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AIX Versions 3.2 and 4 Asynchronous Communications Guide

16-Port Asynchronous Adapters

The family of adapters is based on a common functional design. The individual adapter characteristics, however, are determined by the supported device interfaces. The family consists of two adapters, detailed in the following sections:

The family of 16-port adapters is based on the dual universal asynchronous receiver and transmitter (DUART) chip, which provides two serial communications channels. More information on the DUART chip and its functionality can be found in the "16-Port Asynchronous Adapter Hardware Information".

The following sections contain detailed information about 16-port adapters:

16-Port Asynchronous Adapter - EIA 422A Description

The 16-Port asynchronous adapter - EIA 232 provides support for attaching a maximum of 16 EIA 232 asynchronous serial devices (printers and terminals) to a system unit. Up to eight adapters (any combination within the family) can be used in a single system unit.

This adapter is fully programmable and supports asynchronous communications only. It adds and removes start bits and stop bits. The adapters support even, odd, or no parity on serial data. A programmable baud-rate generator allows operation from 50 to 38400 bps. The adapters support 5-, 6-, 7-, or 8-bit characters with 1, 1.5, or 2 stop bits. A priority interrupt system controls transmit, receive, error, line status, and data set interrupts. The 16 connectors for device attachment are provided in a 16-port cable assembly, EIA 422A, as shown in the following figure:


Installing the 16-Port Asynchronous Adapter

The 16-port asynchronous adapter fits into a single Micro Channel slot in the server. To install the adapter, use the following steps:

  1. Verify that all users are logged off the system and run the following command:
    shutdown -F
  2. When the shutdown command completes, turn the system power switch to the "off" position.
  3. Open the server case and insert the 16-port asynchronous adapter into a free Micro Channel slot.
  4. Attach the 78-pin D-shell connector from the 16-port interface cable to the 16-port adapter (see figure).
  5. Put cover panels back on the system unit
  6. Push the system power switch to the "on" position.

    The system will recognize and configure the 16-port adapter during the boot process.

After the boot completes, login using the root user ID and issue the following command to check the adapter availability:

lsdev -Cc adapter | pg

Only those adapters in the available state are ready for use by the system.

If the newly installed adapter is NOT available, then verify the following:

  1. The adapter is installed correctly into the Micro Channel slot.
  2. All necessary cabling is attached and fitted tightly into place.
  3. Run the command: errpt -a | pg and examine the system error report for problems relating to the adapters.
  4. Run the command: cfgmgr -v | pg. This command will attempt to reconfigure the adapter without rebooting. Watch the paged output for errors.
  5. If running cfgmgr fails, a reboot will be necessary.

16-Port Asynchronous Adapter Hardware Information

The system interface presents a 3-bit address and 8-bit data as well as control lines to the chip. Data from the system interface is serialized for transmission to an external device. The serial data may include a parity bit at the byte boundary. Conversely, data from an external device is deserialized for transmission to the system interface. This data may also include a parity bit, which can be optionally checked. As an option, the channel can operate in first-in-first-out (FIFO) mode.

In FIFO mode, up to 16 bytes can be buffered in both the transmitter and receiver. The serial interface uses start-stop protocol for both data transmission and reception. That is, each byte (plus parity bit) is framed by a start bit and stop bit, which allows synchronization on an individual character (byte) basis.

The DUART chip uses a 12.288 MHz oscillator to generate its internal timing to drive the transmitter and receiver logic. The channel supports full duplex operation. Eight DUART chips are implemented on each 16-port adapter.

Thirteen system-accessible registers are available. Programmable features on each channel include:

The following table is a summary of port (device interface) characteristics for the adapters.

Parameter EIA 232 EIA 422A
Topology Point to Point Point to Point
Maximum data rate (standard) 20Kbps 2Mbps
Maximum data rate (board) 38.4Kbps 38.4Kbps
Transmission media Multiconductor Multiconductor
Number of cable wires 5 including signal ground 5 including signal ground
Maximum cable length 61 m (200 ft) 1200 m < 90Kbps
Device connector 25-pin D 25-pin D
Electrical interface Unbalanced Balanced
Bit encoding Digital bi-level Digital bi-level

16-Port Asynchronous Adapters Adapter Board Priority

The interrupt arbitration logic sets priority for adapters according to the following scheme:

Adapter   Priority
    0     Highest
    1       | 
    2       |
    3       |
    4       |
    5       |
    6       |
    7       |
    8       |
    9       |
    10      |
    11      |
    12      |
    13      |
    14      |
    15    Lowest

Communications Channel Priority

The DUART channels with pending interrupts are serviced according to a fixed-priority scheme. The highest priority is assigned to port 0. Next in priority is port 1, and so forth. The lowest priority is port 15.

16-Port Asynchronous Adapters Interrupt Logic Description

The interrupt logic is divided into two sections:

Both logic sections are implemented on every 16-port adapter. The interrupt generation logic provides the interface to the system. This logic generates the system interrupt requests and contains the interrupt-sharing circuitry.

The function of the interrupt arbitration logic is to identify the 16-port adapter with the highest priority interrupt pending. The logic then places the interrupt information of the highest priority port in the interrupt arbitration register. This is accomplished in one read operation.

The interrupt arbitration logic is unique to the 16-port adapters and should not be confused with the Micro Channel arbitration logic.

Interrupt Generation Logic

The adapter implements the following eight system interrupt request (IRQ) lines:

Only one request line is active during normal operation. All 16-port adapters in one system should use the same interrupt level for optimal system performance. The active line is selected by writing to the appropriate POS register during the setup cycle. The adapter supports interrupt sharing and implements an open collector configuration as defined in the Micro Channel architecture. In this arrangement, the interrupt line is pulled high by a system pull-up resistor. The adapter pulls the line low to indicate an active interrupt request.

Interrupt Arbitration Logic

Up to eight 8-port or 16-port adapters can co-reside and concurrently operate in a system. The interrupt arbitration logic determines the priority for software service when two or more 8-port or 16-port adapters generate interrupts. This logic provides the system with adapter and port identification as well as the interrupt type in a single read operation. Upon detection of an interrupt request, the system reads the 16-bit interrupt arbitration register located at I/O address 0130.

16-Port Asynchronous Adapters EIA 232 Interface Signals

The following interface signals are implemented on each port of the adapter:

Signal Definition
TxD Transmit data
DCD Data carrier detect
DTR Data terminal ready
RxD Receive data
Sig Gnd Signal ground

EIA 232 Signal Voltage Levels

The signal is in the mark state when the voltage on the interchange circuit, measured at the interface point, is less than -3 V dc with respect to the signal ground. The signal is in the space state when the voltage is greater than +3 V dc with respect to the signal ground. The region between +3 V dc and -3 V dc is defined as the transition region and is not a valid level. Voltage less than -15 V dc or greater than +15 V dc is also not a valid level.

During the transmission of data, the mark state denotes binary state 1 and the space state denotes binary state 0.

For interface control circuits, the function is on when the voltage is greater than +3 V dc with respect to the signal ground and is off when the voltage is less than -3 V dc with respect to the signal ground. See the following table for EIA 232 signal levels.

Interchange Voltage Binary State Signal Condition Interface Control Function
+ Voltage 0 Space On
- Voltage 1 Mark Off

Standards Compliance

The electrical characteristics of the 16-Port asynchronous EIA 232 adapter ports conform to the EIA Standard 232C dated October 1969 and to the EIA Standard 232D dated January 1987.

The adapter ports meet the functional requirements for asynchronous operation (start-stop protocol) as described in the EIA Standard 232C dated October 1969 and in the EIA Standard 232D dated January 1987.

16-Port Asynchronous Adapters EIA 422A Interface Signals

The following EIA 422A interface signals are implemented on each port of the adapter:

Signal Definition
TxA Transmit Data
TxB Transmit Data
RxA Receive Data
RxB Receive Data
Sig Gnd Signal Ground

EIA 422A Signal Voltage Levels

The line driver produces a differential voltage in the range of 2 to 6 volts (measured at the generator interface point). The magnitude of the differential voltage at the receiver must be in the range of 200 millivolts to 6 volts (measured at the load interface point).

Measurements are taken at terminal A (positive lead) with respect to terminal B (negative lead). The following table describes the signal states with respect to voltage levels:

Interchange Voltage Binary State Signal Condition
+ Voltage 0 Space
- Voltage 1 Mark

Surge Protection Circuitry

The 16-Port Asynchronous EIA 422A adapter supports indoor cabling up to 1200 m (4000 ft) in length. Cables of such lengths are susceptible to sudden voltage surges due to induced voltages such as indirect lightning strikes. Secondary surge protection circuitry is implemented on the EIA 422A adapter to protect it from these voltage surges. The surge protection circuitry is implemented on the adapter interface data lines.

Fail-Safe Circuitry

Fail-safe circuitry has been added to the input leads of each EIA 422A receiver to prevent fault conditions when the receiver is not connected to a driver (open cable). The fail-safe circuitry sets the receiver to the mark state (binary 1) whenever the receiver is not connected to a driver.

Standards Compliance

The electrical characteristics of the 16-Port asynchronous EIA 422A adapter ports comply with the EIA Standard 422A dated December 1978.

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