As was pointed out earlier, the DIO II Bus is a superset of the DIO Bus. DIO II can only exist in the large system size card slots. The DIO can exist either in the large slots or in the smaller DIO slots. Although the slots are of different sizes and capablities, there is no ordering of address, interrupt capability, etc, due to slot location in the backplane. All timing specifications apply equally to all slots. It is possible, however, that future mainframes may have slot-dependent features (for example: interrupt prioritizing). Thus, designers must be aware of features of each mainframe that their products will operate in.
Note also that timing specifications for the DIO Bus were developed using the 8 Mhz MC68000.
Thus the DIO II Bus must live with these timing constraints. Designs incorporating other processors must ensure that the DIO II Bus specifications are met.
The DIO II Bus is asynchronous. That is, there is no clock on the backplane to reference signals to. While address and data generation are related to the CPU clock, the actual clock does not appear on the bus. The presence of address or data is indicated by various control lines which execute interlocked handshaking protocol to convey address and data to their destination.
Because the address, data, and control lines are not referenced to a clock on the backplane, signal skew must be controlled to maintain the relative timing between these signals.
DIO II Bus Overview 2-13
For example, the MC68000 is guaranteed to drive the address bus lines 30 nsec prior to asserting data strobes, and the read/write signal.
2. The PC board trace length on bus signals should be as short as possible and must not exceed 3 inches.
3. The SN74LS245 (or equivalent SN74LS244) is used to drive the above signals (may use ALS).
Rationale for These Rules
The above guidelines originated during the early development of the HP 9826. It became clear that controlling bus capacitance was essential. Hence Rules 1 and 2 were established. Also, to minimize bus skew, it was decided to specify a "standard" bus driver. Hence Rule 3 was established. Next, further analysis was performed on the 74LS245 to develop detailed timing specifications.
Two efforts were made to define bus skew:
1. The first effort was to model the 74LS245 and determine formulas for worst-case minimum and maximum gate delays as a function of bus capacitance. method to follow. Therefore, the second effort (measuring delays) was investigated.
Using a sample of 74LS245 parts with different date codes, the worst-case difference in gate delays was measured for a 500 pf load. The difference between the fastest gate and the slowest gate (in different packages) was 9.5 nsec. This was derated by 50% to 15 nsec for margin as well as to cover skew on the bus itself (which is caused by different signal loading). Returning to the MC68000 example, if the address precedes the address strobe by 30 nsec at the MC68000 output, then all receiving devices are guaranteed to have 15 nsec (30 - 15) of address setup time prior to the address strobe (assumes 74LS245's are used to drive the bus).
2-14 DIO II Bus Overview
In addition to determining bus skew, the worst-case high-to-Iow and low-to-high delay times were determined for a 74L8245 driving a 500 pf load. Delay times were measured relative to the output reaching the nominal device threshold. For example, experimental and published data indicates that 1.7V is sufficient to be seen as a high for the 74L8245; hence, the low-to-high time delay measurement concluded when the output reached 1.7V. The results, which are shown below, have been used in calculating several timing specifications. Drivers for signals such as DTACK (which has a pullup resistor) require approximately 50 nsec to drive the bus from high to low. Likewise, a buffer without a pullup has approximately 40 nsec of delay (for example:
the DMA Controller's Fold Buffer). These times include propagation delays.
With 1 kohm pull-up resistor Without pull-up resistor
47.5 nsec high to low 37.5 nsec low to high 39.5 nsec high to low 39.5 nsec low to high
Data was not taken for pull up resistors larger than 1 kohm. However, larger pullups will
"group" the rise and fall times closer to the non-pull up rise and fall tilnes.
In summary, the following points can be made:
1. The skews due to the bus drivers and the bus itself are not specified separately. Instead, a "lumped" skew specification of 15 nsecs is provided for the DIO Bus. For the DIO II Bus the skew was calculated at 25 nsecs. Between any two signals driven by 74AL8245 drivers, 25 nsecs of skew can develop between the signal inputs and outputs (where the outputs are measured at the receiving device on the bus). The increased skew is due to potentially light loaded signals on the DIO II backplane compared with potentially heavy loaded signals on the DIO backplane.
2. In many cases, the DIO Bus timing specifications have been derived using the minimum or maximum MC68000 timing specifications (plus or minus the 15 nsec skew, whichever is appropriate).
3. This guideline does not take into account signals driven by devices other than the 74L8245 or the 74AL8245.
DIO II Bus Overview 2-15
2-16 DIO II Bus Overview