower left

corner of the board.

2) Use a digital voltmeter to verify that the oval pad

labeled POS5 is +5V (+4.75V minimum, +5.25V

maximum). Also verify that the pads labeled

SBDAT1, SBCLK1, SBSUS1, ALERT1, and ALERT2

are above +4V.

3) If these DC voltages are correct, MAXSMBus pass-

es the functionality test.

_______________Detailed Description

The MAXSMBus interface board provides all of the

interface signals necessary to interface an IBM PC-

compatible computer with an SMBus-compliant device.

A DB25 right-angle plug connects to the computer

(Table 1). The companion board plugs into a 20-pin

dual-row right-angle header at the edge of the board

(Table 2). Alternatively, connection can be made by

soldering wires to the oval pads as appropriate. This

allows the companion board to be placed in an environ-

mental chamber for evaluation over temperature.

Refer to the documentation of the companion Maxim EV

kit for quick start and operating instructions.

________________General Description

MAXSMBus is an interface between an IBM-compatible

PC and System Management Bus (SMBus™)-

compatible serial-interface devices such as tempera-

ture monitors, voltage regulators, or A/D converters

(ADCs). The MAXSMBus interface board is connected

between the PC parallel port and the device under test,

converting parallel data into 2-wire, open-drain serial

data. The board is provided for use with selected

Maxim products and is not intended to replace com-

mercially available SMBus hardware. MAXSMBus is

shipped with a companion EV kit board, including all

relevant software.

Note that the MAXSMBus board, combined with a PC,

is intended only for the functional evaluation of Maxim

SMBus-interfaced devices. The PC functions as the

only bus master, and does not yield to any other SMBus

master. The SMBus emulation system is not designed

to meet all of the SMBus specifications.

____________________________Features

♦ SMBus-Compatible 2-Wire Interface

♦ SMBus Suspend Output

♦ Two SMBus Alert Inputs

♦ Overvoltage Fault Protection

♦ PC Parallel Port Interface

Evaluates: MAXSMBus

2-Wire Interface Board

________________________________________________________________ Maxim Integrated Products 1

19-1629; Rev 1; 2/00

QTY DESCRIPTION

C1, C2, C3 3 0.1µF ceramic capacitors

C4–C7, C9 5 3.3µF, 20V tantalum capacitors

D1 1 1N4692 zener diode, 6.8V

D2 1 1N4688 zener diode, 4.7V

D3 1 1N4148 small-signal diode

J1 1 DB25 right-angle plug

DESIGNATION

_______________Ordering Information

PART

MAXSMBus

BOARD TYPE

Companion Board for SMBus EV Kits

MAXSMBus Interface Board

____________________________Component List

J2 0 Not installed

P1 1 2x10 right-angle male header

R1, R2, R3,

R8–R11

7 47kΩ ±5% resistors

R4–R7 4 4.7kΩ ±5% resistors

U1 1 74HCT05 hex open-collector inverter

U2 1 74HCT04 hex inverter

U3 1 74HCT08 quad AND gate

U4 1 74HCT74 dual D flip-flop

U5 1

+5V, 100mA regulator

LM78L05ACM

U6 1 MAX865EUA (8-pin µMAX)

U7 1 MAX367CWN (18-pin SO)

NONE 1 PC board

SMBus is a trademark of Intel Corp.

C8 1 2.2µF, 35V tantalum capacitor

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

Evaluates: MAXSMBus

2-Wire Interface Board

2 _______________________________________________________________________________________

SYMPTOM CAUSE SOLUTION

Can’t Find the Interface Board

Board not connected to

parallel printer port

Verify that the cable is a 25-pin parallel port I/O extension cable

with a plug on one end and a socket on the other end.

Verify that the cable is connected to a printer port, not a floppy

disk, SCSI, or serial communications port.

Clock or Data Stuck Low

Board is connected to cor-

rect port, but SMBus is not

functioning

Check power connections on the interface board.

Check clock and data signal connections.

Try operating the interface board without the companion Maxim

evaluation kit—this should cause the address-not-acknowledged

symptom described below.

Power Supply

The interface board is powered by a 78L05 linear regu-

lator. The companion board can draw about 20mA of

+5V power through the circuit protector. Companion

boards that require more power must provide their own

regulator. The unregulated input to the 78L05 is avail-

able on the right-angle header.

Fault-Protection Circuitry

Overvoltage fault protection is provided by a MAX367

fault protector (U7). If any of the SMBus interface sig-

nals exceed the MAX367’s power-supply rails, the

MAX367 increases its resistance to prevent damage to

the user’s computer. A MAX865 dual charge pump

(U6) and two zener diodes (D1 and D2) provide +7V

and -3V supplies to the MAX367, thus allowing 0 and

+5V signals to pass with a nominal resistance of 100Ω.

Bus Driving Circuitry

A 74HCT05 open-drain inverter (U1) is used to pull

down the SMBus interface signals. The 74HCT08 (U3),

74HCT74 (U4), and 74HCT04 (U2) buffer the signal to

the IBM PC and provide the capability to mask the

ALERT interrupts, detect an externally generated start

condition, and capture data sent by an external bus

master. Interface connections are listed in Table 1 and

Table 2.

Bus Monitoring Circuitry

Flip-flop U4A detects the start condition (falling edge of

SMBDATA when SMBCLK is high). Flip-flop U4B

detects the falling SMBCLK edge when enabled, and

U1F holds SMBCLK low until the software releases it.

To advance to the next data bit, the software uses U1A

to assert and then release SMBCLK. A logic high at the

input of U1A also resets flip-flop U4B. Because the IBM

PC parallel port has a limited number of inputs, the

start-detect circuit and the two alert inputs share a sin-

gle interrupt input. The source of the interrupt is distin-

guished using U3A, U3B, and U3C.

________________________________________________________Troubleshooting Guide

Address Not Acknowledged

SMBus is OK, but no

response at expected

SMBus address

Verify that the companion board is connected to the MAXSMBus

interface board.

Verify that the companion board is powered.

If the companion offers a choice of addresses, confirm that the soft-

ware and hardware addresses match. Some devices only read the

address select pins at device power-up.

Conflict with local printer

driver

Disable print manager in the Windows printer control panel. Disable

printer driver.

Operating system conflict

1) Use computer with commercially available BIOS.

2) Make a bootable floppy disk, remove unnecessary device dri-

vers from A:config.sys, and boot system from floppy.

Erratic Operation

Evaluates: MAXSMBus

2-Wire Interface Board

_______________________________________________________________________________________ 3

Table 1. DB25 Connector Signals

PIN NAME FUNCTION

1 SPARE OUTPUT A Spare output

SMBCLK_

OUT

When high, drives SMBCLK signal low

SMBDATA_

OUT

When high, drives SMBDATA signal low

4 SMBUS_OUT When high, drives SMBSUS signal low

5 LOOPBACK Loopback connection for port verification

MASK_

ALERT1 When high, allows ALERT1 to trigger INT low

MASK_

ALERT2 When high, allows ALERT2 to trigger INT low

MASK_

START When high, allows a start condition to trigger INT low

9 CAPTURE_ENABLE

When high, enables slave / bus monitor circuitry. This circuit waits until SMBCLK

is pulled low, and then it holds SMBCLK until the software resets it.

INT

Active low interrupt input

SMBDATA_

When high, indicates that SMBDATA is low

SMBCLK_

When high, indicates that SMBCLK is low

13 LOOPBACK Loopback connection for port verification

14 SPARE OUTPUT B Spare output

HOLDING_

CLOCK

When low, indicates that interface board is holding SMBCLK low

16 UNUSED Not used

17 UNUSED Not used

18–25 GND Signal ground return

P1-P3 P4-P6 P7-P8

MAXSMBUS

Mfr. #:

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Manufacturer:

Maxim Integrated

Description:

Interface Development Tools Eval Kit MAXSMBUS (Two-Wire Interface Board)

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