Originally Posted by
H.O
Hi Paul,
Lets start with that third drive that doesn't see the encoder....Is that the "only" problem with it? I mean, it does power up and you can "talk" to it with the terminal program, it just doesn't read the encoder? If that's the case then....
First check that you do have 5V in the DSUB-connector for the encoder, measure between pin1 and pin15 for example. Then check the SN75115 IC - is it mounted in the correct orientation? The little notch, indicating pin1, should be facing away from the encoder-connector. Also check for solderbridges at or around the SN75115 as well as the DSUB connector for the encoder.
(The following is only valid if you have a differential encoder, skip this part if using a single ended encoder)
If that checks out OK, take your volt-meter and attach the negative lead to GND, the negative of the incoming 15V for example but any GND on the board is fine. Then, with the positive lead, measure the voltage at pin 6 on the SN75115 while very very slowly rotating the motor. The voltage should toggle between 0-0.5V and 3-5V roughly. Whenever you have a "high" on pin 6 you should have a "low" on pin 5. If that checks out OK do the same procedure with pin 10 and 11 on the SN75115, when one is "high" the other must be "low".
If the above doesn't check out OK try removing the SN75115 from its socket and redo the measurements without the IC mounted. If it's OK now you probably have a bad SN75115.
(Continue here if all of the above is OK and/or if you have a single ended encoder and don't use the SN75115)
With the negative lead still connected to GND measure the voltage at the J1- and J2-pads on the board (two square solder pads right next the SN75115) while again, very very, slowly rotating the motor. The voltage should toggle between 0-0.5V and 4.5-5V. If it doesn't, check R10 and R11 for correct value and solder joints etc.
If still no go remove U3 (if it's in a socket) and redo the measurements. If you still don't get correct readings on the J1 and J2-pads the SN75115 is again a suspect. If you DO get the correct voltages now (with U3 removed) try with a new 7414.
HTH
/Henrik.