Motorola MCS 2000 Service Instructions Manual page 132

9-16
Serial Bus Communication
A general serial bus communication problem in the radio is typically indicated by the radio remaining
inactive on power up, and the display showing "FL 01/90".
Make sure that no accessory is connected to P0651 (MIC Connector) and 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 D0677 (most common error), Q0655 and Q0656.
2. Check that U0653 pin 52 is 5 VDC with pulses to ground while transmitting data. Then check
U0655 (disconnect pin 1). If problem disappears and OPTION SW B+ circuit is OK (see 5.),
replace U0655. If the OPTION SW B+ circuit is defect, D0684 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 D0678 (most common error), Q0657 and Q0656.
2. Check that U0653 pin 52 is 5 VDC with pulses to ground while transmitting data. Then check
U0655 (disconnect pin 12). If problem disappears and OPTION SW B+ circuit is OK (see 5.),
replace U0655. If the OPTION SW B+ circuit is defect, D0683 should also be checked.
SB9600 BUSY, J650 pin 14
1. This signal is 0 V with pulses to 5 VDC when data are sent on the bus. If it is constantly 5
VDC or 0 V, check D0675 (most common error) and Q0653. Check that U0653 pin 43 is 5
VDC with pulses to ground while sending data.
2. Then check U0656 (disconnect pins 1 and 12). If problem disappears and OPTION SW B+
circuit is OK (see 5.), replace U0656. If the OPTION SW B+ circuit is defect, D0685 should
also be checked.
SB9600 data in comparator
If the above voltages are correct, check U0652, Q0658 and Q0656. Finally check U0653.
Communications stops only with accessory on MIC Conn
If bus communication stops working 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 pins 17 and 18, check that the DC volt-
age on P0651 pin 1 is approx. 12 VDC. If not check Q0662.
Multiplexer in normal position
Check that the voltage on U0655/U0656 pin 10 is 5 VDC. If this fails, check Q0662 and Q0663.
December 6, 2004
Troubleshooting Troubleshooting Chart 1-8A, Control Head Model I
68P81083C20-D