Modem status register – Measurement Computing COM232/8AT User Manual

Page 14

14 COM232/8AT User’s Guide

March 1998

1000-0901, rev 1.1

MODEM Status Register

The bit definitions for MODEM Status Register are as follows:

MODEM Status Register Definitions

DCD

Data Carrier Detect

Ring Indicator

DSR

Data Set Ready

CTS

Clear To Send

DDCD

Delta Data Carrier Detect

TERI

Trailing Edge Ring Indicator

DDSR

Delta Data Set Ready

DCTS

Data Clear To Send

DCD Data Carrier Detect

− Complement of the DCD input, pin 38.

RI Ring Indicator

− Complement of the RI input, pin 39.

DSR Data Set Ready

− Complement of the DSR input, pin 37.

CTS Cleat To Send

− Complement of the CTS input, pin 36.

Note:

This note pertains to the bits DDCD, TERI, DDSR, and DCTS. These bits are the sources of
the MODEM Status Interrupts. These bits are reset when the MODEM Status Interrupt is
read.

DDCD Delta Data Carrier Detect

− Indicates the Data Carrier Detect (DCD) input [pin 38] has

changed state.
TERI Trailing Edge Ring Indicator

− Indicates the Ring Indicator (RI) input [pin 39] has changed

from a low state to a high state.
DDSR Delta Data Set Ready

− Indicates the Data Set Ready (DSR) input [pin 37] has changed state.

DCTS Delta Clear To Send

− Indicates the Clear To Send (CTS) input [pin 36] has changed state−

Indicates the FIFO is full and another character has been shifted in. The character in the shift register is
destroyed, but is not transferred to the FIFO.

Scratchpad Register (Interrupt ID Register not accessible)

Each 16550 ACE (Asynchronous Communication Element) has a Scratchpad Register at Base+7. The
Scratchpad Register is a read/write register which is not used by the 16550 ACE to perform
communication transfers. However, you can use the Scratchpad Register for general purpose data
storage. The Scratchpad Register is selected by positioning the jumper on JP10 to pins 1 and 2. The
location of JP10 is shown on page 6.

Note: The JP10 jumper should be positioned to Scratchpad for Windows95 applications.

Interrupt ID Register (Scratchpad Register not accessible)

Note: Do not confuse this register with the Interrupt Identification Register.

The Interrupt ID Register is accessed by reading Base+7 of any 16550 ACE. This
returns the interrupt status of all eight ACEs. IPX indicates Interrupt Pending on
the associated channel. The Interrupt ID Register is selected by positioning the
jumper on JP10 to pins 2 and 3. The location of JP10 is shown on page 6.

Note: The JP10 jumper should be positioned to ID Interrupt for

WindowsNT version 4.0 applications.

Interrupt ID Register

IP8

IP7

IP6

IP5

IP4

IP3

IP2

IP1