Data transmission refers to the structure and timing of data, and to flow control between the 7318 and the terminal or modem. It includes the following parameters:
Data rate is the speed at which data travels between devices, measured in bits per second. To successfully transmit data, two devices must transmit at the same data rate; otherwise you may get distorted data or no data at all.
The physics of cables dictates that the faster you transmit data on a cable, the less reliable is its reception at the other end. Many other factors also reduce data integrity. These include:
Since you generally have little control over these factors, you can minimize their effect by using a slower data rate.
A terminal interface usually supports higher communications data rates than the speed at which the terminal processes data. Therefore, if you set the line speed to a substantially higher data rate than the terminal can process, you reduce the reliability of your connection without improving the observed response time.
To choose the optimal speed for the terminal, transmit data at the highest possible rate without introducing errors, but no faster than the terminal can actually process. You can use parity checking to verify the integrity of the data.
Slew rate is the rate of change on a communications line as it transitions from 1 to 0 and from 0 to 1.
Note: Data rate specifies the duration of the ones and zeroes, not the transition time.
The 7318 allows you to program the slew rate in groups of four ports (0-3, 4-7, 8-11, 12-15) under software control. If all your devices operate at speeds of 38.4 Kbps or below, you don't need to set the slew rate, because the default values of the 7318 are appropriate. If, however, you want to run at high speeds, such as 115.2 Kbps, you will need to increase the slew rate.
Slew rate is not directly related to data rate, since theoretically a line should be able to transition from 0 to 1 infinitely fast regardless of the actual data rate.
In practice, however, there is a relationship. Signals with a fast slew rate tend to generate interference with other signals, and this interference can limit the transmission distance of a communications signal. If the slew rate is too slow relative to the data rate, the signal does not reach a sufficiently high voltage to be recognized by the receiver on a 1 bit before it starts transitioning to a 0 bit. Consequently, there is an optimal slew rate for each data rate.
Parity is a coding scheme for checking the validity of data characters. An extra bit transmitted with each character indicates whether the sum of the other bits in the character is even or odd. You can configure most terminals to transmit data, using even, odd, or no parity bits (none).
Parity checking detects reliability problems when transferring data from the terminal to the 7318. However, because most terminals ignore parity errors on data sent to the terminal, parity checking on data sent to a terminal is useless.
The 7318 monitors and logs parity errors on data it receives. You can use the cnsview command to report on the number of errors observed on a port. If you observe parity errors on a port, you should take corrective action, such as:
Stop bits delineate the end of a character in the data stream. Supported stop bits are one and two.
Flow control is the process of pacing data transmission so that the receiver has a chance to process all incoming characters before the transmitter sends additional data. Terminals usually require flow control to display data as fast as possible because some operations like clearing the screen take longer than simply adding a character to the screen. If you set the speed slow enough to accommodate the slowest operation, you unnecessarily delay more common operations. If you set the speed to match the fastest operation, the terminal may drop characters if it encounters a series of slow operations. Flow control enables you to set a high speed without dropping characters when the terminal is doing slow operations.
You can manage flow control using either software or hardware. Software flow control uses ASCII control characters to start and stop character transmission. Hardware flow control uses modem control signals to start and stop transmission.
| Flow Control | |
| Advantages | Disadvantages |
| Software flow control uses only transmit and receive data circuits and does not require circuits in your cable. | Software flow control uses the same Ctrl-S and Ctrl-Q characters that some applications use. |
| Hardware flow control is completely independent of any application or interpretation of the data stream. | Hardware flow control requires additional wires and cables. |
A modem is a data communication equipment (DCE) that uses EIA 232. EIA 232 defines many different circuits as part of the interface between the DCE and the data terminal equipment (DTE). Modems commonly use the circuits shown in the following table.
| Modems | |||
| Signal | Name | Direction | Function |
| TD, TxD XMT | Transmit Data | To modem | The modem receives information on this circuit. |
| RD, RxD RCV | Receive Data | From modem | The modem sends information on this circuit. |
| DTR | Data Terminal Ready | To modem | The 7318 asserts DTR when it wants the modem to answer or dial out and lowers DTR when it wants the modem to hang up. |
| DSR | Data Set Ready | From modem | The modem asserts DSR when it is on. The 7318 assumes the modem is always on; therefore this circuit is not used (except in buddy mode). |
| RTS | Request to Send | To modem | The modem uses RTS for flow control. The 7318 asserts RTS when it is ready to receive more data. |
| CTS | Clear to Send | From modem | The modem also uses CTS for flow control. The modem asserts CTS when it is ready to send more data. |
| DCD, CD | Data Carrier Detect | From modem | Indicates when a phone call connects. The modem lowers DCD when a dial-in connection hangs up or a dial-out connection is lost. |
| RI | Ring Indicator | From modem | Not used on Hayes-style modems and not supported by the 7318 (except in buddy mode). |
| GND, SGND | Signal Ground | Common reference point for all data and control circuits. | |
| RD, REF RXD | Receive Reference | A 7318 circuit that attaches to a signal ground on the modem. | |
| PGND | Protective Ground | Used for the shield of a shielded cable. | |
Control signals provide additional functionality, which is useful with some applications. "Connector Pinouts" explains the pinouts for the control signals.
Modems use DB-25 connectors. Using a modem adapter (DB-25 to RJ-45) to convert the DB-25 connector on your modem to an RJ-45, enables you to use twisted-pair cable between the modem and the 7318.
Serial printers are normally configured as DTEs; that is, they expect to receive data on the receive data line and transmit data on the transmit data line. Like terminals, DTE-configured serial printers require a null modem cable to attach to a 7318.
Serial printers default to EIA 232 connections and use DB-25 D-type connectors. Many printers also support EIA 422 connections.