There are 2 methods that you can use to get XMODEM support onto your Elf/OS
system.  The easiest, if you have the ability to make your own ROMs, is to get
the XR/XS install ROM image from my site, burn that onto a ROM and replace the
ROM on your PEV2 with the XR/XS ROM.  The XR/XS ROM already contains the PEV2
BIOS as well as the initial jump set to FF00, so it will boot straight to
Elf/OS instead of MINIMON.  Boot your system with this ROM installed and then
do:

     CD /BIN
     INSTALL

  And then select Y for both XR and XS and when it is complete you will have
both the XMODEM sender and receiver programs in your BIN directory ready for
use.

  If you do not have the ability to burn your own ROMs, Elf/OS does contain
tools that would still allow you to get XR onto your system, just needs a bit
of copy/paste.  Here is a terminal session of how to get XR onto your system

$ MINIMON

>!2000 30 1B 0B 02 07 E4 00 00 00 00 70 00 00 00 00 00
>!2010 00 00 00 00 00 00 00 00 00 00 00 9A BF 8A AF 4A
>!2020 FF 21 33 1F 2A F8 00 5A F8 20 BD F8 06 AD F8 03
>!2030 A7 D4 03 06 3B 4F F8 20 BF F8 42 AF D4 03 33 C0
>!2040 03 03 66 69 6C 65 20 65 72 72 6F 72 0A 0D 00 D4
>!2050 20 79 F8 24 BF F8 D3 AF F8 80 AC F8 00 BC D4 20
>!2060 B3 C3 20 70 F8 24 BF F8 D3 AF D4 03 0C C0 20 52
>!2070 D4 03 12 D4 21 AB C0 03 03 8F 73 9F 73 F8 D3 AF
>!2080 F8 22 BF 9E 5F FA FE BE 1F F8 15 5F 1F F8 01 5F
>!2090 1F F8 80 5F 1F F8 00 5F F8 00 AF BF F8 10 AE 2F
>!20A0 8F CA 20 9F 9F CA 20 9F 2E 8E CA 20 9F 60 72 BF
>!20B0 F0 AF D5 8A 73 9A 73 89 73 99 73 88 73 98 73 F8
>!20C0 00 A8 F8 00 B8 F8 D6 AA F8 22 BA 0A A9 52 F8 DB
>!20D0 F4 AA F8 22 7C 00 BA 89 FA 80 C2 20 EC D4 21 31
>!20E0 C3 21 19 F8 DB AA F8 22 BA F8 00 A9 4A 5F 1F 19
>!20F0 18 2C 8C CA 20 D7 9C CA 20 D7 F8 D6 AA F8 22 BA
>!2100 89 5A 88 AC 98 BC 60 72 B8 F0 A8 60 72 B9 F0 A9
>!2110 60 72 BA F0 AA F8 00 F6 D5 88 AC 98 BC 60 72 B8
>!2120 F0 A8 60 72 B9 F0 A9 60 72 BA F0 AA F8 01 C0 21
>!2130 17 8F 73 9F 73 8C 73 9C 73 D4 22 00 C3 21 99 F8
>!2140 D9 AF F8 22 BF 0F 52 F8 D5 AF F8 22 BF 0F F7 CA
>!2150 21 8D F8 D8 AF F8 22 BF F8 00 BC F8 83 AC 4F 52
>!2160 9C F4 BC 2C 8C CA 21 5E 0F 52 9C F7 CA 21 8D F8
>!2170 D4 AF F8 22 BF F8 06 5F 1F 0F FC 01 5F 1F F8 00
>!2180 5F F6 60 72 BC F0 AC 60 72 BF F0 AF D5 F8 D4 AF
>!2190 F8 22 BF F8 15 5F C0 21 39 F8 06 D4 22 9C F8 D7
>!21A0 AF F8 22 BF F8 01 5F F6 C0 21 82 F8 D3 AF F8 22
>!21B0 BF 0F BE D4 FF 66 0A 0D 58 4D 4F 44 45 4D 20 72
>!21C0 65 63 65 69 76 65 20 63 6F 6D 70 6C 65 74 65 0A
>!21D0 0D 0A 0D 00 D5
>!2200 8C 73 9C 73 8A 73 9A 73 8D 73 9D 73 89 73 99 73
>!2210 F8 84 AA F8 01 BA F8 D4 AF F8 22 BF 0F B9 F8 D8
>!2220 AF F8 22 BF F8 92 AD F8 22 BD F8 09 A9 F8 00 F6
>!2230 33 35 7A 30 38 7B 30 38 DD E2 E2 99 76 B9 29 89
>!2240 3A 30 7B F8 08 AC 9E BC F6 F6 BE 3D 4B DD 9C BE
>!2250 DD 30 55 30 5B 9C F6 35 53 F9 80 BC DD 2C C4 C4
>!2260 8C 3A 55 9C 5F 1F 9A F6 BA 3B 74 9C FF 04 3A 74
>!2270 F8 01 30 7B 2A 8A CA 22 43 F8 00 F6 60 72 B9 F0
>!2280 A9 60 72 BD F0 AD 60 72 BA F0 AA 60 72 BC F0 AC
>!2290 D5 D3 9E F6 AE E2 2E 8E 32 91 30 96 AE 8F 73 9F
>!22A0 73 8D 73 9D 73 8E BF F8 09 AF F8 92 AD F8 22 BD
>!22B0 F8 00 F6 33 B8 7A 30 BB 7B 30 BB DD E2 E2 9F 76
>!22C0 BF 2F 8F 3A B3 7B DD DD 60 72 BD F0 AD 60 72 BF
>!22D0 F0 AF D5
>/

$ SAVE
File dump utility
Start address : 2000
End address   : 22D3
Exec address  : 2000
Filename      : XR