Serial Communication Interface (SCI) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 453

9.3.6 Data Transmission Errors
The following errors may occur during the data transfer process using asynchronous serial
transmission:
• Framing error. A framing error occurs when a received character is improperly
framed by the start and stop bits; it is detected by the absence of the stop bit. This
error indicates a synchronization problem, faulty transmission, or a break condition.
A break is defined as the transmission or reception of a logic 0 for one frame or more.
• Receiver overrun. One or more characters in the data stream were received but
were not read from the buffer before subsequent characters were received.
• Parity error. A parity error occurs when an odd number of bits change value. It can
be detected by a parity error detecting circuit.

9.3.7 Null Modem Connection
When two DTE devices are located side by side and use the TIA-232E interface to exchange
data, there is really no reason to use two modems to connect them. However, the TIA-232E stan-
dard does not allow the direct connection of two DTEs. In order to make this scheme work, a null
modem is needed. The null modem interconnects leads in such a way as to fool both DTEs into
thinking that they are connected to modems. The null modem connection is shown in Table 9.2.

DTE 1 DTE 2

Signal Name Signal Name

DB25 Pin DB9 Pin DB9 Pin DB25 Pin

FG (frame ground)
TD (transmit data)
RD (receive data)
RTS (request to send)
CTS (clear to send)
SG (signal ground)
DSR (data set ready)
CD (carrier detect)
DTR (data terminal ready)
DTR (data terminal ready)

1
2
3
4
5
7
6
8
20
20

-
3
2
7
8
5
6
1
4
4

-
2
3
8
7
5
4
4
1
6

1
3
2
5
4
7
20
20
8
6

FG
RD
TD
CTS
RTS
SG
DTR
DTR
CD
DSR

Table 9.2 ■ Null modem connection

In Table 9.2, the signals of DTE1 and DTE2 that are to be wired together are listed in the
same row. The transmitter timing and receiver timing signals are not needed in asynchronous
data transmission. A ring indicator is not needed either because the transmission is not through
a public phone line.

9.4 The HCS12 Serial Communication Interface

An HCS12 device may have one or two serial communication interfaces. These two SCI
modules are referred to as SCI0 and SCI1, respectively. The SCI0 module shares the use of the
Port S pins PS1 (TxD0) and PS0 (RxD0); the SCI1 shares the use of the Port S pins PS3 (TxD1)
and PS2 (RxD1). The block diagram of the HCS12 SCI module is shown in Figure 9.7.