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19-1306 Rev 0 3 / 98

Two-Wire Interface Board

19-1306 Rev 0 3 / 98
Two-Wire Interface Board
MAXSMBus is an interface between an IBM-compatible PC and System Management Bus (SMBus)TMcompatible 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 two-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.
o SMBus-Compatible Two-Wire Interface o SMBus Suspend Output o Two SMBus Alert Inputs o Overvoltage Fault Protection o PC Parallel Port Interface
Evaluates: MAXSMBus
PART MAXSMBus BOARD TYPE Companion Board for SMBus EV Kits
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 +9V DC supply (+7V minimum, +20V 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.
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.
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Two-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 74HC05 open-drain inverter (U1) is used to pull down the SMBus interface signals. The 74HC08 (U3), 74HC74 (U4), and 74HC04 (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
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.
SYMPTOMS 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 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.
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
Two-Wire Interface Board Evaluates: MAXSMBus
Table 1. DB25 Connector Signals
Two-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 DUTSMBSUS GND DUTSMBALERT GND DUTALERT2 GND SPARE OUTPUT A GND SPARE OUTPUT B GND GND RAW POWER FUNCTION +5V at 20mA 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.
Figure 1. MAXSMBus Schematic
SMBCLK
R5 4.7k
J1-12
R6 4.7k
Evaluates: MAXSMBus
J1-11
Two-Wire Interface Board
Evaluates: MAXSMBus
POS5 R8 1k +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 1k SPARE OUTPUT J1-14 D2 4.3V -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
Two-Wire Interface Board
Figure 1. MAXSMBus Schematic (continued)
POS9 +5V D3 U5 LM78L05ACM +5V 1 C8 3.3µF 25V +9V C6 3.3µF 25V U6 1 C5 3.3µF 25V C1+ 2 C2+ 3 C24 V8 C1V+ 7 IN 6 5 GND MAX865 +10V C4 3.3µF 25V +5V C9 3.3µF 25V OUT 2 GND 3 GND 4 N.C. 8 +9V IN GND 7 GND 6 5 N.C. GND1 C7 3.3µF 25V -10V GND
Two-Wire Interface Board Evaluates: MAXSMBus
Figure 2. MAXSMBus Component Placement Guide
Figure 3. MAXSMBus PC Board Layout-Component Side
Two-Wire Interface Board Evaluates: MAXSMBus
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.