A Higher Speed Operations Proposal for IEC 625-1

By Friedel Hacker, ines GmbH

In the world of test and measurement, the IEC 625-1 (GPIB) standard (also referred to as IEC 60625-1) has proven its reliability over the last two decades. Thousands of instruments are equipped with this widely accepted and popular interface that meets the requirements of the major classes of instruments.

Certain applications exist, however, that require a higher data transmission rate than is possible with the current IEC 625-1 specifications. But there may be requirements for these applications to use the 625-1 interface standard. Any solutions must be fully compatible with past and future implementations to protect the investments of instrument manufacturers and end users.

One solution for applications requiring faster data exchange than the 625-1 specifications, does not modify any of the existing protocols and does not require the implementation of additional circuitry. Instead, it explicitly states the conditions that allow for a higher speed operation. These conditions are a limitation of the conditions stated in IEC 625-1, Clauses 24 and 31.3 (ANSI IEEE 488.1 Standard, Section 3.8 and 5.2.3, respectively) on high-speed operation. Calculations show that a limited cable length and adding resistive loads, up to 15, reduce the data settling time and cause an increase of the transmission speed.

Theoretical Background
There is a theoretical basis for an increase of the data transfer rate beyond the maximum level established by the current standard. The conditions required by the IEC 625-1 standard for high-speed operations that are assumed include:

• All signal lines use 48 mA three state drivers except the NDAC, NRFD and SRQ lines. These three lines use 48 mA open-collector drivers.

• Maximum cabling is 15 meters.

• Device capacitance is <50 pF.

• Cable capacitance is <150 pF/m.

The maximum possible data transfer rate depends on the timing of the bus signals (refer to Figure 1).

Data Settling Time T₁
The data settling time T₁ is the length of time the source waits before it asserts the DAV = True message (DAV line low). The value of T₁ is specified by the IEC 625 standard. It is a fixed value that is based on worst case settling time conditions. The data signal lines are driven by three-state drivers. Consequently, the minimum required settling time value T₁ is determined by the maximum values for t_hl and t_lh3s. The specified 350 ns value for T₁ is based on the largest expected values for t_hl and t_lh3s. These values decrease considerably when the cable length is significantly less than 15 meters or resistive loads are a maximum of 15 per system.

Therefore, the lower values of t_hl and t_lh3s allow a decrease in the currently specified 350 ns data settling time.

Calculation of Transition Times
Examples of the transition times for device driver circuitry are shown in Figure 2 and Figure 3. Where:

• V_s (Supply voltage): V_s = 5 V

• R1, R2 (Terminator resistors): R1 = 3 kW; R2 = 6.2 kW

• C (Total device and cable capacitance): C_dev = 50 pF; C_cable = 150 pF/m

• V_I (Initial voltage over the capacitance C): initial line voltage

• I (Drive current - source/sink)

The circuit of Figure 2 can be simplified to the calculation model shown for Figure 3:

R_p = R₁ x R₂ / R₁ + R₂ = 2 kW

V_D = (R₂ / R₁ + R₂)V_s = 3.4 V

The time needed for the voltage to reach the low and high detection levels, and to determine the signal transition times, can be calculated. Table 1 lists the initial voltage and current values used for calculating the signal transition times on the bus.

Additional Resistive Loads
To improve the performance speed, the IEC 625-1 standard recommends using additional resistive loads. This means that up to 15 loads resistive loads may be added. For example, a system with at least 1 but no more than 15 connected devices (1 < n < 15) has 1 meter of cabling placed between each device, and has loads equal to 15 minus the number of devices (15 - n) added. The values for the parallel resistance and capacitance would be:

• The parallel resistance R_p = 2 x 10^3/ / n + (15 - n) = 133 W

• The capacitance C = n x 50 x 10^-12 + (n-1) x 150 x 10^-12 = 50(4n-3) pF

By substituting these values into the equation:

T₀ = -tln{IR_p + V_D - V₀ / IR_p + V_D - V_I} with t= R_pC

The transition time is:

T₀ = -133 x 50 x (4n-3) x 10^-12 ln {-133 x I + V_D - V₀ / -133 x I + V_D - V_i = 6.65 x (4n - 3) x 10^-9) x ln{133 x I + V_D - V₀ / 133 x I + V_D - V_i.

Substituting the values from Table 1 the values of the transition times can be calculated:

t_hl (high - low transition time), t_hl = 3.48(4n-3) x 10^-9

t_lhRC (low - high transition time), t_lhRC = 5.07 x (4n-3) x 10^-9

t_lh3s (low - high transition on the three-state driver line), t_lh3s = 3.78 x (4n-3) x 10^-9

The optimal value for T₁ is determined by t_lh3s. Using it for T₁, the cycle time, t_cycle can be calculated from:

T_cycle = (3.48 + 5.07 + 2 x 3.78) x (4n - 3) x 10^-9 = 16.11 x (4n - 3) ns.

The transmission frequency F = 1 / t_cycle = 62 / (4n - 3) MB/s

Recommended Changes to IEC 625-1
Some changes recommended include the addition of a note five to Table XXXIX. It should state that the settling time for data signals also should have an adjustable value of >50 ns for all bytes following the first byte, after each false transition of ATN.

To achieve the maximum possible data transfer ratio (nominally to 5 MB/s) within a system, the designer should:

• Use 48 mA three-state drivers.

• Use cable lengths as short as possible to a maximum of 15 m, and with at least one equivalent load per meter to ensure all devices have power.

• Minimize device capacitance on the leads to <50 pF per device.

• Use a settling time (T₁) value consistent with the number and capacitance of devices, cabling length and settling time of the data I/O and the EOI lines.

• Add multiple resistive powered loads, more than one per signal line per device and as many as 15 loads per interconnected system.

Conclusion
Data transfer rates to 10 MB/s are possible based on these concepts, and they require no additional functions. These recommendations also promote full compatibility with existing and previous versions of the IEC 625-1 standard.

ines (innovative Electronik Systeme) GmbH, Neuenhöfer Allee 45, 50935 Köln, Germany. Phone: (011) 49-511-943-810.

For a complete derivation of the settling times and resistive loads go to: www.inesinc.com/iec_dkex.pdf.