___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, +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 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.
____________________________Features
♦SMBus-Compatible 2-Wire Interface
♦SMBus Suspend Output
♦Two SMBus Alert Inputs
♦Overvoltage Fault Protection
♦PC Parallel Port Interface
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
________________________________________________________________ Maxim Integrated Products 1
19-1306; Rev 0; 3/98
QTY DESCRIPTION
C1, C2, C3 30.1µF ceramic capacitors
C4–C9 63.3µF, 25V tantalum capacitors
D1 11N5235B zener diode, 6.8V
D2 11N5229B zener diode, 4.3V
D3 11N4148 small-signal diode
J1 1DB25 right-angle plug
DESIGNATION
_______________Ordering Information
PART
MAXSMBus
BOARD TYPE
Companion Board for SMBus EV Kits
Maxim SMBus Interface Board
____________________________Component List
J2 1Not installed
P1 12x10 right-angle male header
R1, R2, R3,
R10, R11 547kΩ±5% resistors
R4–R7 4 4.7kΩ±5% resistors
R8, R9 21kΩ±5% resistors
U1 174HC05 hex open-collector inverter
U2 174HC04 hex inverter
U3 174HC08 quad AND gate
U4 174HC74 dual D flip-flop
U5 1+5V, 100mA regulator
LM78L05ACM
U6 1MAX865EUA (8-pin µMAX)
U7 1MAX367CWN (18-pin SO)
NONE 1PC board
SMBus is a trademark of Intel Corp.
For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
Evaluates: MAXSMBus
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 0V and
+5V signals to pass with a nominal resistance of 100Ω.
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 condi-
tion, 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 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
MAXSMBus
2-Wire Interface Board
_______________________________________________________________________________________ 3
Table 1. DB25 Connector Signals
PIN NAME FUNCTION
1SPARE OUTPUT A Spare output
2SMBCLK_
-
OUT When high, drives SMBCLK signal low
3SMBDATA_
-
OUT When high, drives SMBDATA signal low
4SMBUS_OUT When high, drives SMBSUS signal low
5 LOOPBACK Loopback connection for port verification
6MASK_
-
ALERT1 When high, allows ALERT1 to trigger INT low
7MASK_
-
ALERT2 When high, allows ALERT2 to trigger INT low
8MASK_
-
START When high, allows a start condition to trigger INT low
9CAPTURE_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.
10 INT Active low interrupt input
11 SMBDATA_
-
IN When high, indicates that SMBDATA is low
12 SMBCLK_
-
IN When high, indicates that SMBCLK is low
13 LOOPBACK Loopback connection for port verification
14 SPARE OUTPUT B Spare output
15 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
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
4 _______________________________________________________________________________________
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.
Table 2. Right-Angle Header P1 Signals
PIN NAME FUNCTION
1DUT +5V +5V at 20mA power supply to Maxim companion board
2GND Signal ground return
3DUT SDA SMBDATA interface signal
4GND Signal ground return
5GND Signal ground return
6GND Signal ground return
7DUT SCL SMBCLK interface signal
8GND Signal ground return
9DUTSMBSUS SMBSUS interface signal
10 GND Signal ground return
11 DUTSMBALERT Primary ALERT interface signal
12 GND Signal ground return
13 DUTALERT2 Secondary ALERT interface signal
14 GND Signal ground return
15 SPARE OUTPUT A Spare output from pin 1 of the DB25 connector
16 GND Signal ground return
17 SPARE OUTPUT B Spare output from pin 14 of the DB25 connector
18 GND Signal ground return
19 GND Signal ground return
20 RAW POWER Unregulated, unprotected power-supply input to MAXSMBus interface board
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
_______________________________________________________________________________________ 5
OC
U1C
74HC04
74HC08 6
53
1
10
U2E U3A
11
J1–6 2
ALERT1ALERT1
R10
47k
R11
47k
+5V
ALERT1
MASK_ALERT1
OC
U1D
74HC05
+5V
74HC05
74HC05
INT
J1–10
74HC04
74HC04
74HC08 8
10
R3
47k
96
4
12
U2F
U2A
U3B
13
J1–7 5
ALERT2ALERT2
ALERT2
Q
S
4
3
6
START DETECTED
5
2
1
12 SMBSUS SMBSUS
CLK
74HC74
U4A
D
R
CAPTURE_ENABLE
SMBCLK
SMBDATA_IN
Q
MASK_ALERT2
J1–4
GND
R7
4.7k
SMBSUS_DUT
U1E
74HC08 118
9
U3C
J1–8 10
START_DETECTED
MASK_START
CAPTURE_ENABLE
SMBCLK_IN
DRIVE_SCL_LOW
+5V
74HC05
SMBCLK
+5V
DECOUPLING CAPACITOR
C1
0.1µF
C2
0.1µF
C3
0.1µF
J1–13
J1–5 LOOPBACK
Q
S
10
11
8
9
12
13
34
SMBCLK
2
5
U2B
U1A
U2C
74HC05
1
6
CLK
74HC74
U4B
D
R
Q
U1F
74HC08 13 12
11
12
GRAB_SCL
CAPTURE_ENABLE
U3D
HOLDING_CLOCK
J1–15
13
J1–18
J1–19
OC
OC
J1–9
R4
4.7k
R1
47k
+5V
SMBCLK
CAPTURE_ENABLE
J1–2 R5
4.7k
SMBCLK_OUT
74HC04
J1–12
OC
74HC04
SMBCLK_IN
SMBDATA
SMBDATA
4
9
U2D
U1B
74HC05
3
8
R2
47k
+5V
SMBDATA
J1–3 R6
4.7k
SMBDATA_OUT
J1–11
OC
74HC04
SMBDATA_IN
J2–1
INTERFACE TEST POINTS
HOLDING_CLOCK
J2–2 SMBCLK_OUT
J2–3 SMBDATA_OUT
J2–4 SMBSUS_OUT
J2–5 MASK_ALERT1
J2–6 MASK_ALERT2
J2–7 MASK_START
J2–8 CAPTURE_ENABLE
J2–9 INT
J2–10 SMBDATA_IN
J2–11 SMBCLK_IN
J2–12 GND
J1–20
J1–21
J1–22
J1–23
J1–24
J1–25
Figure 1. MAXSMBus Schematic
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
6 _______________________________________________________________________________________
P1–17
P1–15
P1–1
DUT +5V
DUT SDA
DUT SCL
P1–3
P1–5
P1–7
P1–2
P1–4
P1–6
P1–8
P1–10
P1–12
P1–14
P1–16
P1–18
P1–20
+9V
+5V
+10V
+5V
+9V
+5V
POS9
GND1
+9V
D3
U5
LM78L05ACM
C8
3.3µF
25V
C9
3.3µF
25V
C4
3.3µF
25V
1
2
3
4
8
7
6
5
OUT
GND
GND
N.C.
IN
GND
GND
N.C.
D1
6.8V GND
R8
1k
1
2
3
4
5
6
7
8
9
SMBDATA
SMBCLK
SMBSUS
ALERT1
ALERT2
J1–1
J1–14
R9
1k
D2
4.3V
-10V
17
POS5
SBDAT1
18
16
15
14
13
12
11
10
MAX367
SBCLK1
SBSUS1
ALERT1
ALERT2
GND0
GND
P1 2 x 10 RIGHT-ANGLE HEADER
DUT SMBSUS
DUT SMBALERT P1–11
P1–9
DUT ALERT2
SPARE OUTPUT J1–1
SPARE OUTPUT J1–14
P1–13
P1–19
C5
3.3µF
25V
C6
3.3µF
25V
C7
3.3µF
25V
+10V
+5V
-10V
GND
MAX865
U6
1
2
3
4
8
7
6
5
C1+
C2+
C2-
V-
C1-
V+
IN
GND
U7
Figure 1. MAXSMBus Schematic (continued)
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
_______________________________________________________________________________________ 7
Figure 2. MAXSMBus Component Placement Guide Figure 3. MAXSMBus PC Board Layout—Component Side
1.0"
1.0"
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
© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Evaluates: MAXSMBus
MAXSMBus
2-Wire Interface Board
Figure 4. MAXSMBus PC Board Layout—Solder Side
1.0"
