Introduction
We require a device to interface digital signals between the National Instruments TTL hardware and the Q meters and Yale cards. The existing apparatus for performing this task (the “DIO box”) is no longer suitable for several reasons:
•	The device interfaces directly with a National Instruments DIO card, preventing us from upgrading the NI hardware (e.g. to a PXI system)
•	The device supports many features, such as shim heaters and a target mover, that we no longer need and never intend to use.
•	The box is large and unwieldy, consisting mainly of DIP parts mounted on breadboards.
We set out to design a new DIO module that meets the following product requirements:
•	RF switching for six Q meters
•	Two TTL signals for Yale gain control
•	DC convert signal, both computer-controlled and on manual button press
•	No reliance on specific DIO hardware (i.e. can interface with any TTL source)
New DIO module
We have implemented the new DIO module on a four-layer printed circuit board (PCB), using surface mount components to save space wherever possible. The new board is 4.8” x 1.5” in size and mounts directly to the front panel of a 1U rack mount enclosure; thus, no additional wiring for a user interface is necessary. A 2.5 mm barrel jack must connect to a 5 V power supply (a current rating of 1 A is sufficient). TTL inputs come in through a 10-pin ribbon header, and a 20-pin header contains all Q meter and Yale card outputs. The circuit does not use optical isolation and must share a ground reference with both the input and output signals.
We break out the input header to a DB-25 female connector with the pin assignment in Table 1. The input connector can interface with any 5 V TTL signal source, such as an NI DIO card or PXI module. We break out the output header to a DB-25 male connector with the pin assignment in Table 2, which is the same assignment the old DIO box used.
Table 1 – Input pin assignment
ribbon pin	DB-25F pin	input	
1	1	Q meter 1 enable	
2	2	Q meter 2 enable	
3	3	Q meter 3 enable	
4	4	Q meter 4 enable	
5	5	Q meter 5 enable	
6	6	Q meter 6 enable	
7	14	Yale gain 20	
8	15	Yale gain 50	
9	16	Yale DC convert	
10	17	ground	

Table 2 - Output ribbon header pin assignment
ribbon pin	DB-25M pin	output	
1	21	ground	
2	1	Q meter 1 +	
3	14	Q meter 1 -	
4	2	Q meter 2 +	
5	15	Q meter 2 -	
6	3	Q meter 3 +	
7	16	Q meter 3 -	
8	4	Q meter 4 +	
9	17	Q meter 4 -	
10	5	Q meter 5 +	
11	18	Q meter 5 -	
12	6	Q meter 6 +	
13	19	Q meter 6 -	
14	23	Yale gain 20	
15	24	Yale gain 50	
16	21	Yale DC convert	
17	25	ground	
18	N/C	N/C	
19	N/C	N/C	
20	N/C	N/C	

Q meters
The primary function of the DIO module is to select an active Q meter. Figure 1 shows the schematic for this circuit. Two outputs (QSHUNT+ and QSHUNT-) go to each of the six potential Q meters – one line carries the RF signal while the other carries a ground reference. We use reed relays to short these lines, shunting the RF signal to ground, when the Q meter is disabled. The relays are normally closed, meaning all Q meters are disabled by default. To enable a Q meter, a TTL signal on the corresponding input (QSELECT) opens the relay, removing the short and allowing the RF signal to enter the resonator. An LED on the front panel indicates when a Q meter is enabled.
We are uncertain whether we actually need reed relays here. Although conventional wisdom dictates that they are one of the few effective ways to switch an RF signal, since are only shunting the signal to ground, an engineer here believes analog switches would be sufficient. The old DIO box used reed relays, however, and to minimize our chance of problems, we still use them here.

Figure 1 - Q meter selection circuit
Yale card signals
The DIO box must also provide several TTL signals to the Yale cards. Figure 2 details this part of the circuit. Two inputs (GAIN20 and GAIN50) control these gain settings on the Yale cards (YALE20 and YALE50). Note that, to achieve a gain of 50, both gain 20 and gain 50 must be enabled, since these signals control series-connected amplifier stages. Unlike the old box, the new DIO module buffers the signals to the Yale card, meaning the TTL input does not necessarily need to source any current.
Another important signal to the Yale card is DC convert. A 10 ms TTL pulse here resets the amount of DC subtraction on the Yale card. In previous cooldowns, operators have found it tedious to return to the computer every time they wanted to perform this function, so we added a debounced pushbutton that performs the same task. Now, either a signal on the DCCONV input or a button press will assert the YALEDC output.

Figure 2 - Yale card interface circuit