BCR TTL SERIAL HACK

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What is this all about?

External serial bidirectional communication on the Model 100 today is limited to (1) the RS-232 port and (2) the modem port, both of which use the single UART in the computer.

This hacks shown here enable the BCR port to send and receive half duplex serial TTL signals. The BCR port is already set up for TTL receive function as this is used for the Bar Code Reader function. So these hacks provides facility to transmit as well. In addition I provide some software routines that demonstrate the capability.

There are 2 modifications described. I recommend the improved hack, as it works with both 8085 and NSC800 processors. For CP/M users, a patch is available which can be applied to CPM410/210 to update the CASS functionality.

BCR port serial TTL is supported in Model 100 CP/M as well as with the MVT100 Driver, for use with MVT100 Terminal or other VT100 CRT solution.

Get in touch with me at Twospruces at --the google mail service.

Acknowledgements

I would like to acknowledge and thank those that have contributed to the project:

Philip Avery - for efforts with CP/M and attached Video using the MVT100 Adapter


Improved BCR Hack

Status

  • CP/M: currently supported in CPM210/410 as CASS video option (via patch from me)
  • MVT100: working on implementing support for this new hack

Supported Models

  • TRS-80 Model 100
  • a hardware hack for T102 is likely however not explored at this time


The Hardware Modification

This improved implementation is more universal. SOD pin is available on 80C85 but not Z80. The original modification therefore cannot work on NSC800. By utilizing the /REMOTE signal on the CASSETTE port, we can accomplish the same thing...making this hack usable with both NSC800 and 80C85 processors.

The core of the modification is to connect pin 3 of the BCR port to the REMOTE signal. This is accomplished by adding a single wire to the PCB. Also, we want to maintain functionality of /REMOTE for cassette use. To prevent the cassette relay from toggling as the rapid serial data is transmitted, an single capacitor is added to eliminate the high speed signals, while still enabling /REMOTE to control the cassette motor.

  This page details the hardware change required to transmit at 57600 baud with minimal changes.
  It can be demonstrated that, in the M100, both serial Tx and Rx is possible at 57600 over the BCR port.

The following image shows the modification to the M100 Cassette/BCR port circuitry.

Ckt hack2.png

Here we see a strap added from pin 3 of M34 to pin 3 of the BCR port, defining this pin as Tx data. Also, a single capacitor to ground is added to the base of T6, to remove AC from the /REMOTE signal. I recommend a surface mount 68 uF tantalum capacitor, but other similar capacitors can work. If you hear the relay buzzing, you need a larger capacitor. ;)

The following image shows the placement of the strap and capacitor on the secondary side of the M100 PCB.


Layout hack2.png

The Software

As mentioned, the use of the BCR port to send TTL data to the external CRT is supported in Model 100 CP/M, as well as by using the VT100 Driver, which integrates the external CRT with BASIC, based on Microsoft Disk BASIC for the Model 100.

Additionally, I include some code here as examples. (Rx routines are the same as posted below on the original hack.)

57600 Baud Tx Routines

The following code has been used to patch CP/M to support this new modification. It can also be used in routines on M100.

Tx routine:

   keysnd_routine_v3.zip

Original BCR Hack

Status

  • currently supported in CPM210/410 as CASS video option
  • MVT100 support has been implemented.

Supported Models

  • TRS-80 Model 100
  • a hardware hack for T102 is likely however not explored at this time
  • a similar hack should be possible for all ModelT versions.


The Hardware Modification

The core of the modification is to connect pin 3 of the BCR port to the SOD output of the CPU. This is accomplished by adding a single wire to the PCB.

  This page details the hardware change required to transmit at 57600 baud with minimal changes.
  It can be demonstrated that, in the M100, both serial Tx and Rx is possible at 57600 over the BCR port.
  Tx and Rx at higher speeds is more challenging.  Tx speeds of up to 128000 baud have been demonstrated.
  Transmission at this rate requires disconnection of the cassette port Tx filter at C63.
  If you remove C63 you get better signal quality for TTL serial, but the cassette port is no longer usable.
  If, like me, you never use the cassette port they it may not be a big deal.

The following image shows the modification to the M100 Cassette/BCR port circuitry.

Ckt2.png

Here we see a strap added from pin 12 of M34 to pin 3 of the BCR port, defining this pin as Tx data.

The following image shows the placement of the strap on the secondary side of the M100 PCB.


Layout2.png

The Software

As mentioned, the use of the BCR port to send TTL data to the external CRT is supported in Model 100 CP/M, as well as by using the VT100 Driver, which integrates the external CRT with BASIC, based on Microsoft Disk BASIC for the Model 100.

Additionally, I include some code here as examples.

57600 Baud Routines

Currently both CP/M and my own VT100 driver user 57600, using the modification posted here.

The following code can be compiled, and run in Model 100 to demonstrate transmission between 2 machines at 57600.

Tx routine:

   keysnd_57600.asm

Rx routine:

   bcrx_57600.asm


I may have done the M100 <--> M100 testing using an improved circuit (IE remove C63). I'm not sure; if you do try this experiment keep it in mind that the Tx signal quality is better if you pull C63. This of course makes the cassette port useless.

Higher speed Tx Routines

Faster transmission is possible; both 115600 and 128000 baud have been demonstrated. The core Tx routines are provided here.

for Tx at 57600 baud:

   bcrtx_57600.asm

for Tx at 115600 baud:

   bcrtx_115600.asm

for Tx at 128000 baud:

   bcrtx_128000.asm


Timing for 57600 Baud

Since the processor is 100% busy reading in bits during Rx functions, the requirement for the BCR port receiver is to read and place the byte into memory before the stop bit is completed. The faster the Rx can process the received byte, the faster the link rate can be. At 57600 baud that is about 17 microseconds. Typical processor instruction time is between 1.2 and 1.6 microseconds, so this is about 10-12 instructions max (42.65 clock cycles). I think that is enough time to write to memory and increment a counter or a parity, and a jump or 2. 57600 baud then looks feasible, even for large memory transfers so long as the buffer can be continuously written to without interruption while the PC is sending data. It also means that the RS-232 port could run flat-out at 5.7 kBytes/sec in the Rx direction. In the Tx direction it similar, but it takes a small amount of time to prepare the data before sending the start bit and following data bits. I think it is probably better than 5.5 kbytes/sec.

For CP/M and REXCPM backup/restore this means

  4MB backup restore time could be about 12 minutes @57600 baud
   -- assuming "large packet support" on LaddieAlpha, and direct writes to block RAM.


What about 115600 or 128000 baud? While the Tx side has been shown to work, the Rx side is the challenge. Challenge 1. The M100 ROM requires an initial interrupt and then 3 jumps to get to the RST6.5 hook. The M100 has to make it to the RX code before the data arrives. This places a practical limit of 57600 on the rate. 76800 is too fast. However if an option ROM was used this could potentially be sped up.

A quick look at BCRX.ASM above suggests the RX bit reception time could be reduced to 29 clock cycles, supporting ~85000 baud.