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10.1.4. Host Interface Troubleshooting
The methods described in this section are useful as a starting point for debugging a prototype system or as a
continuation of the troubleshooting process described previously. The procedures presented in this section require
a known good ISOModem evaluation board and data sheet. This section describes how to substitute the evaluation
board for the entire modem circuitry in the prototype system. Substituting a known operational modem can help
isolate problems quickly. The first step is to substitute the evaluation board for the complete modem solution in the
prototype system. This immediately demonstrates whether any modem functionality problems are in the prototype
modem circuitry or in the host processor, interface, or software.
Verify ISOModem Evaluation Board Functionality
Connect the evaluation board to a PC and a phone line or telephone line simulator. Using a program, such as
HyperTerminal, make a data connection between the evaluation board and a remote modem. Remove power
and the RS232 cable from the evaluation board and proceed to the next step.
Connect Evaluation Board to Prototype System
Completely disconnect the embedded modem from the host interface in the prototype system. Connect the
ISOModem evaluation board to the host interface as described in the ISOModem evaluation board data sheet
section titled Direct Access Interface. This connection is illustrated in Figure 42. Be sure to connect the
evaluation board ground to the prototype system ground. Power up and manually reset the evaluation board,
then power up the prototype system and send AT<CR>. If an OK response is received, make a connection to
the remote modem as in the previous step. If no OK response is received, debug host interface and/or software.
If a connection is successful, go to the next step to isolate the problem in the prototype modem.
An alternative approach is to connect the prototype modem to the ISOModem EVB motherboard in place of the
daughter card and use a PC and HyperTerminal to test the prototype modem. See Figure 43 for details.
10.1.5. Isolation Capacitor Troubleshooting
Connect the evaluation board isolation capacitor to Prototype Modem Si3018/10. Remove C1 on the evaluation
board and on the prototype system. Solder one end of the evaluation board, C1, to the ISOmodem-side pad
leaving the other end of C1 unconnected. Next, solder a short jumper wire from the unconnected side of C1 on the
evaluation board to the Si3018/10-side C1 pad on the prototype system. This connection is illustrated in Figure 44.
Connect the phone line to the prototype system RJ-11 jack.
Power up and manually reset the evaluation board, then power up the prototype system. Attempt to make a
connection using the host processor and software, the evaluation board ISOmodem, and the prototype system
Si3018/10 and associated external components. If this connection is successful, the problem lies with the PCB
layout, the external components associated with the ISOmodem or the ISOmodem device itself.
If the connection attempt is not successful, the problem lies with the Si3018/10 and/or associated components.
Proceed to “Si3018/10 Troubleshooting”.
This diagnosis can be validated by connecting the Host ISOcap capacitors to the Si3018/10 on the evaluation
board as shown in Figure 45.
10.1.6. Si3018/10 Troubleshooting
Start by measuring the on-hook and off-hook voltages at the Si3018/10 pins with respect to IGND (pin 15).
Compare these voltages to those in Figure 46. This may indicate an area of circuitry to investigate further using the
Component Troubleshooting techniques in the following section. The voltages measured should be reasonably
close to those in the figure.
If any of the on-hook or off-hook Si3018/10 pin voltages are significantly different than those in Figure 46 and
nothing seems wrong with the external circuitry after using the Component Troubleshooting techniques, replace
the Si3018/10.
10.1.7. Component Troubleshooting
A digital multimeter is a valuable tool for verifying resistances across components, diode directions, transistor
polarities and node voltages. During this phase of troubleshooting, it is very useful to have a known, good
ISOModem evaluation board to compare against measurements taken from the prototype system. The resistance
values and voltages listed in Tables 163, 164, and 165 will generally be enough to troubleshoot all but the most
unusual problems.
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