IA8044/IA8344
SDLC COMMUNICATIONS CONTROLLER
Data Sheet
Data Clocking Options
The SIU may be clocked in one of two ways, with an external clock or in a self-clocked mode.
In the external clocked mode a serial clock must be provided on SCLK. This clock must be
synchronized to the serial data. Incoming data is sampled at the rising edge of SCLK. Outgoing
data is shifted out at the falling edge of SCLK.
In the self-clocked mode the SIU uses a reference clock and the serial data to reproduce the serial
data clock. The reference clock can be an external source applied to SCLK, the IA8044/IA8344’s
internal clock or the timer 1 overflow. The reference clock must be 16x or 32x the data rate. A
DPLL uses the reference clock and the serial data to adjust the sample time to the center of the
serial bit. It does this by adjusting from a serial data transition in increments of 1/16 of a bit time.
The maximum data rate in the externally clocked mode is 2.4Mbps in a point-to-point configuration
and 1.0Mbps in a loop configuration. With a 12 MHz cpu clock the maximum data rate in the self-
clocked mode with an external clock is 375Kbps. The maximum data rate in the self-clocked mode
with an internal clock will depend on the frequency of the IA8044/IA8344’s input clock. An
IA8044/IA8344 using a 12MHz input clock can operate at a maximum data rate of 375Kbps.
The Serial mode register bits 5, 6, and 7 select the clocking option for the SIU. (see SMD register
description)
Operational Modes
The SIU operates in one of two modes, AUTO or FLEXIBLE. The mode selected determines how
much intervention is required by the CPU when receiving and transmitting frames. In both modes
short frames, aborted frames, and frames with CRC errors will be ignored.
AUTO mode allows the SIU to recognize and respond to specific SDLC frames without the CPUs
intervention. This provides for a faster turnaround time but restricts the operation of the SIU.
When in AUTO mode the SIU can only act as a normal response secondary station and responses
will adhere to IBM’s SDLC definitions.
When receiving in the AUTO mode the SIU receives the frame and examines the control byte. It
will then take the appropriate action for that frame. If the frame is an information frame the SIU
will load the receive buffer, interrupt the CPU and make the required response to the primary
station. The SIU in AUTO mode can also respond to the following commands from the primary
station.
RR (Receive ready),
RNR (Receive Not Ready),
REJ (Reject),
UP (Unnumbered Poll) also called NSP (Non-Sequenced Poll) or ORP (Optional Response Poll).
In AUTO mode when the transmit buffer is full the SIU can transmit an information frame when
polled for information. After transmission the SIU waits for acknowledgement from the receiving
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