UNIBUS

Parts of this topic may be machine translated.

(see in another language - Deutsch, Polski, Русский)

The PDP-11 computer bus, the predecessor of the more modern QBUS.

It was also used in the Eastern bloc (SMEP, JPR 12R, ...), but its physical connection is incompatible.

UNIBUS bus signals

The L at the end indicates an inverted signal ("active low"). Some schemes (JPR 12R, etc.) use the prefix "BUS" for the UNIBUS bus, i.e. e.g. "BUS A 05 L" is "A 05 L", somewhere "B" is also used ("B INIT L" etc.). In modern diagrams created in CAD, you can also find inversion marking at the beginning instead of L at the end - e.g. /A05.

Connection of UNIBUS cards PDP/11

5.18" (13.5 cm) x 4.68" (11.88 cm) card with two side connectors (A,B). The second letter indicates the pin (A..V), the third number in the sequence indicates the side of the card.

Involvement in SMEP

The UBM (universal interface block, such a "backplane") is unified and consists of four positions (R1 - R4) for three-connector (K1, K2, K3) boards. And the first and last positions are only for double connectors (so-called 2/3 boards), if the bus goes to and from the next UBM (K1). So it is realistically possible to install three-connector (so-called 3/3 boards) only in the two middle positions. We are talking about general boards with a common bus, such as ASAD, PAD, QASAD, etc. Most of the other specialists had to have their own "BM" - bus boards. E.g. older tape drive controller, cassette disk controller, etc. These could not be inserted into standard UBM.

Bus drivers

Commonly used DS8641, or compatible DL8641, DS3662. A frequently mentioned replacement for the DS8641 is the AM26S10: it is designed to drive/receive similar bus lines like UNIBUS/QBUS and also contains 4 transmitter/receiver pairs. However, the pinout is different. The 75138 circuit is similar to the AM26S10.

Tesla produced the 120 ohm Unibus MH8641 bus driver (apparently equivalent to the National DS8641). The Russian КР559ИП3 was also used.

parameter DEC spec National Semiconductor DS8641 National Semiconductor DS3662 TI AM26S10 TI SN75138
driver Vol max 0.7V at 70mA 0.7V at 50mA 0.9V at 100mA 0.7V at 70mA 0.45V at 100mA
leakage current max 25 uA driver, 80 uA receiver 100 uA max 100 uA max 100 uA max 25 uA typ, 300 uA max
leakage current max, Vcc = 0V 25 uA driver, 80 uA receiver 100 uA max 100 uA max 100 uA max 1500 uA max
capacitive load 10 pF max ? ? ? ?
driver rise time min 5 ns (Qbus), min 10 ns (Unibus) type 17 ns type 20 ns min 4 ns, type 10 ns ?
driver fall time min 5 ns (Qbus), min 10 ns (Unibus) type 17 ns type 20 ns min 2 ns, type 10 ns ?
receiver Vil 1.3V min 1.3V min 1.5V min 1.75V min 1.8V min
receiver Vih 1.7V max 1.7V max 1.9V max 2.25V max 2.9V max

General circuits in the role of UNIBUS driver

In SAPI-R general TTL: MH8438 for driver and UCY7402 for receiver.

A 74S38 would probably be fine as a driver (V_ol=0.5V at 60mA) and a 74HCT14 or 74LVC14 would probably be fine as a receiver (V_ih around 2.0V), although neither would meet the no-supply drain specifications.

You can approach the drivers with the 74ABT126, connecting the inputs to ground and using the ENABLE signal for data. To really guarantee meeting Qbus specifications, you would have to run two drivers in parallel.

References

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