Programmable Four-Channel Communications Controller
The CD2481 is a four-channel synchronous/asynchronous communications controller
specifically designed to reduce host-system processing overhead and increase efficiency in a
wide variety of communications applications. A special member of the CD24X1 family, the
device allows easy field upgrades and enhancement with an on-chip 8K-word microcode store
for downloaded control code. The CD2481 is packaged in a 100-pin MQFP package that offers
10 data/clock/modem pins per channel. The device has four fully independent serial channels to
support standard asynchronous, PPP, MNP4, SLIP, bit-synchronous (HDLC), and byte-
synchronous (bisync, X.21) protocols. The device is non-functional until the microcode is
downloaded; only a small boot ROM with code to perform device initialization is included.
The device is based on a proprietary on-chip RISC processor that performs all the time-critical,
low-level tasks otherwise performed by the host system.
The CD2481 boosts system efficiency with eight on-chip DMA channels, on-chip FIFOs (16
bytes/direction), intelligent vectored interrupts, and intelligent protocol processing. The on-chip
DMA controller provides ‘fire-and-forget’ transmit support — the host need only inform the
CD2481 of the location of the packet to send. Similarly, on receive, the CD2481 automatically
receives a complete packet with no host intervention or assistance. The DMA controller also has
a transmit ‘Append mode’ for use in asynchronous applications.
The DMA controller uses a dual-buffer scheme that easily implements simple or complex buffer
schemes. Each channel and direction in the dual-buffer scheme has two active buffers.
The CD2481 can be programmed to interrupt the host at the completion of a frame or buffer. In
applications where buffers are of a small, fixed size, the dual-buffer scheme allows large frames
to be divided into multiple buffers.
For applications where a DMA interface is not desired, the device can be operated as either
interrupt-driven or polled. This choice is available for each channel and each direction. For
example, a channel can be programmed for DMA transmit and interrupt-driven receive. In either
case, 16-byte FIFOs on each channel and in each direction reduce latency time requirements,
making both software and hardware designs less time-critical. Threshold levels on the FIFOs are
Vectored interrupts are another way the CD2481 helps system efficiency. Separate interrupts are
generated for transmit, receive, and modem-signal/timer changes with unique, user-defined
vectors for each type and channel. This allows very flexible interfacing and fast, efficient
interrupt coding. For example, the Good Data interrupt allows the host to vector directly to a
routine that transfers the receive data — no status or error checking is required.
As of May 2001, this document replaces the Basis Communications Corp.
document CL-CD2481 — Programmable Four-Channel Communications Controller.