Motorola MC68824 User Manual page 65

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3.6.4 TRANSMITIER TEST Command
The transmitter test is used by the host processor to check the path from the TX queue in memory
to the transmitter and back to the receiver. During this test, the TBC acts as if it is in_ring and
has just received a token. The transmitter test may be run with the option of waiting for non-
silence (bit 1 of the command) on the bus before transmission begins, thus deliberately creating
a collision. Note that if this test is run in internal loopback mode, it must also be run with the
silence/noise bit set to one, that is the TBC will wait for reported silence to start transmitting. If
desired, a cyclic transmit queue can be used, creating an almost unlimited number of frames for
transmission (see SECTION 4 BUFFER STRUCTURES). The number of frames is only limited by
the high_priority_token_hold time when in externalloopback mode, while there are no limitations
in internalloopback mode. During the transmitter test, the receiver is in a special mode in which
it checks incoming frames for CRC (FCS) and underrun errors but does not write the contents of
the frame to the FIFO. To perform this test, the host must prepare a class 6 transmission queue
and set the high_priority_token_hold_time to 'FFFF' for maximum transmitter test time. When
either the last transmission frame has been confirmed, or an interrupt on empty transmission
queue has occurred (bit three of interrupt status word 0, TXQE), the test should be ended by
issuing the OFFLINE command. Command confirmation and status are returned to the command
return area in the initialization table. The OFFLINE command must be given before the token hold
timer expires while running in external loopback mode. To check the tests results, the host can
check the FD confirmation word for TX queue access class 6. If detected, CRC/underrun errors
are indicated by the TBC setting bit 1 of the command result area after confirmation was given
for the OFFLINE command.
The following routine may be used to perform the transmitter test:
Prepare the Initialization Table
Issue RESET Command
Wait until Semaphore Register is 'FF'
Initialize the Interrupt Vector Register
Load the Function Code of the Init Table Pointer into ORO
Issue LOAD INITIALIZATION TABLE FC Command
Wait until Semaphore Register is 'FF'
Load the Init Table Pointer into DR
Issue INITIALIZE Command
Issue SET MODE 3 Command to Set SWAP, HLEN, PS3 and TCDS Bits
Prepare Access Class 6 TX Queue
Issue SET ONE WORD Command to Set High_Priority_ Token_Hold_Time if Different from
Value Set in Initialization Table
Issue SET FC BD, FD, and RXITX Data Buffers if needed
Issue SET INTERNAL/EXTERNAL LOOPBACK MODE
Initialize Modem if in External Loopback Mode
Issue START Command for Access Class 6
Clear Done Bit in CPA Status Word
Issue TRANSMITTER TEST (Code is B4 or B6)
Wait for Command Confirmation which Indicates Acceptance of the Test
Wait until Last Known TX Frame has been Confirmed or an Interrupt has been Generated by
the TBC for TXQE
MOTOROLA
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MC68824 USER'S MANUAL