Radio Shack TRS-80 Model 100 Basic Manual page 188

On the other hand, you might see every character you type displayed twice. This
would happen if you have the Model 100 set for Half Duplex and the remote system is
set for Full Duplex. If this happens, press
~
to toggle from Half to Full Duplex.
You can transfer'fiIes over the modem using the Upload and Download capabilities of
the terminal. Refer to the previous lesson on serial communications which discusses
how to do this and how to use the other features of the terminal mode. The only
change required is to specify "M" for the baud rate, R, so that communication is
through the built-in modem.
Experiment #3 Log-On from a BASIC Program
The built-in modem can be used from BASIC to communicate with other devices. The
procedure, however, is not especially easy, and is therefore recommended only for
advanced or adventuresome programmers. While it is probably easiest to transfer files
and communicate with other systems using TELCOM, there are times when it is
desirable to use BASIC. For example, you can write a BASIC program to call a stock
quotation service on a periodic basis, say every hour, and save the hourly quotes in a
data file for later analysis.
The following program segment will dial and log-on to the hypothetical IS
(Information Service) from the previous two experiments:
100 0$
=
"123-45G7<'·WM?U87GB,453'·M?PT . . . . l o-tone
h
M?
SNewshM)"
110 M = VARPTR(O$)
120 D
=
PEEKCM+1l + PEEKCM+2l*25G
130 CALL 21200 : CALL 21283,0,0
Line
166
The telephone number and-log on sequence are stored in the string variable
D$.
Line
116
The VARPTR function returns an address M which helps locate the string
D$. Location M contains the length of the string variable, location M
+
1 contains the
least significant byte of the two byte starting address of the string, and location M
+
2
contains the corresponding most significant byte of the address.
Line 128 The address of the string variable D$ is computed and stored in the numeric
variable D. The function PEEK(x) returns the decimal value of the contents of
memory location x.
Line
136
The CALL statement is used to call a machine language subroutine. The
general form is
CALL adr,A,HL
where adr is the starting address of the subroutine, A is an optional eight bit value to
be
passed through the A register, and HL is an optional 16 bit value to be passed
through the HL register. The first statement
CALL 21200
calls a machine language subroutine in the Model 100 ROM which "takes the
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