I MODEM CONTROL REGISTER FORMATS

The modem signals made available by the DMVII can be examined or modified by the user program if needed. This supplies the flexibility needed to meet the various modem interface requirements of dif-ferent countries.

READ MODEM STATUS BSEL4:

Bit

o

2

3

4

Name CARRIER

NOT USED

CLEAR TO SEND

MODEM READY

HALF-DUPLEX

Description

Received line signal detector, commonly referred to as carrier de-tect, indicates that there is an appropriate audio tone being re-ceived from the remote modem. Typically, in full- and half-dupl-ex applications, carrier detect is on whenever the communications line is intact and the remote modem has the sig-nal request to send asserted (the modem is transmitting). This signal is also applicable to the DMVl1 integral modem.

ALWAYS READ AS ZERO.

This signal is generated by the local modem to indicate whether or not it is ready to transmit data. Clear to send is the local modem's response to the asserting of request to send. This signal has a slightly different meaning with different modems. With some modems it indicates that the carrier is being received from the remote modem, and, therefore, is an indication that a suitable communications channel exists.

This signal indicates that the modem is ready to operate. The ON condition indicates that the local modem is connected to the com-munications line and is ready to exchange further control signals with the DMVII. The OFF condition indicates that the local modem is not ready to operate. This signal, when implemented by the modem, is used by the DMVII to detect either a power-off condition or a cable-related modem malfunction.

This signal, when asserted, indicates that the DMVII is in the half-duplex mode. This means that the DMVII is connected to a communications line designed for transmission in either direc-tion, but not in both directions simultaneously. When cleared, it

Bit

This signal serves to control the data channel transmit function of the local modem, and on a half-duplex channel, to control the di-rection of data transmission of the local modem. On a fuii-dupiex channel, the ON condition maintains the modem in the transmit mode, and the OFF condition maintains the modem in the non-transmit mode. On a half-duplex channel, the ON condition maintains the modem in the transmit mode and inhibits the ceive mode. The OFF condition maintains the modem in the re-ceive mode. A transition from OFF to ON instructs the modem to enter the transmit mode. The modem responds by taking such ac-tion as may be necessary and indicates compleac-tion of such acac-tions by asserting clear to send, thereby, indicating to the DMVll that data may be transferred across the communications channel. A transition from ON to OFF instructs the modem to complete the transmission of all data that was previously transferred to the modem and then assume a nontransmit or receive mode, which-ever is appropriate. The modem responds to this instruction by turning OFF the signal clear to send when it is again prepared to respond to a subsequent ON condition of request to send.

This signal controls the switching of the local modem to and from the communications line. When asserted, this signal serves to in-form the local modem that the DMVll is ready to operate. This modem. The ON state of ring must appear approximately at the same time as the ON segment of the ringing cycle (during rings) dicates character-oriented protocol operation, and a zero in-dicates bit-oriented protocol operation. The DMVll initializes this bit to one.

Bit Name DMVII that the local modem has been placed in a test condition.

The ON condition can also be in response to either local or re-mote activation by means of any other modem test condition. Ac-tivation of a telecommunications network test condition (for ex-ampie, faciiity loop back) that is known to the modem can also cause this signal to be ON. In the OFF condition, this signal in-dicates that the modem is not in the test mode and is available for normal operation. definition command. The DMY 11 is equipped with a software in-terlock that prevents simultaneous transmission and reception when in the half-duplex mode. While the transmitter is trans-mitting, the receiver is disabled from receiving data via a

hard-ware interlock. ^I

Bit

6

7

Name Description

band is selected for transmission to the communications channel, and the lower freauencv band is selected for receotion from the communications chann~l. When OFF, the lower f~equency ^band is selected for transmission to the communications channel, and the higher frequency band is selected for reception from the com-munications channel.

This signal controls switching of the local modem to and from the communications line. When asserted, this signal serves to inform the local modem that the DMVll is ready to operate. This signal also prepares the modem for connection to the communications line and maintains this connection as long as it is ON. When turned OFF, this signal causes the local modem to disconnect af-ter all data previously transferred to the modem has been trans-mitted. This signal can be used by the local modem to detect a power-off condition at the DMVll or a cable-related modem malfunction.

This signal determines whether or not the local modem will rapid-ly respond to new data on the communications line. This signal is used at control stations in multipoint networks where the remote modems operate in switched-carrier mode. This incoming signal to the control station appears as a series of short message bursts transmitted by each tributary as it responds to the poll from the control station. In order to permit rapid accommodation to sig-nals from several tributaries appearing in quick succession, the control station informs the local modem when a new signal is about to begin by asserting polling for a brief interval. For syn-chronous systems, clock timing on the incoming message varies from message to message because the remote modems are in no way synchronized to each other. If the time interval between mes-sages is too short, the clock holdover after the end of one message may preclude rapid synchronization on the following message.

The use of this signal allows the control station to reset the modem receiver timing recovery circuit, enabling it to respond more quickly to the line signal present after polling has been turned OFF. This signal applies only to modems that support pol-ling.

C.2 RS-449 VERSUS RS-232-C

The most common interface standard in use during recent years is RS-232-C. However, when used in modern communications systems it has critical limitations; the most serious being speed and distance.

For this reason, the interface standard RS-449 was developed to replace RS-232-C. This standard main-tains a degree of compatibility with RS-232-C to accommodate an upward transition to RS-449.

The most significant difference between RS-449 and RS-232-C is the electrical characteristics of sig-nals used between the data communications equipment (DCE) and the data terminal equipment

(DTE). The RS-232-C standard specifies only unbalanced circuits, whereas, RS-449 specifies both bal-anced and unbalbal-anced circuits. The specifications for these two circuit types supported by RS-449 are contained in EIA standards RS-422-A for balanced circuits and RS-423-A for unbalanced circuits.

These new standards permit greater transmission speeds and allow greater distance between the DTE and DCE. The maximum transmission speeds supported by RS-422-A and RS-423-A specified circuits vary with circuit length. The normal transmission speed limits are 20K bls for RS-423-A at 61 m (200 feet) and 2M bls for RS-422-A also at 61 m (200 feet). These normal transmission speeds can be varied by tradeoffs between speed and distance.

Another major difference between RS-449 and RS-232-C is the specification of two new connectors to accommodate the leads required to support additional circuit functions and the balanced interface cir-cuits. One connector is a 37-pin cinch used to accommodate the majority of data communications appli-cations. The other is a 9-pin cinch for applications requiring secondary channel functions. Some of the new circuits implemented by RS-449 support local and remote loopback testing and standby channel selection.

The transition from RS-232-C to RS-449 will not happen immediately. Therefore, applications that re-quire connection between RS-232-C and RS-449 interfaces must adhere to the limitations of RS-232-C, which specifies a normal transmission speed of 20K bls at a maximum distance of 15.2 m (50 feet).

DMV11 does not support RS-422-A balanced circuits.