The LGP-21 is a transistorized version of the LGP-30, first made in the early 1950's. The LGP-21 is somewhat -- but not completely -- software compatible with the LGP-30. The LGP-21 is somewhat of a superset of the LGP-30, with a few exceptions. A note by Bob Supnik on Ed Thelen's website prodded me to make this LGP 21 and 30 order comparison chart outlining the differences in order code.

Generally speaking, LGP-30 programs are potentially "close" to LGP-21 programs, and may work if the LGP-30 programs are modified; there are substantial differences in the I/O area. However, LGP-21 programs will almost certainly not work on the LGP-30. In addition there are these issues:

• Optimum coding for the LGP-30 will be decidedly non-optimum on the LGP-21. Since the LGP-21 is 3X slower than the LGP-30, this will not help things any.
• The P command in the LGP-30 does not "block"; the I/O hardware has no knowledge of output device timing. It is up to the programmer to delay printing sufficiently to produce the correct character-output timing. The LGP-21 however blocks until device-ready [Sep 2004 note: not sure if it blocks before/after print start]. Programmatic delays just-sufficient for the LGP-30 will likely be excessive on the LGP-21 (though it should work).
• The P command in the LGP-30 is incompatible with the LGP-21 -- the LGP-30's P command operand contains the character to print ('immediate operand'). The LGP-21 has no equivalent order; it has device address in the track portion of the order, and generally outputs the character stored in the track portion of ACC. The LGP-30 requires order-modification to output variables. It would be very difficult to modify LGP-30 binaries to be LGP-21 compatible. "P 000000" (start input) is the same in both machines.
• All forms of the Z command on the LGP-30 are "STOP", with an argument of 0 listed in the LGP-30 manual. Assuming that LGP-30 programmers actually put "0" in the track/sector fields, the LGP-21 will STOP properly.
• The I (input) order on the LGP-30 has no variants, and is the equivelant to the LGP-21's I command when track/sector is not 6200, 6-bits from the device are input to the accumulator. If the track/sector argument for the LGP-30 I command is 0, as suggested in the manual, then LGP-21 programs will input from device 0 [Sep 2004 I think this is the Friden, but I'm not sure].

Here's a chart of LGP-21 vs. LGP-30 order code.