Hewlett Packard 34721B BCD Module

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.

HP 34721B BCD mod­ule attached to a 34740A dis­play module

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.

HP 34721B BCD mod­ule board bot­tom view

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.

HP 34721B BCD mod­ule range and func­tion rib­bon cable

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.

HP 34721B BCD mod­ule back side of front panel

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.

HP 34721B BCD mod­ule back pan­el with dis­play module

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.

HP 34721B BCD mod­ule over­all view
HP 34721B BCD mod­ule top of cir­cuit board and rib­bon cable

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.

Calibrating the full stack of mod­ules 34740A, 34721B, and 34703A

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