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1)
Dial Request Bit (bit 0)
In order to trigger a dial on demand action, the PLC program needs only to set
the Dial Request (RQ) bit in the first register of the command block. When
the RQ bit is set the telephone and action data is retrieved by the modem in the
same poll, so remember to set these values before the bit!
2)
Stopped Polling bit (bit 1)
The Stopped Polling bit (STP) is set by the modem while it is performing a
dial out operation. It may be used in conjunction with the poll flag below to
indicate that polling has been temporarily suspended.
3)
Polling Flag bit (bit 2)
The poll bit is toggled by the modem after every successful poll has been
made. This feature may be used by the PLC (particularly one at a remote site)
to determine if the modem has stopped polling it for some reason.
CAUTIONARY NOTE:
The polling bit is set and reset by a word write command. The remaining bits
in the register are overwritten with the data they contained at the time of the
poll read request. Therefore, take care when setting the RQ bit; perhaps by
triggering on the rising/falling edge of the poll bit.
4)
Transmission Complete bit (bit 3)
The Transmission Complete bit (TC) is set by the modem when it has; made a
successful dial out, connected, and has performed all the transactions indicated
in the message registers. Used by the PLC as an indicator that the modem has
dealt with the request, and may be used to trigger a new set of transactions.
5)
Error bits (bits 15-7)
The first address of the command block has a number of bits allocated for
error reporting by the modem. These bits are set by the modem in the event of
an error and presented to the PLC on an information only basis.
Bit 15 – General error – set when any error occurs
Bit 13 – Syntax error – set when either the local or remote PLC responds with
three consecutive NAK messages.
Bit 12 – No response – set when the remote PLC has not responded in three
seconds to a WAT request, in three consecutive attempts.
Bits 11 to 8 – PLC communication error – accompanies the syntax error flag,
the least two significant characters returned by a NAK response are copied
here. (AnS PLC only).
Bits 4 to 7 – dial errors – these flags are set when a dial attempt has failed for
one of the following reasons: Line busy, no carrier, no dial tone, or no answer.
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