This is revision A of the Servo4 module.

Servo4 Module (Revision A)

1. Introduction
2. Programming
3. Hardware
- 3.1 Circuit Schematic
- 3.2 Printed Circuit Board
4. Software
5. Issues

1. Introduction

The Servo4 module can control up to 4 servos with complete opto isolation. The reason for the optoisolation is to prevent power surges from the servos from accidentally triggering voltage spikes on the logic power bus.

2. Programming

Command
Name Action Bit Description
8 7 6 5 4 3 2 1 0

High Low Set Send 0 0 0 0 0 0 0 s s Set servo ss to position hhhh hhhh llll llll, which specifies the pulse width in multiples of .20µS.

Send 0 h h h h h h h h
Send 0 l l l l l l l l

Low Set Send 0 0 0 0 0 0 1 s s Set the low order 8-bits of servo ss to llll llll.
Send 0 l l l l l l l l

High Get Send 0 0 0 0 0 1 0 s s Get the high order 8-bits of servo ss and return as hhhh hhhh.
Receive 0 h h h h h h h h

Low Get Send 0 0 0 0 0 1 1 s s Get the low order 8-bits of servo ss and return as llll llll.
Receive 0 l l l l l l l l

Enables Set Send 0 0 0 0 1 e e e e Set the enables for the servos to eeee.

Enables Get Send 0 0 0 1 0 0 0 0 0 Return the current enable bits as eeee.
Receive 0 0 0 0 0 e e e e

Common
Commands Send 0 1 1 1 1 1 1 x x Standard common command. See specifications for details.
Receive 0 d d d d d d d d

Select Send 1 a a a a a a a a Select the module with an address of aaaa aaaa. A time-out indicates that the module is not selected.
Receive 0 0 0 0 0 0 0 0 0

Command Name	Action	Bit	Description
8	7	6	5	4	3	2	1	0
High Low Set	Send	0	0	0	0	0	0	0	s	s	Set servo ss to position hhhh hhhh llll llll, which specifies the pulse width in multiples of .20µS.
Send	0	h	h	h	h	h	h	h	h
Send	0	l	l	l	l	l	l	l	l
Low Set	Send	0	0	0	0	0	0	1	s	s	Set the low order 8-bits of servo ss to llll llll.
Send	0	l	l	l	l	l	l	l	l
High Get	Send	0	0	0	0	0	1	0	s	s	Get the high order 8-bits of servo ss and return as hhhh hhhh.
Receive	0	h	h	h	h	h	h	h	h
Low Get	Send	0	0	0	0	0	1	1	s	s	Get the low order 8-bits of servo ss and return as llll llll.
Receive	0	l	l	l	l	l	l	l	l
Enables Set	Send	0	0	0	0	1	e	e	e	e	Set the enables for the servos to eeee.
Enables Get	Send	0	0	0	1	0	0	0	0	0	Return the current enable bits as eeee.
Receive	0	0	0	0	0	e	e	e	e
Common Commands	Send	0	1	1	1	1	1	1	x	x	Standard common command. See specifications for details.
Receive	0	d	d	d	d	d	d	d	d
Select	Send	1	a	a	a	a	a	a	a	a	Select the module with an address of aaaa aaaa. A time-out indicates that the module is not selected.
Receive	0	0	0	0	0	0	0	0	0

3. Hardware

The hardware consists of a circuit schematic and a printed circuit board.

The parts list kept in a separate file -- servo4.ptl.

3.1 Circuit Schematic

The schematic for the Servo4 module is shown below:

The bus is connected to the 2×5 shrouded conenctor N5. The two CAN bus signals are fed into the PCP2551 CAN bus transceiver U2. The output of U2 is fed to the UART transmit and receive signals on the PIC16F688 microcontroller U1. The four signal wires RC0-RC3 are connected to the light emmitting diodes of U3 through 470 Ohm current limiting resistors R1-4. BPWR (pins 1 and 10) and BGND (pins 2 and 9) from N5 are routed to pins 2 and 1 respectively on connectors N1-4 to provide power for the servos. The servo signal wires are pulled to BGND via the 10K Ohm resistors R1-4. Whenever the one of the light emitting diodes in U3 is activated, the corresponding NPN transistor in U3 pulls the servo signal up to BPWR.

3.2 Printed Circuit Board

The printed circuit board files are listed below:

servo4_back.png: The solder side layer.
servo4_front.png: The component side layer.
servo4_artwork.png: The artwork layer.
servo4.gbl: The RS-274X "Gerber" back (solder side) layer.
servo4.gtl: The RS-274X "Gerber" top (component side) layer.
servo4.gal: The RS-274X "Gerber" artwork layer.
servo4.gml: The RS-274X "Gerber" mask layer.
servo4.drl: The "Excellon" NC drill file.
servo4.tol: The "Excellon" tool rack file.

4. Software

The following files are available:

servo4.ucl: The µCL source code for the Servo4 module.
servo4.asm: The Servo4 module assembly code file.
servo4.lst: The Servo4 module listing file.
servo4 .hex: The µCL Servo4 Intel^® Hex file.

5. Issues

The following fabrication issues came up:

Both the emitters and collects of the opto-isolators are all tied to BPWR. The emmitters need to be connected to BGND instead.
Having R1-4 be 470 Ohms may be too small. It would probalby work just as well up at 1K or 2K Ohms.