Low Power RS-232C Driver And Receiver In CMOS Datasheet

Low Power RS-232C Driver and Receiver in CMOS

This article sets out to describe the new innovative low power CMOS RS-232C driver and receiver IC’s introduced by National Semiconductor with particular reference to the EIA RS-232C standard Comparison will also be made with existing bipolar driver and receiver circuits The DS14C88 and DS14C89A are monolithic MOS circuits utilizing a standard CMOS process. Important features are a wide operating voltage range (4 5V-12 6V) together with ESD and latch up protection and proven reliability. The Electronics Industries Association released Data Terminal Equipment (DTE) to Data Communications Equipment (DCE) interface standards to cover the electrical mechanical and functional interface between among terminals (i e teletypewriters CRT’s etc ) and communications equipment (i e modems cryptographic sets etc ). The EIA RS-232C is the oldest and most widely known DTE DCE standard Its European version is CCITT V 24 specification It provides for one-way non-reversible single ended (unbalanced) non-terminated line serial digital data transmission. The DS14C88 quad CMOS driver and its companion circuit the DS14C89A quad CMOS receiver combine to provide an efficient low power system for RS-232C or CCITT V 24 applications.

THE DRIVER

The DS14C88 quad CMOS line driver is a pin replacement of the existing bipolar circuit DS1488 MC1488

The DS14C88 is fabricated in CMOS technology and therefore has an inherent advantage over the bipolar DS1488 MC1488 line driver in terms of current consumption Under worst case static conditions the DS14C88 is a miser when it comes to current consumption In comparison with the DS1488 MC1488 line driver a current consumption reduction to 500 mA max versus 25 mA can be achieved.

The RS-232C specification states that the required driver output voltage is defined as being between a5V and a15V and is positive for a logic ‘‘0’’ (a5V to a15V) and negative for a logic ‘‘1’’ (b5V to b15V) These voltage levels are defined when driver is loaded (3000X k RL k 7000X) The DS14C88 meets this voltage requirement by converting HC or TTL LSTTL levels into RS-232C levels through one stage of inversion.

In applications where strict compliance to RS-232C voltage levels is not essential a g5V power supply to the driver may be used The output voltage of the DS14C88 will be high enough to be recognized by either the 1489 or 14C89A receiver as valid data.

The RS-232C specification further states that during transitions the driver output slew rate must not exceed 30V ms. The inherent slew rate of the equivalent bipolar circuit DS1488 MC1488 is much too fast and requires the connection of one external capacitor (330 - 400 pF) to each driver output in order to limit the slew rate to the specified value. However the DS14C88 does not require any external components. The DS14C88 has a novel feature in that unique internal slew rate control circuitry has been incorporated which eliminates the need for external capacitors to be precise a saving of four capacitors per package The 14C88 minimizes RFI and transition noise spikes by typically setting the slew rate at 5V - 6V ms This will enable optimum noise performance but will restrict data rates to below 40k baud.

The DS14C88 can also withstand an accidental short circuit from a conductor in the interconnecting cable to any one of four outputs in a package without sustaining damage to itself or its associated equipment.

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