8.1.3 Modem Connection
8.1.3.1 Modem required:
External Hayes compatible (or one compatible with the calling modem) with a female DB-25 con
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nector and configured as a DCE (Data Communications Equipment) is required. The DB-25 port on
the modem should conform to the EIA RS-232 standard. The modem should be able to run in full
duplex mode. The modem must support a fixed data link rate on the modem's serial port of 1200,
2400, 4800, or 9600 baud - 300 baud modems are not supported. The modem must hang up the
phone line after a defined time period with no carrier signal. The modem must not change the link
rate when answering. For example, if an intelligent 2400 baud modem is used on the module and
a 1200 baud modem is used to initiate a call to the 2400 baud modem, the 2400 baud modem will
attempt to connect at 1200 and then set the data link rate at 1200 baud. However the module will
still be set at 2400 baud since it currently does not recognize the CONNECT or CONNECT 1200
string returned by the modem. If you wish to allow variable baud rates upon connect, you will have
to use a modem that allows a fixed data link rate at 9600 baud (or 4800, 2400, or 1200) and a
floating connect rate to match the calling modem and a buffer sufficient to store 2KB of data. The
buffer is required since the SCI module does not support handshaking at this time so when a status
command is sent, the modem will have to be able to store the contents of the status screen while
sending it at a lower speed to the calling modem. One example of a modem that supports a fixed
data rate and has a 3.25KB buffer would be the US Robotics Courier V32bis modem (fixed data link
rate of 9600 baud, error control enabled). Please note that these types of modems are complex in
thier setup and have not been tested extensively with the RSM.
8.1.3.2 Modem setup:
The baud rate of the modem is normally set to match the highest baud rate programmed in the
RSM module that it is capable of achieving. The SCI module is currently not programmed to send
initialization strings to the modem. The modem may or may not have indicator lights for transmitted
data (usually SD or TD), received data (usually RD), data set ready (usually DSR or MR), data ter-
minal ready (DTR or TR), and automatic answer (usually AA - sometimes doubles as a ring indica-
tor). The wiring jumpers incorporated into the modem cable (P/N 877-006-10) connect DSR to DTR
and RTS to CTS and DCD so it should cause the DTR indicator (if any) to come on, and it also
forces the hardware handshaking to be bypassed.
A critical setting is the time period required for the modem to disconnect and hang up the line after
carrier is lost (ie when the calling modem hangs up). A Hayes compatible modem uses the S10
register to define the time period in 0.1 second increments. A typical value would be 2 seconds
(S10=20). If this is set to a value where the modem never hangs up the line after loss of carrier,
then the modem will not answer more than one call.
If the modem does not power up in automatic answer mode (either set via external switches or in
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ternal non-volatile memory) then the following steps will have to be followed:
1) If your modem has external setup switches for answer mode and baud rate:
a) Connect the modem's power input to the power source that will be used during its normal op
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eration (necessary for step 3d below, otherwise any convenient power source).
b) Set the necessary switch such that the modem will power up in automatic answer mode.
c) Set the necessary switch such that the modem will operate at the desired baud rate (see
section 8.1.3.2)
ARGUS TECHNOLOGIES 010-030-C0 Rev E Page 37 of 42
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