7.10 Correction for OPEN COM Transfer & Receive Buffer; TB, RB (59431)

SUMMARY

This article corrects three documentation errors in the description of the Transfer Buffer (TB[n]) and Receive Buffer (RB[n]) options for the OPEN COM statement on page 240 of the "Microsoft Basic 7.0: Language Reference" manual for Basic PDS versions 7.00 and 7.10.

Also, page 559 of "Microsoft Basic 7.0: Programmer's Guide" incorrectly states that "the transmission buffer is allocated 128 bytes for each communications port." This should be changed to "512 bytes" for the default transmission buffer size.

To illustrate the documentation corrections (listed below), consider the following program sequence. PROG1.EXE is started from the MS-DOS or OS/2 command line. PROG1 sets the receive-buffer size to 4096 in the OPEN COM statement, CLOSEs the communications port, and CHAINS to PROG2.EXE. When PROG2 OPENs the COM port, if PROG2 does not use the RB parameter, the size of the receive buffer will still be 4096. This is true whether you compile with /O or use the BRTxxx.EXE run-time module.

Note that the size of the transmit buffer (TB) in PROG1.EXE never affects the size of the transmit buffer in PROG2.EXE. The size of the transmit-buffer is not transferred across a CHAIN. (This is by design.)

This information applies to Microsoft Basic Professional Development System (PDS) versions 7.00 and 7.10 for MS-DOS and MS OS/2.

MORE INFORMATION

The following are three documentation errors in the description of the Transfer Buffer (TB[n]) and Receive Buffer (RB[n]) options for the OPEN COM statement on page 240 of the "Microsoft Basic 7.0: Language Reference" manual:

For the TB[n] option, the first sentence is incorrect, as follows:
Sets the size of the receive buffer to n bytes.
This sentence should be changed to read as follows:
Sets the size of the transmit buffer to n bytes.
For the TB[n] option, the last sentence incorrectly describes the default transmit buffer size, as follows:
The default value, if n or the TB option is omitted, is the current receive buffer size.
This sentence should be changed to read as follows:
The default value, if n or the TB option is omitted, is 512 bytes.
(NOTE: The default size for the transmit buffer is 512 bytes; the only way to change this size is with the TB option in the OPEN COM statement. The transmit-buffer size is never affected by any receive-buffer options.)
The description for the RB[n] option should be supplemented to say that the receive buffer size of the program currently being executed, if not changed by the RB in the OPEN COM statement or by the /C:n option on the QB or BC command line, will be the same as for the program from which the current program was CHAINed (if any) with the CHAIN statement.

The OPEN COM statement can open the device names "COM1:" and "COM2:".