STD computer cards

The STD Bus is a computer bus that was used primarily for industrial control systems, but has also found applications in computing. The STD Bus has also been designated as STD-80, referring to its relation to the Zilog Z80 series microprocessors. The term STD is in reference to "standard", but several marketing terms were also promulgated, including simple to design, simple to debug, and swift to deliver.


The STD Bus uses 6.5" by 4.5" expansion card with an edge connector with 56 pins. Many different types of cards have been available for the STD Bus, from processing cards, RAM cards, I/O cards, and specialized cards for various applications.

The use of the STD bus has declined. From the over one hundred manufacturers of components during its peak, vendor numbers have dwindled to under a dozen, but it is still used by hobbyists, manufacturers and in industrial applications.

Connector pin assignments

The STD Bus has a card edge connector with 56 contacts. The pin configuration is as follows. Flow is relative using an STD Bus processor card.[1]

Pin Mnemonic Signal flow Description Pin Mnemonic Signal flow Description
1 +5V In Logic power 2 +5V In Logic power
3 GND In Logic ground 4 GND In Logic ground
5 -5V In Negative logic power 6 -5V In Negative logic power
7 D3 In/out Data bus 8 D7 In/out Data bus
9 D2 In/out Data bus 10 D6 In/out Data bus
11 D1 In/out Data bus 12 D5 In/out Data bus
13 D0 In/out Data bus 14 D4 In/out Data bus
15 A7 Out Address bus 16 A15 Out Address bus
17 A6 Out Address bus 18 A14 Out Address bus
19 A5 Out Address bus 20 A13 Out Address bus
21 A4 Out Address bus 22 A12 Out Address bus
23 A3 Out Address bus 24 A11 Out Address bus
25 A2 Out Address bus 26 A10 Out Address bus
27 A1 Out Address bus 28 A9 Out Address bus
29 A0 Out Address bus 30 A8 Out Address bus
31 WR Out Write to memory or I/O 32 RD Out Read to memory or I/O
33 IORQ Out I/O address select 34 MEMRQ Out Memory address select
35 IOEX Out I/O expansion 36 MEMEX Out Memory expansion
37 REFRESH Out Refresh timing 38 MCSYNC Out CPU machine cycle sync
39 STATUS 1 Out CPU status 40 STATUS 0 Out CPU status
41 BUSAK Out Bus acknowledge 42 BUSRQ In Bus request
43 INTAK Out Interrupt acknowledge 44 INTRQ In Interrupt request
45 WAITRQ In Wait request 46 NMIRQ In Non-maskable interrupt
47 SYSRESET Out System reset 48 PBRESET In Push button reset
49 CLK Out Clock from processor 50 CNTRL In Aux timing
51 PCO Out Priority chain out 52 PCI In Priority chain in
53 AUX GND In Aux ground 54 AUX GND In Aux ground
55 AUX +12V In Aux positive 56 AUX -12V In Aux negative


A focus of the STD bus was its ability to build a system using the exact bus cards required for an application. The compact size of a card made the STD bus system more adaptable to various applications than the contemporary computer buses of the mid-1980s such as the S-100 and the SS-50, because it could use servo control cards along with a fully programmable computer for mathematical operations.

In applications for running an astronomical observatory, the large industrial base of cards, and the system's expandability, made the system desirable for use in a photometry lab to control the telescope as well as do the data logging and computations required.[2]

In typical university laboratory settings of the mid - late 80's, STD bus data acquisition systems were commonplace using Z80 or similar processor cards for the data capture, processing and control, parallel I/O cards for experiment control as well as analogue to digital conversion cards for reading experiment analogue parameters. Such systems would only occupy minimal rack space, while providing full CP/M processing features.[3]


The STD-32 is a pin compatible STD interface that allows the co-existence of both 8-bit and 32-bit systems on a single bus. This is accomplished by the addition of pins between the normal pins that do not connect, nor do they interfere with the original specification. This allows with the proper STD-32 backplane the ability to run legacy cards used for specific applications on the same bus without having to upgrade the complete system.


  1. ^ Prolog 7801 8085A Processor Card Specifications September 1981
  2. ^ The STD Bus and other microcomputer buses for photometrists. By Russell M. Genet and Douglass J. Sauer. From the Fairborn Observatory in Fairborn Ohio.
  3. ^ MICRO-LEARN: A low cost microprocessor development system for laboratory use based on the STD bus, Z-80 CPU and CP/M Operating system. By D. Crosetto(INFN, Turin), Zhong-Ren Gao(Beijing, Inst. Phys.)