Lesson plan
Modified Modular Jack
The Modified Modular Jack (MMJ) is a small form-factor serial port connector developed by Digital Equipment Corporation (DEC). It uses a modified version of the 6P6C modular connector with the latch displaced off-center so that standard modular connectors for telephone jacks cannot accidentally be plugged in. MMJ connections are used on Digital minicomputers, such as the PDP-11, VAX and Alpha systems, and to connect terminals, printers, and serial console servers. The MMJ connector has six conductors. As defined by DEC, the six pins are Tx and Rx for the data transmission, their return paths, and DSR and DTR for handshaking. The transmit and receive signals are differential: each signal is the voltage difference between the line and its associated ground, as opposed to a voltage on a single connector relative to a common reference. The electrical signaling is defined by the EIA RS-422 standard. The system can interoperate with RS-232 signaling by shorting the lower voltage sides of each signal to the RS-232 signal ground line. In this case, RS-232 limits apply to data rate and cable length. When connecting two DTE devices such as a computer and a printer, the Digital BC16E crossover cable is used. The DEC VT320 and TI/HP-928 were terminals offering an MMJ option. Using RS-422 signaling over 6-conductor RJ12, cables could be up to 1000 meters.…
Article summary
The Modified Modular Jack (MMJ) is a small form-factor serial port connector developed by Digital Equipment Corporation (DEC). It uses a modified version of the 6P6C modular connector with the latch displaced off-center so that standard modular connectors for telephone jacks cannot accidentally be plugged in. MMJ connections are used on Digital minicomputers, such as the PDP-11, VAX and Alpha systems, and to connect terminals, printers, and serial console servers. The MMJ connector has six conductors. As defined by DEC, the six pins are Tx and Rx for the data transmission, their return paths, and DSR and DTR for handshaking. The transmit and receive signals are differential: each signal is the voltage difference between the line and its associated ground, as opposed to a voltage on a single connector relative to a common reference. The electrical signaling is defined by the EIA RS-422 standard. The system can interoperate with RS-232 signaling by shorting the lower voltage sides of each signal to the RS-232 signal ground line. In this case, RS-232 limits apply to data rate and cable length. When connecting two DTE devices such as a computer and a printer, the Digital BC16E crossover cable is used. The DEC VT320 and TI/HP-928 were terminals offering an MMJ option. Using RS-422 signaling over 6-conductor RJ12, cables could be up to 1000 meters.…
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.