IBM System 360/370 Interfacing Circuits

The purpose of the interface is to provide a ready physical connection to System/360 and System/370 control units. Information, in the form of data, status and sense information, control signals, and 110 addresses is transmitted over the time- and function-shared lines of this interface. The I/O interface (channel to control unit) is the communication link between a channel, or line, and the various I/O control units in the IBM System/360 and System/370. Information on the complete design of this interface is available from:

IBM SYSTEM PRODUCTS DIVISION
PRODUCT PUBLICATIONS DEPT. B98
PO BOX 390
POUGHKEEPSIE. N.Y. 12602

The design of the interface provides several important features:

  • A degree of consistency in input/output programming for a wide range of control units.
  • Ready physical connection to System/360 and System/370 control units designed by any manufacturer.
  • Ability to physically accommodate future control units designed to meet the parameters of this interface specification.
  • An interlocked interface operation that is, in most cases, time independent: this feature increases the variety of control units that may be
  • An operation applicable to both multiplex and burst mode operations as well as many control operations and channel-to-channel transmissions.
  • Up to eight control units may be serviced per set of lines.

DRIVER AND RECEIVER REQUIREMENTS 

Driver Requirements 

A driver at one extreme end of the data line must be able to drive up to 10 receivers. Up to 10 drivers must be able to be wire-OR connected to drive one receiver. The receiver would be located at one extreme end of the data line.

In the logic zero state:

  • The driver output voltage must not exceed 0.15 V at a load of 240 µ (See Figure 9-120 for the definition of driver current polarity.)
Figure 9-120. Driver Current Polarity

In the logic one state:

  • The driver output voltage must be 3.11 V or greater at a load of 59.3 mA.
  • The output voltage must not exceed 5.85 V at a load of 30 µ
  • The output voltage must not exceed 7.0 V at a load of 123.0 mA during an overvoltage internal to the driver. 

Drivers must be designed to ensure that no spurious noise is generated on the line during a normal power-up or power-down sequence. For the driver this may be accomplished by one of the following methods:

  • Sequencing the power supplies.
  • Building noise suppression into the circuit.
  • Providing an externally controlled gate. (See Figure 9-121.)
Figure 9-121. Driver Control Gate Switch (S)

For a normal power-down sequence: 

  • Logically ensure that the driver is in the zero state.
  • Close contact S. (See Figure 9-121.)
  • Turn power off. 

For a normal power-up sequence: 

  • Ensure that contact S is closed.
  • Turn power on.
  • Logically ensure that the input level will cause the driver output to be in the zero state.
  • Open contact S.

Receiver Requirements 

If there are multiple receivers on a single data line they must be spaced a minimum of 3 feet apart. An input voltage (relative to receiver circuit ground) of 1.7 V or greater is interpreted as a logic one; an input of 0.70 V or less is interpreted as a logic zero. The receiver should not be damaged by:

  • A dc input of 7.0 V with the receiver power on.
  • A dc input of 6.0 V with the receiver power off.
  • A dc input of -0.15 V with power on or off.

To reduce the loading effect on the line, the receiver input resistance must be greater than 7400Ω with any input voltage from 0.15 V to 3.9 V. and the negative receiver input current must not exceed – 0.24 mA at an input voltage of 0.15 V. (See Figure 9-122 for the definition of receiver current polarity.)

Figure 9-122. Receiver Input Current Polarity

Receivers must be designed to ensure that no spurious noise is generated on the line during a normal power-up or power-down sequence.

GENERAL PHYSICAL AND ELECTRICAL

REQUIREMENTS 

Line Terminations

The terminating network must present a resistance of 95Ω ± 2.5% between the signal line and ground, and must be capable of dissipating 390 mW. An end-of-line driver or receiver may be placed beyond the terminator. In this case the distance between the end-of-line driver or receiver and the terminator must be less than 6 inches. No minimum requirement is set for the spacing between drivers. No minimum requirement is set for the spacing between a terminator and driver or receiver if the terminator is placed on the extreme end of the line. The maximum stub length from the line to a driver or receiver on the circuit card is 6 inches.

Voltage Levels

There are two logic voltage levels. A dc line voltage of 2.25 V or more denotes a logic one state, and a dc voltage of 0.15 V or less denotes a logic zero state. These voltages are relative to the driver ground.

Cable

All data lines must have a characteristic impedance of 92Ω ± 10% and, with the exception of “select out”, must be terminated at each extreme end in their characteristic impedance by a terminating network. For “select out/select in” see Select out Circuitry in following paragraphs. Cable length may be limited under special conditions, but is never to exceed a maximum line resistance of 33Ω. The 33Ω limit includes all contact resistance, internal line resistance and interunit line resistance.

Ground Shift and Noise 

The maximum noise (measured at the receiver input) coupled onto any signal line must not exceed 400 mV. The maximum allowable ground shift, between an active driver and any receiver on the same interface line, is 150 mV. Therefore the maximum shift (coupled noise plus ground shift) allowed on any line is 550 mV. The line logic levels and receiver threshold levels allow for a 550 mV shift. That is, a worst-case 550 mV shift during a logic one state of 2.25 V (minimum) still guarantees a receiver input of at least 1.7 V. (See Figure 9-123.) Also during a logic zero state of 0.15 V (maximum) there is a guaranteed receiver input of less than 700 mV. (See Figure 9-124)

Figure 9-123. Negative Noise Spikes

Fault Conditions 

A grounded signal line must not result in damage to drivers, receivers, or line terminators. With one driver transmitting a logic one, loss of power in any other driver or receiver on the line must not result in any damage to circuits on that line. Data transmission must not be affected by a power-off condition of any driver or receiver on the line.

ELECTRICAL CHARACTERISTICS FOR SELECT

OUT INTERFACE 

General 

The “Select” line has a single-driver to single-receiver configuration, with only the receiver end of the line terminated in its characteristic impedance. A dc line voltage of 1.85 V or greater denotes a logic one state, and a dc line voltage of 0.15 V or less denotes a logic zero state. These voltages are relative to the driver ground. Because of the nature of the select out/select in line, negative noise tolerance has been neglected. All data line requirements not covered here are also applicable to the Select line.

Receiver Requirements 

The Select line receiver must meet all the requirements given for the data line receivers.

Driver Requirements 

The Select line driver must be capable of withstanding an output short to ground, while in the logic one or logic zero state, without damage to the driver circuit.

For the Select line logic zero state:

  • The driver output voltage must not exceed 0.15 V at a load of 1.0 mA.

For the Select line logic one state:

  • The driver output voltage must exceed 3.7 V at a load of 41 mA.
  • The driver output must not exceed 5.8 V at a load of 0.3 mA.
  • The driver output must not exceed 7.0 V at a load of 72 mA during an overvoltage internal to the driver.