While almost every Pocket PC device that is available today supports RS-232C serial communications, each has its own unique connector (located on the device), which is used for hooking it to another serial device. Typically, most device manufactures provide a cable that enables it to connect it to a standard DB-9 or DB-25 serial connector.
Two different classifications of devices use serial connections: Data Communications Equipment (DCE) devices and Data Terminal Equipment (DTE) devices. Computers and data terminals are usually classified as DTE devices, whereas DCE devices are typically modems and printers. When a serial connection is established, one typically acts as the DCE, while the other acts as the DTE. For example, a Pocket PC device that is connected to a modem would be considered the DTE; and the modem is considered the DCE.
The RS-232C specification states that serial cables should not be longer than 75 feet at 9,600 bps.
Figure 5.3 shows the standard DB-25 male and female connectors:
Table 5.13 shows the pin assignments for a standard 25-pin serial connector.
Pin | Description |
---|---|
2 | Transmit Data (TD) |
3 | Receive Data (RD) |
4 | Request to Send (RTS) |
5 | Clear to Send (CTS) |
6 | Data Set Ready (DSR) |
7 | Signal Ground (SG) |
8 | Data Carrier Detect (DCD) |
20 | Data Terminal Ready (DTR) |
22 | Ring Indicator (Ring) |
Because asynchronous serial connections typically use only nine wires, most of the newer serial devices use the DB-9 connector (except modems, which always seem to use DB-25). Figure 5.4 shows two examples of what the DB-9 male and female connectors look like.
Table 5.14 shows the pin assignments for a standard 9-pin serial connector.
Pin | Description |
---|---|
1 | Data Carrier Detect (DCD) |
2 | Receive Data (RD) |
3 | Transmit Data (TD) |
4 | Data Terminal Ready (DTR) |
5 | Signal Ground (SG) |
6 | Data Set Ready (DSR) |
7 | Request To Send (RTS) |
8 | Clear to Send (CTS) |
9 | Ring Indicator (Ring) |
When you want to connect two DTE devices (such as two Pocket PC devices or two computers), you need to use what is known as a crossover cable, or null modem cable. A null modem cable makes the DTE device you are directly connecting to look like a DCE connector by reversing the pin contacts between the two devices.
To make a null connector between two standard DB-25 connectors, you need to cross over the wires as shown in Table 5.15.
To make a null connector between two standard DB-9 connectors, you will need to cross over the wires as shown in Table 5.16.
Pin | Pin | Signal |
---|---|---|
2 | 3 | Transmit Data |
3 | 2 | Receive Data |
4 | 5 | Request to Send |
5 | 4 | Clear to Send |
6, 8 | 20 | Data Set Ready and Data Carrier Detect |
7 | 7 | Signal Ground |
20 | 6, 8 | Data Terminal Ready |
Pin | Pin | Signal |
---|---|---|
3 | 2 | Transmit Data |
2 | 3 | Receive Data |
7 | 8 | Request to Send |
8 | 7 | Clear to Send |
6, 1 | 4 | Data Set Ready and Data Carrier Detect |
5 | 5 | Signal Ground |
4 | 6, 1 | Data Terminal Ready |
Finally, if you need to make a null connector between a DB-25 connector and a DB-9 connector, you can cross over the wires as shown in Table 5.17.
25-Pin | 9-Pin | Signal |
---|---|---|
2 | 2 | Transmit Data |
3 | 3 | Receive Data |
4 | 8 | Request to Send |
5 | 7 | Clear to Send |
6, 8 | 4 | Data Set Ready and Data Carrier Detect |
7 | 5 | Signal Ground |
20 | 6, 1 | Data Terminal Ready |