User's Manual

PMAC User Manual

Setting Up a Coordinate System 141

What Is Coordinate System Time-Base?

Each coordinate system has its own time base that helps control the speed of interpolated moves in that

coordinate system. The PMAC interpolation routines increment an elapsed-time register every servo

cycle. While the true time for the servo cycle is set in hardware for the card (by jumpers E98, E29-E33,

and E3-E6) and does not change, the value of time added to the elapsed-time register each servo cycle is

just a number in a memory register. It does not have to match the true physical time for the cycle.

The units for the time base register are such that 2

(8,388,608) equals 1 millisecond. The default value

for the time-base register is equal to the value of I10. The factory default value for I10 of 3,713,707

represents the default physical servo cycle time of 442 microseconds.

If the value of the time base register is changed from I10, interpolated moves will move at a different

speed from that programmed. Many people call this capability feedrate override. The physical time does

not change, so servo loop dynamics remain unchanged.

Each coordinate system has a variable Ix93 that contains the address of the register that the coordinate

system uses for its time base. With the default value of Ix93, the coordinate system gets its time base

information from a register that is set by % commands from the host computer. A %100 command puts a

value equal to I10 in this register; a %50 command puts a value equal to I10/2 in this register.

Regardless of the source of the time base information, a % query command causes PMAC to report back

the value of the present time base expressed as a percentage of I10.

Time base information can come from other sources. The most common alternative to command-sourced

time base is external frequency-sourced time base, where the time base value is proportional to the

frequency of a master encoder. This provides a powerful position-synchronized slaving mechanism that

is commonly called electronic cam.