COM20019IP datasheet(43/81 Pages) SMSC

43 / 81 page

DISCONTINUED DATASHEET

The "Disable Transmitter" command may be used

to cancel any pending transmit command when

the COM20019 next receives the token. Normally,

in an active network, this command will set the TA

status bit to a logic "1" when the token is received.

If the "Disable Transmitter" command does not

cause the TA bit to be set in the time it takes the

token to make a round trip through the network,

one of three situations exists. Either the node is

disconnected from the network, or there are no

other nodes on the network, or the external

receive circuitry has failed. These situations can

be determined by either using the improved

diagnostic features of the COM20019 or using

another software timeout which is greater than the

worst case time for a round trip token pass, which

occurs when all nodes transmit a maximum length

message.

Receive Sequence

A receive sequence begins with the RI status bit

becoming a logic "1", which indicates that a

reception

has

concluded.

The

microcontroller

will

interrupted

the

corresponding bit in the Interrupt Mask Register is

set to logic "1". Otherwise, the microcontroller

must periodically check the Status Register. Once

the microcontroller is alerted to the fact that the

previous reception has concluded, it may issue

the "Enable Receive to Page fnn" command,

which resets the RI bit to logic "0" and selects a

new page in

the RAM buffer. Again, the appropriate buffer size

specified

the

"Define

Configuration"

command. Typically, the page which just received

the data packet will be read by the microcontroller

at this point. Once the "Enable Receive to Page

fnn" command is issued, the microcontroller

attends to other duties.

There is no way of

knowing how long the new reception will take,

since another node may transmit a packet at any

time. When another

node

does

transmit a

packet

this

node,

and

the

"Define

Configuration"

command

has

enabled

the

reception

long

packets,

the

COM20019

interprets the packet as either a long or short

packet, depending on whether the content of the

buffer location 2 is zero or non-zero. The format

of the buffer is shown in Figure 9. Address 0

contains the Source Identifier (SID), Address 1

contains the Destination Identifier (DID), and

Address 2 contains, for short packets, the value

256-N, where N represents the message length,

or for long packets, the value 0, indicating that it is

indeed a long packet. In the latter case, Address

3 contains the value 512-N, where N represents

the message length. Note that on reception, the

COM20019 deposits packets into the RAM buffer

in the same format that the transmitting node

arranges them, which allows for a message to be

received

and

then

retransmitted

without

rearranging any bytes in the RAM buffer other

than the SID and DID. Once the packet is

received and stored correctly in the selected

buffer, the COM20019 sets the RI bit to logic "1"

to signal the microcontroller that the reception is

complete.

TRI

POR

TEST

RECON

TMA

TTA

TMA

TTA

TRI

MSB

LSB

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