Motorola MCS 2000 Service Instructions Manual page 138

9-22
Serial Bus Communication
A general serial bus communication problem in the radio, when the control head is connected, is
typically indicated by an inactive radio on power up or the display showing "FAIL 01/90".
Make sure no accessory is connected to P0651 (MIC Connector). Check the following lines with an
Oscilloscope while turning the radio on and off.
SB9600 BUS+, J650 pin 15
1. This signal is normally 5 VDC with short pulses to 0V when data are sent on the bus. If it is
constantly 0 V or 5 VDC check D0665 (most common error), Q0663 and Q0662. Check that
U0653 pin 52 is 5 VDC with pulses to ground while sending data.
2. Then check U0659 (disconnect pins 1 and 12). If problem disappears and OPTION SW B+
circuit is OK (see 5.), replace U0659.
3. If the OPTION SW B+ circuit is defect, D0669 should also be checked.
SB9600 BUS-, J650 pin 4
1. This signal is normally 0 V with short pulses to 5 VDC when data are sent on the bus. If it is
constantly 5 VDC or 0 V, check D0664 (most common error), Q0661 and Q0662. Check that
U0653 pin 52 is 5 VDC with pulses to ground while sending data.
2. Then check U0657 (disconnect pins 1 and 12). If problem disappears and OPTION BW B+
circuit is OK (see 5.), replace U0657.
3. If the OPTION SW B+ circuit is defect, D0667 should also be checked.
SB9600 BUSY, J650 pin 14
1. This signal is normally 0 V with pulses to 5 VDC when data are sent on the bus. If it is con-
stantly 5 VDC, check D0659 (most common error) and Q0653. Check that U0653 pin 43 is 5
VDC with pulses to ground while sending data.
2. Then check U0658 (disconnect pins 1 and 12). If problem disappears and OPTION BW B+
circuit is OK (see 5.), replace U0658.
3. If the OPTION SW B+ circuit is defect, D0668 should also be checked.
SB9600 Data in Comparator
1. If the above voltages are correct, check U0656, Q0660 and Q0662.
2. Finally check U0653.
Communication stops only with accessory on MIC Conn
If bus communication stops only when an accessory is connected to the MIC Connector (P0651),
check the OPTION SW B+ circuit (See 5.). The accessory is either defect or incompatible.
OPTION SW B+/Multiplexing of MIC Connector
Normal mode = No or simple microphone connected.
SB9600 mode = Smart accessory with serial bus connected.
SW B+ in Normal mode
1. Make sure no accessory is connected to the MIC Connector (P0651).
2. With power connected to the control head on J0650 pin 17 and 18, check that the DC voltage
on P0651 pin 1 is approx. 12 VDC. If not check Q0664.
December 6, 2004
Troubleshooting Troubleshooting Chart 1-8B, Control Head Models II & III
68P81083C20-D