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Table 6 - Command Register
DATA COMMAND DESCRIPTION
0000 0000 Clear
Transmit
Interrupt
This command is used only in the Command Chaining operation. Please
refer to the Command Chaining section for definition of this command.
0000 0001 Disable
Transmitter
This command will cancel any pending transmit command (transmission
that has not yet started) and will set the TA (Transmitter Available) status
bit to logic "1" when the COM20020 next receives the token.
0000 0010 Disable
Receiver
This command will cancel any pending receive command. If the
COM20020 is not yet receiving a packet, the RI (Receiver Inhibited) bit will
be set to logic "1" the next time the token is received. If packet reception is
already underway, reception will run to its normal conclusion.
b0fn n100 Enable
Receive to
Page fnn
This command allows the COM20020 to receive data packets into RAM
buffer page fnn and resets the RI status bit to logic "0". The values placed
in the "nn" bits indicate the page that the data will be received into (page 0,
1, 2, or 3). If the value of "f" is a logic "1", an offset of 256 bytes will be
added to that page specified in "nn", allowing a finer resolution of the
buffer. Refer to the Selecting RAM Page Size section for further detail. If
the value of "b" is logic "1", the device will also receive broadcasts
(transmissions to ID zero). The RI status bit is set to logic "1" upon
successful reception of a message.
00fn n011 Enable
Transmit from
Page fnn
This command prepares the COM20020 to begin a transmit sequence
from RAM buffer page fnn the next time it receives the token. The values
of the "nn" bits indicate which page to transmit from (0, 1, 2, or 3). If "f" is
logic "1", an offset of 256 bytes is the start of the page specified in "nn",
allowing a finer resolution of the buffer. Refer to the Selecting RAM Page
Size section for further detail. When this command is loaded, the TA and
TMA bits are reset to logic "0". The TA bit is set to logic "1" upon
completion of the transmit sequence. The TMA bit will have been set by
this time if the device has received an ACK from the destination node. The
ACK is strictly hardware level, sent by the receiving node before its
microcontroller is even aware of message reception. Refer to Figure 1 for
details of the transmit sequence and its relation to the TA and TMA status
bits.
0000 c101 Define
Configuration
This command defines the maximum length of packets that may be
handled by the device. If "c" is a logic "1", the device handles both long
and short packets. If "c" is a logic "0", the device handles only short
packets.
000r p110 Clear Flags This command resets certain status bits of the COM20020. A logic "1" on
"p" resets the POR status bit and the EXCNAK Diagnostic status bit. A
logic "1" on "r" resets the RECON status bit.
0000 1000 Clear
Receive
Interrupt
This command is used only in the Command Chaining operation. Please
refer to the Command Chaining section for definition of this command.