AC37 High-Speed Communication Adapter Card
4-1
MODES OF OPERATION
The AC37 has two operating modes. Mode 0 is selected by removing the OPT0 (OPTION 0) jumper. Mode 1 is
selected by installing the OPT0 jumper. Option jumper 1 is reserved for future use. The option jumpers are read
only after the AC37 has been powered up or upon reset.
Mode 0:
While in mode 0, the AC37 will operate very much like a standard serial COM port. It also will not receive what
it has transmitted. When data is received, it is put into the receive FIFO and the Receive Data Available bit is set
immediately. This bit remains set until all data has been read from the receive FIFO or the FIFO is reset. When
data is put into the transmit FIFO, it is read and transmitted as room becomes available in the output shift
register.
Mode 1:
Mode 1 is intended to facilitate the command-response transactions necessary to operate Remote I/O bricks.
This is done primarily by holding the Receive Data Available flag reset until the complete response string is put
into the receive FIFO. By doing so, the AC37 minimizes the time required for the host CPU to collect the data,
since only one interrupt is necessary to read the entire response. Both ASCII and binary protocols are supported
and the operation of the AC37 will be slightly different depending on which is being used.
ASCII Protocol:
The AC37 will send and receive data much the same way as it does in mode 0, except that after transmitting a
carriage return (the ASCII protocol end-of-message character), the AC37 will stop transmitting and wait for a
response from the Remote I/O brick or controller. As the response string is received, it is put into the receive
FIFO but the Receive Data Available bit is held clear until a carriage return is received. If a receive error occurs,
or two seconds expire with no data having been received, the Receive Data Available flag is allowed to operate
in it’s normal manner.
Binary Protocol:
The binary protocol uses 9 bit characters to transfer data to and from the Remote I/O bricks and controllers. The
9th bit is used to differentiate data bytes from address bytes. Address bytes are normally only sent at the
beginning of a command string.
In mode 1, when the AC37 reads an address byte from the transmit FIFO it will wait to transmit the address byte
until the second byte (count byte) is put into the transmit FIFO. This has the effect of holding off the Transmit
Driver Active flag (normally the Transmit Shift Register Empty flag), and allowing the software to change the
state of the 9th bit without waiting for the first byte to be completely shifted out. While this is not necessary
when using the AC37, it may be done to keep software compatible with other serial cards.
CHAPTER 4
Updated: 10-28-02
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