2-Wire Interface Board

2

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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 100

Ω.

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