LM628, LM629

SNVS781C – JUNE 1999 – REVISED MARCH 2013

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RDDV COMMAND: READ DESIRED VELOCITY

Command Code: 07 Hex

Bytes Read: Four

Data Range: C0000001 to 3FFFFFFF

Executable During Motion: Yes

This command reads the integer and fractional portions of the instantaneous desired (current temporal ) velocity,

as used to generate the desired position profile. The bytes are read in most-to-least significant order. The value

read is properly scaled for numerical comparison with the user-supplied (commanded) velocity; however,

because the two least-significant bytes represent fractional velocity, only the two most-significant bytes are

appropriate for comparison with the data obtained via command RDRV (see below). Also note that, although the

velocity input data is constrained to positive numbers (see command LTRJ), the data returned by command

RDDV represents a signed quantity where negative numbers represent operation in the reverse direction.

RDRV COMMAND: READ REAL VELOCITY

Command Code: 0B Hex

Bytes Read: Two

Data Range: C000 to 3FFF Hex, See Text

Executable During Motion: Yes

This command reads the integer portion of the instantaneous actual velocity of the motor. The internally

maintained fractional portion of velocity is not reported because the reported data is derived by reading the

incremental encoder, which produces only integer data. For comparison with the result obtained by executing

command RDDV (or the user-supplied input value), the value returned by command RDRV must be multiplied by

signed quantity, with negative values representing reverse-direction motion.

RDSUM COMMAND: READ INTEGRATION-TERM SUMMATION VALUE

Command Code: 0D Hex

Bytes Read: Two

Data Range: 00000 Hex to ± the Current Value of the Integration Limit

Executable During Motion: Yes

This command reads the value to which the integration term has accumulated. The ability to read this value may

be helpful in initially or adaptively tuning the system.

Typical Applications

PROGRAMMING LM628 HOST HANDSHAKING (INTERRUPTS)

A few words regarding the LM628 host handshaking will be helpful to the system programmer. As indicated in

various portions of the above text, the LM628 handshakes with the host computer in two ways: via the host

interrupt output (Pin 17), or via polling the status byte for “interrupt” conditions. When the hardwired interrupt is

used, the status byte is also read and parsed to determine which of six possible conditions caused the interrupt.

When using the hardwired interrupt it is very important that the host interrupt service routine does not interfere

with a command sequence which might have been in progress when the interrupt occurred. If the host interrupt

service routine were to issue a command to the LM628 while it is in the middle of an ongoing command

sequence, the ongoing command will be aborted (which could be detrimental to the application).

Two approaches exist for avoiding this problem. If one is using hardwired interrupts, they should be disabled at

the host prior to issuing any LM628 command sequence, and re-enabled after each command sequence. The

second approach is to avoid hardwired interrupts and poll the LM628 status byte for “interrupt” status. The status

byte always reflects the interrupt-condition status, independent of whether or not the interrupts have been

masked.

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