|C H A P T E R 3|
EIA-232 Connectors and Cables
This chapter describes the EIA-232 connectors and cables. It includes the following sections:
SunSAI/P boards are shipped with a connector box (FIGURE 3-1) that provides eight female DB-25 EIA-232 connectors wired for data terminal equipment (DTE) operation.
The pin assignments for the DB-25 connectors are shown in TABLE 3-1 and follow the usual conventions for EIA-232 wiring.
EIA-232 serial interface cables should be shielded, low capacitance cables, ideally designed specifically for serial data transmission.
The shield should be grounded at both ends of the cable. Chassis Ground, available on the shell of the DB-25 connectors, is ideal for this purpose.
While good shielding provides reasonable protection against "noise" (Electro-Magnetic Interference, or EMI), cables should still be routed away from noise sources wherever possible. Avoid laying cables in close proximity to transformers, generators, motors, or fluorescent lights.
The total capacitance of a cable affects the integrity of transmitted data. As a rule of thumb, the total capacitance of a cable (including the connectors) should not exceed 2500 pF for baud rates of up to 57,600 (1200pF for 115K baud and 600 pF for 230K baud). Serial interface cable is usually rated in pico Farads per foot. Therefore, if a cable has a capacitance of 50 pF/ft, and the connectors are 100 pF each, the maximum recommended cable length is 46 feet for baud rates of up to 57,600. If the cable is rated at 12.5 pF/ft, the maximum recommended cable length is 184 feet, and 5 pF/ft cable can be run up to 460 feet.
In situations where low-capacitance cable is unavailable, or very long cable runs are required, "short-haul" modems, available from suppliers such as Black Box, can be used to increase the effective range of the EIA-232 interface. Short-haul modems are similar to standard modems, except that they are connected directly to each other via a cable instead of going through a telephone circuit.