HP 59306A Relay Actuator – Frontansicht

HP 59306A "Relay Actuator" (1973)
(click images to enlarge)

The model 59306 is a simple interface with six SPDT relay outputs that can be controlled via front panel pushbuttons or the HPIB.

Small light bulbs (not yet LEDs) in the buttons indicate the relay positions (lit – contact C and A connected; off – contact C and B connected).

The back panel carries 18 binding posts (with 4 mm banana jacks) for the relay contacts, the power and HPIB sockets, and the HPIB address switch.

Inside, the relays are mounted on a carrier board together with the driver transistors. The main board contains only SSI TTL ICs, nothing more complicated than a 7485 (4 bit comparator) or 74138 (3-to-8 decoder).

Controlling the 59306 is easy - send a letter "A" for On or "B" for Off, followed by one or more digits from "1" to "6" indicating which relays to switch.

HP 59306A, rear view

HP 59306A, rear view

HP 59306A, interior view

HP 59306A, interior view

However, owing to the simplistic decoding of characters received from the bus, the 59306 will also respond to characters other than "0"–"9" range as digits — unfortunately, this is also true for carriage return and line feed characters.

Therefore, if e.g. in an HP-85 program, the 59306 (assumed on interface 7, HPIB address 10) were controlled as usual, with

OUTPUT 710; "B1"

then the computer will silently add CR LF to the end of the string, and the wrong output will switch.

HP 59306A main logic board

HP 59306A main logic board

There are several ways to sidestep this issue, one of them being to send the command "manually":


This code explicitly addresses the 59306 (Unlisten, My Talker Address, Listen Address 10) and sends exactly 2 data bytes to the unit.

A more elegant solution uses a feature of the OUTPUT USING statement that is a little hidden in HP's documentation – prefixing the format string with # suppresses the line feed:

OUTPUT 710 USING "#,A,D" ; "B", 1

This and many more details are described in a Programming Note for using the HP59501A with the HP-85.

Here's a little surprise - a 59306 bought via ebay turned out to be a Frankenstein unit:

Apparently, someone had ripped out the original heavy-duty relays and replaced them with mercury-wetted reed relays (mounted vertically as they must be).

Probably this person was in need of a switch for very low voltage signals, which paradoxically the fat contacts in the original relays are not reliable for.

A modified HP 59306A

A modified HP 59306A

A close relative of the 59306 is the 2 × 4 – 59307A VHF Switch.

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