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19-1629; Rev 1; 2/00
2-Wire Interface Board
________________General Description
MAXSMBUS is an interface between an IBM-compatible PC and System Management Bus (SMBusTM)compatible serial-interface devices such as temperature 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 commercially 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
o SMBus-Compatible 2-Wire Interface o SMBus Suspend Output o Two SMBus Alert Inputs o Overvoltage Fault Protection o PC Parallel Port Interface
Evaluates: MAXSMBUS
_______________Ordering Information
PART MAXSMBUS BOARD TYPE Companion Board for SMBus EV Kits
___MAXSMBUS Functionality Check
Follow these steps to verify that the MAXSMBUS interface board is functioning properly. All necessary software 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 passes the functionality test.
MAXSMBUS Interface Board ____________________________Component List
DESIGNATION C1, C2, C3 C4-C7, C9 C8 D1 D2 D3 J1 J2 P1 R1, R2, R3, R8-R11 R4-R7 U1 U2 U3 U4 U5 U6 U7 NONE QTY 3 5 1 1 1 1 1 0 1 7 4 1 1 1 1 1 1 1 1 DESCRIPTION 0.1F ceramic capacitors 3.3F, 20V tantalum capacitors 2.2F, 35V tantalum capacitor 1N4692 zener diode, 6.8V 1N4688 zener diode, 4.7V 1N4148 small-signal diode DB25 right-angle plug Not installed 2x10 right-angle male header 47k 5% resistors 4.7k 5% resistors 74HCT05 hex open-collector inverter 74HCT04 hex inverter 74HCT08 quad AND gate 74HCT74 dual D flip-flop +5V, 100mA regulator LM78L05ACM MAX865EUA (8-pin MAX) MAX367CWN (18-pin SO) PC board
_______________Detailed Description
The MAXSMBUS interface board provides all of the interface signals necessary to interface an IBM PCcompatible 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 environmental chamber for evaluation over temperature. Refer to the documentation of the companion Maxim EV kit for quick start and operating instructions.
SMBus is a trademark of Intel Corp. ________________________________________________________________ Maxim Integrated Products 1
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.
2-Wire Interface Board Evaluates: MAXSMBUS
Power Supply
The interface board is powered by a 78L05 linear regulator. 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 available on the right-angle header.
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.
Fault-Protection Circuitry
Overvoltage fault protection is provided by a MAX367 fault protector (U7). If any of the SMBus interface signals 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.
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 single interrupt input. The source of the interrupt is distinguished using U3A, U3B, and U3C.
________________________________________________________Troubleshooting Guide
SYMPTOM CAUSE Board not connected to parallel printer port SOLUTION 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. 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. 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 software and hardware addresses match. Some devices only read the address select pins at device power-up. Disable print manager in the Windows printer control panel. Disable printer driver. 1) Use computer with commercially available BIOS. 2) Make a bootable floppy disk, remove unnecessary device drivers from A:config.sys, and boot system from floppy.
Can't Find the Interface Board
Clock or Data Stuck Low
Board is connected to correct port, but SMBus is not functioning
Address Not Acknowledged
SMBus is OK, but no response at expected SMBus address
Conflict with local printer driver Erratic Operation Operating system conflict
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2-Wire Interface Board Evaluates: MAXSMBUS
Table 1. DB25 Connector Signals
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18-25 NAME SPARE OUTPUT A SMBCLK_ OUT SMBDATA_ OUT SMBUS_OUT LOOPBACK MASK_ ALERT1 MASK_ ALERT2 MASK_ START CAPTURE_ENABLE INT SMBDATA_ IN SMBCLK_ IN LOOPBACK SPARE OUTPUT B HOLDING_ CLOCK UNUSED UNUSED GND Spare output When high, drives SMBCLK signal low When high, drives SMBDATA signal low When high, drives SMBSUS signal low Loopback connection for port verification When high, allows ALERT1 to trigger INT low When high, allows ALERT2 to trigger INT low When high, allows a start condition to trigger INT low 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. Active low interrupt input When high, indicates that SMBDATA is low When high, indicates that SMBCLK is low Loopback connection for port verification Spare output When low, indicates that interface board is holding SMBCLK low Not used Not used Signal ground return FUNCTION
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2-Wire Interface Board Evaluates: MAXSMBUS
Table 2. Right-Angle Header P1 Signals
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 NAME DUT +5V GND DUT SDA GND GND GND DUT SCL GND DUT SMBSUS GND DUT SMBALERT GND DUT ALERT2 GND SPARE OUTPUT A GND SPARE OUTPUT B GND GND RAW POWER FUNCTION +5V at 1mA power supply to Maxim companion board Signal ground return SMBDATA interface signal Signal ground return Signal ground return Signal ground return SMBCLK interface signal Signal ground return SMBSUS interface signal Signal ground return Primary ALERT interface signal Signal ground return Secondary ALERT interface signal Signal ground return Spare output from pin 1 of the DB25 connector Signal ground return Spare output from pin 14 of the DB25 connector Signal ground return Signal ground return Unregulated, unprotected power-supply input to MAXSMBUS interface board
Note: Odd-numbered pins are on the outer row. Even-numbered pins are on the inner row. All right-angle header signals pass through the MAX367 circuit protector, except 20.
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U2E U3A U1C ALERT1 74HCT04 J1-6 MASK_ALERT1 74HCT05 +5V U2F U3B U1D ALERT2 74HCT04 5 74HCT08 74HCT05 U3C U1E 8 11 OC 74HCT05 10 Q 1 R 74HCT74 U2A 1 74HCT04 SMBSUS 2 SMBSUS J1-8 MASK_START 10 74HCT08 5 START DETECTED START_DETECTED 9 69 OC 8 INT J1-10 J1-7 MASK_ALERT2 13 12 ALERT2 4 U4A S Q CLK D 6 ALERT2 R11 47k R3 47k 2 74HCT08 +5V 35 OC 6 R10 47k ALERT1 11 10 ALERT1 1
4 3 2
Figure 1a. MAXSMBUS Schematic
INTERFACE TEST POINTS HOLDING_CLOCK J2-1 SMBCLK_OUT J2-2 SMBDATA_OUT J2-3 SMBSUS_OUT J2-4 J2-5 J2-6 J2-7 +5V SMBCLK_IN U2B 3 74HCT04 R 74HCT74 U1A 1 OC +5V 74HCT05 U2C 6 74HCT04 U1B 3 OC 74HCT05 U2D 8 74HCT04 9 SMBDATA SMBDATA 4 SMBDATA +5V R2 47k 5 SMBCLK J1-18 J1-19 J1-20 J1-21 J1-22 R1 47k J1-5 LOOPBACK J1-13 2 SMBCLK C1 0.1F C2 0.1F C3 0.1F 4 CAPTURE_ENABLE 13 74HCT08 DRIVE_SCL_LOW 13 Q 11 13 9 GRAB_SCL 12 CAPTURE_ENABLE 12 D U3D OC 74HCT05 U1F 12 SMBCLK DECOUPLING CAPACITOR +5V 11 CLK Q 8 HOLDING_CLOCK J1-15 10 S U4B J2-8 J2-9 J2-10 J2-11 J2-12 MASK_ALERT1 MASK_ALERT2 MASK_START CAPTURE_ENABLE INT SMBDATA_IN SMBCLK_IN GND J1-23 J1-24 J1-25
SMBDATA_IN
SMBCLK
CAPTURE_ENABLE
SMBSUS_OUT J1-4
R7 4.7k
GND
J1-9
CAPTURE_ENABLE
R4 4.7k
SMBCLK_OUT
J1-2
R5 4.7k
J1-12
SMBCLK_IN
SMBDATA_OUT
J1-3
R6 4.7k
Evaluates: MAXSMBUS
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J1-11
SMBDATA_IN
2-Wire Interface Board
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Evaluates: MAXSMBUS
POS5 R8 47k +10V SBCLK1 D1 6.8V GND SBSUS1 ALERT1 ALERT2 17 16 15 GND P1 2 x 10 RIGHT-ANGLE HEADER 5 13 DUT +5V P1-1 P1-3 P1-5 DUT SCL DUT SMBSUS 9 DUT SMBALERT DUT ALERT2 SPARE OUTPUT J1-1 R9 47k SPARE OUTPUT J1-14 D2 4.7V -10V P1-7 P1-9 P1-11 P1-13 P1-15 P1-17 P1-19 +9V DUT SDA 12 11 10 6 J1-1 J1-14 8 7 P1-2 P1-4 P1-6 P1-8 P1-10 P1-12 P1-14 P1-16 P1-18 P1-20 14 GND0 18 U7 MAX367 +5V SMBDATA SMBCLK SMBSUS ALERT1 ALERT2 4 3 2 1 SBDAT1
2-Wire Interface Board
Figure 1b. MAXSMBUS Schematic (continued)
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POS9 +5V D3 U5 LM78L05ACM +5V 1 C8 2.2F 35V +9V C6 3.3F 20V U6 1 C5 3.3F 20V C1+ 2 C2+ 3 C24 V8 C1V+ 7 IN 6 5 GND MAX865 +10V C4 3.3F 25V +5V C9 3.3F 20V OUT 2 GND 3 GND 4 N.C. 8 +9V IN GND 7 GND 6 5 N.C. GND1 C7 3.3F 20V -10V GND
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2-Wire Interface Board Evaluates: MAXSMBUS
1.0"
1.0"
Figure 2. MAXSMBUS Component Placement Guide-- Component Side
Figure 3. MAXSMBUS PC Board Layout--Component Side
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2-Wire Interface Board Evaluates: MAXSMBUS
1.0"
Figure 4. MAXSMBUS PC Board Layout--Solder Side
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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