STAMP 2 Communications And Control Projects.pdf

SERIAL PORT INTERFACING 27
components, but most RS-232 devices also regard a signal close to 0 V as a logic 1. So if
our 5-V logic’s 0 is close to 0 V, it will suffice as an RS-232 logic 1.
The STAMP 2 will support this type of “fooling”; you simply program the STAMP 2’s
serial output instruction to send inverted serial data. So now when the STAMP 2 receives
data from conventional RS-232 sources, it is receiving a ±15-V signal. In order to protect
the STAMP 2 from destruction, a 22-k resistor is placed in series with the STAMP 2 I/O
line for incoming RS-232 data.
Many projects presented in this book use the alternative STAMP 2, which utilizes
MAX232 serial line drivers to get serial data to and from the STAMP 2. The baudmode
command is a 16-bit number that specifies all the important characteristics of the serial
transmission, i.e., bit time, data and parity bits, polarity, etc. Table 2-1 illustrates more detail
on decimal baud rate number information and direct versus line driver interfacing. Also
check App. 2 and the STAMP 2 programming manual included on the enclosed CD-ROM.
The Serin and Serout STAMP 2 serial command lines are typically quite similar in their
appearance. The Serin command to send/receive asynchronous data is represented as Serin
rpin{\fpin},baudmode,{plabel,}{timeout,label,}[inputData], and often the command can be
as simple as Serin rpin,baudmode,[inputData]. The Serout command structure for transmitting
asynchronous data is represented as Serout tpin,baudmode,{pace,}[outputData] or
Serout tpin\fpin,baudmode,{timeout,tlabel,}[outputData]. Often the Serout command can
be as simple as Serout tpin,baudmode,[outputData].
Some typical STAMP 2 SERIN commands are shown below.
Serin rpin,baudmode, [inputdata]
In the above example the rpin is the STAMP 2 I/O pin that will be receiving serial data.
Baudmode is the setting for the bit rate, data bits, parity, and polarity. The inputdata specifies
what do to with the incoming data—to compare it, ignore it, store it, or convert it from
text to a numeric value, for example.
TABLE 2-1 SINGLE FRAME OF SERIAL DATA—BAUDMODE (Decimal Numbers)
8 BITS 7 BITS 8 BITS 7 BITS
BAUD RATE NO PARITY EVEN PARITY NO PARITY EVEN PARITY
300 19697 27889 3313 11505
600 18030 26222 1646 9838
1200 17197 25389 813 9005
2400 16780 24972 396 8588
4800 16572 24764 188 8380
9600 16468 24660 84 8276
19200 16416 24608 32 8224
38400 16390 24582 6 8198