I don't know whether the SR-56 calculator was designed before or after the SR-52, but it certainly looks like a little brother of Texas Instrument's first magnetic card programmable. The SR-56 has no magnetic cards, only 10 registers and 100 program steps, but it has the same 10+2 digit display, same appearance, and a keyboard layout similar to that of the SR-52. On the other hand, it has a few features the SR-52 lacks, such as a t-register for comparison tests, which became standard in later Texas Instruments units; this, along with the model number, suggests that the SR-56 came out sometime after the SR-52. It wasn't a long-lived model either; it was replaced by the TI-57 in a relatively short time, which perhaps explains why there are few SR-56s around these days. I was lucky to obtain one in very good condition recently (albeit without any manuals or accessories.)

Like other first-generation Texas Instruments programmable calculators, the SR-56 was also compatible with the PC-100 printer/security cradle.

100 unmerged program steps is not a heck of a lot. I was wondering whether this calculator had the horsepower to support a moderately complex program, such as an implementation of the Gamma function. In the end, although I had to use a shoehorn, I was able to squeeze in a program that calculated the Gamma function's natural logarithm for positive arguments. In order to do this, I had to rearrange the formula used and introduce a few optimizations specifically designed for the algebraic logic (with operator precedence) of TI calculators.

The program presented here requires that memory registers 3-9 be filled with a predefined set of values. These are presented here with 12 digits of precision. You can only key in ten digits in direct entry, but you can generate 12-digit constants as shown by the following two examples:

11 EE 11 +/- + 2.506628275 = STO 3

.8676043424 + 83 = STO 4

To use this program, key in an argument and hit RST R/S. Remember, the result is the Gamma function's logarithm; to calculate the actual value, hit e^x.

M3: 2.50662827511
M4: 83.8676043424
M5: 1168.92649479
M6: 8687.24529705
M7: 36308.2951477
M8: 80916.6278952
M9: 75122.6331530

00 33	STO
01 01	1
02 64	×
03 34	RCL
04 03	3
05 84	+
06 34	RCL
07 04	4
08 94	=
09 64	×
10 34	RCL
11 01	1
12 84	+
13 34	RCL
14 05	5
15 94	=
16 64	×
17 34	RCL
18 01	1
19 84	+
20 34	RCL
21 06	6
22 94	=
23 64	×
24 34	RCL
25 01	1
26 84	+
27 34	RCL
28 07	7
29 94	=
30 64	×
31 34	RCL
32 01	1
33 84	+
34 34	RCL
35 08	8
36 84	+
37 34	RCL
38 09	9
39 54	÷
40 34	RCL
41 01	1
42 94	=
43 54	÷
44 01	1
45 35	SUM
46 01	1
47 34	RCL
48 01	1
49 54	÷
50 01	1
51 35	SUM
52 01	1
53 34	RCL
54 01	1
55 54	÷
56 01	1
57 35	SUM
58 01	1
59 34	RCL
60 01	1
61 54	÷
62 01	1
63 35	SUM
64 01	1
65 34	RCL
66 01	1
67 54	÷
68 01	1
69 35	SUM
70 01	1
71 34	RCL
72 01	1
73 54	÷
74 52	(
75 34	RCL
76 01	1
77 84	+
78 01	1
79 94	=
80 64	×
81 92	.
82 05	5
83 35	SUM
84 01	1
85 34	RCL
86 01	1
87 45	yx
88 52	(
89 34	RCL
90 01	1
91 74	-
92 05	5
93 94	=
94 13	lnx
95 74	-
96 34	RCL
97 01	1
98 94	=
99 41	R/S