User's Manual

PMAC2 User Manual
Setting Up PMAC2 for Pulse-and-Direction Control 57
SETTING UP PMAC2 FOR PULSE-AND-DIRECTION CONTROL
Pulse-And-Direction Format Input
The PMAC2 is capable of commanding stepper-motor drives that require pulse-and-direction format
input, or stepper-replacement servo drives that require this format. PMAC2 can command these drives
either in open-loop fashion, in which case it internally routes the pulse train into its own encoder counters
to create a pseudo-closed loop, or in closed-loop fashion, in which case am external feedback device is
wired to the PMAC2 to create a true feedback loop.
PMAC2 creates its pulse-and-direction output signal with an on-board fully digital pulse frequency
modulation (PFM) circuit. This circuit repeatedly adds the latest command frequency value into an
accumulator at a programmable rate of up to 40 MHz. When the accumulator overflows, an output pulse
is generated with a positive direction signal; when the accumulator underflows, and output pulse is
generated with a negative direction signal. This creates a pulse train whose frequency is directly
proportional to the command value, with virtually no harmonic distortion, and none of the offset problems
that affect analog pulse generation schemes.
Hardware Connection
PMAC2 has interface connections for 4 or 8 axes, depending on hardware configuration, numbered 1 to 4
or 1 to 8. Each axis interface has three output command registers, labeled A, B, and C (e.g. 3A, 3B, and
3C), which each drive their own output lines. The PFM circuitry uses the output command register C for
each axis interface, and therefore the output signal lines for register C.
There are two differential signal pairs (four lines) for each output command register. For register C these
signal pairs can either hold a pulse-width modulated (PWM) signal or a PFM signal. The selection
between the two types of signals on these pins is made with PMAC software parameters; see below.
The signal lines for the PFM signals for each axis interface are labeled:
1. PWMCTOPn+/DIRn+
2. PWMCTOPn-/DIRn-
3. PWMCBOTn+/PULSEn+
4. PWMCBOTn-/PULSEn-
where n is the number of the axis interface (1-8). That is, the pins can be used for the third (C) PWM
outputs for axis n, top and bottom, for a PWM-driven amplifier, or the PFM outputs for a pulse-and-
direction amplifier. Some pinout listings may only show the 'PWM' names for the pins, but they can
always be used for PFM signals as well, because the selection is made in software.
The outputs are 5V differential line driver pairs, with RS-422-style drivers that are capable of transmitting
the signal significant distances particularly with shielded, twisted-pair cables. These outputs can be used
directly to drive the inputs of most stepper drives, whether the inputs are electrically or optically coupled
to the rest of the drive circuitry.
On the PMAC 100-pin JMACH connectors, these signals are brought out on pins 43-46 for odd-
numbered axes, and pins 93-96 for even-numbered axes.
On the Acc-8S stepper interface board, which is recommended as the most cost-effective breakout board
for stepper systems, these signals are brought out on pins 1-4 of TB6 for odd-numbered axes, and on pins
1-4 of TB7 for even-numbered axes.
On the Acc-8E analog interface board, they are brought out on points 9-12 of TB2 for the odd-numbered
axes, and points 9-12 of TB3 for the even-numbered axes. These are available on the -101 and newer
versions of the board, but not on the prototype -100 version
On the Acc-8F digital interface board, these signals are brought out on pins 11-14 of the DB-37 connector
for each axis.