Adding a new HP 34700 series module to the collection. This time it is the 34721B BCD interface plug-between module, which converts character serial data from the 34740A or 34750A Display Module to parallel character information for output to a printer.

I was really surprised when this unit popped up on Ebay, as it seems to be an uncommon module when compared to other plug-between modules such as the 34720A battery module.
It can output data to a printer at a maximum rate of 5 samples per second, and as slow as one sample every 3.2 seconds. It also has a hold setting and manual print button.

Seventeen of the IC’s on the circuit board are DM74L74N Dual edge triggered D flip-flop’s, which are mainly used to store digital data for each printing cycle. The ROM and many of the other IC’s are part of the Algorithmic State Machine (ASM), which controls the operation of the module.

This module also has a 14 pin pass-thru connector in the back cutout for connecting the 34720A battery plug-between module. There are two 14 pin sockets on the board labeled XA2 and XA3 which are used for connecting a ribbon cable to the signal conditioning module. Connector XA2 which is closest to the back panel is used for the 34703A DCV/DCA/OHMS module, and connector XA3 is used for connecting the 34701A Voltmeter and 34702A Multimeter modules. The ribbon cable passes information about the signal conditioning module’s function and range settings.

The front panel is fairly simple and includes the rate selection rotary switch, a manual trigger switch, and an output rate LED indicator. My manual trigger switch only operated intermittently, so it was replaced with a SPDT toggle switch.

The back panel consists of a serial number tag and a 50 pin Centronics style connector for attaching a printer. The recommended printer for this module is the HP 5055A 10 column Digital Recorder.


I have performed some general checks of the module, and it seems to working properly, but I will need a printer to fully check it out.
